Download Ultrafast Electron and Molecular Dynamics in Photoinduced and

crystals
Review
Ultrafast Electron and Molecular Dynamics in
Photoinduced and Electric-Field-Induced
Neutral–Ionic Transitions
Takeshi Morimoto 1 , Tatsuya Miyamoto 1,2 and Hiroshi Okamoto 1,2, *
1
2
*
Department of Advanced Materials Science, University of Tokyo, Chiba 277-8561, Japan;
[email protected] (T.Mo.); [email protected] (T.Mi.)
AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory,
National Institute of Advanced Industrial Science and Technology, Chiba 277-8568, Japan
Correspondence: [email protected]; Tel.: +81-471-363-771
Academic Editors: Anna Painelli and Alberto Girlando
Received: 1 April 2017; Accepted: 8 May 2017; Published: 11 May 2017
Abstract: Mixed-stacked organic molecular compounds near the neutral–ionic phase boundary,
represented by tetrathiafulvalene-p-chloranil (TTF-CA), show a unique phase transition from a
paraelectric neutral (N) phase to a ferroelectric ionic (I) phase when subjected to decreasing temperature
or applied pressure, which is called an NI transition. This NI transition can also be induced by
photoirradiation, in which case it is known as a prototypical ‘photoinduced phase transition’. In this
paper, we focus on the ultrafast electron and molecular dynamics in the transition between the N
and I states induced by irradiation by a femtosecond laser pulse and a terahertz electric-field pulse
in TTF-CA. In the first half of the paper, we review the photoinduced N-to-I transition in TTF-CA
studied by femtosecond-pump-probe reflection spectroscopy. We show that in the early stage of the
transition, collective charge transfers occur within 20 fs after the photoirradiation, and microscopic
one-dimensional (1D) I domains are produced. These ultrafast I-domain formations are followed
by molecular deformations and displacements, which play important roles in the stabilization of
photogenerated I domains. In the photoinduced I-to-N transition, microscopic 1D N domains are
also produced and stabilized by molecular deformations and displacements. However, the time
characteristics of the photoinduced N-to-I and I-to-N transitions in the picosecond time domain are
considerably different from each other. In the second half of this paper, we review two phenomena
induced by a strong terahertz electric-field pulse in TTF-CA: the modulation of a ferroelectric polarization
in the I phase and the generation of a large macroscopic polarization in the N phase.
Keywords: photoinduced phase transition; neutral to ionic transition; organic molecular compound;
ultrafast laser spectroscopy; terahertz spectroscopy; ferroelectricity
1. Introduction
The control of an electronic phase and related macroscopic properties by photoirradiation has
recently attracted much attention [1]. This phenomenon is called a photoinduced phase transition
(PIPT), and is important not only as a new phenomenon in the fields of solid-state physics and materials
science, but also as a useful mechanism applicable to future optical switching and memory devices.
When we aim to realize a PIPT in the subpicosecond time scale, promising targets are correlated electron
materials, in which a photoexcited state causes a change in the surrounding electron (spin) systems
via strong electron–electron (e–e) interactions and gives rise to a conversion to another electronic
phase. In real materials, structural changes are sometimes produced through electron–lattice (e–l) or
spin–lattice (s–l) interactions. Research on PIPTs occurring on an ultrafast time scale is based upon
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the development of femtosecond laser technology, in which the photon energy of a femtosecond laser
femtosecond
laser
pulseAcan
be widely varied.
pump-probe optical
using
such a
pulsea can
be widely
varied.
pump-probe
opticalAspectroscopy
using spectroscopy
such a tuneable
femtosecond
tuneable
femtosecond
laser
system
enables
the
measurement
of
transient
changes
in
optical
laser system enables the measurement of transient changes in optical absorption or reflectivity spectra
absorption or reflectivity spectra induced by photoirradiation, from which we can extract the
induced by photoirradiation, from which we can extract the electronic-state changes and the dynamics
electronic-state changes and the dynamics of electron, spin, and lattice degrees of freedom during
of electron, spin, and lattice degrees of freedom during and after the PIPT.
and after the PIPT.
The The
PIPTs
discovered
thus
farfarinincorrelated
electronmaterials
materials
can
classified
classes
PIPTs
discovered
thus
correlated electron
can
be be
classified
intointo
threethree
classes
(I–III),
as
shown
in
Figure
1
[2,3].
In
this
classification
scheme,
we
consider
a
PIPT
from
an
original
(I–III), as shown in Figure 1 [2,3]. In this classification scheme, we consider a PIPT from an original
phasephase
A with
an electron
system
AA
and
a alattice
anotherphase
phaseB Bwith
with
electron
system B
A with
an electron
system
and
latticesystem
system A
A to another
anan
electron
system
and a lattice
system
B. These
three
classes
PIPTs
arecharacterized
characterized as follows.
and aB lattice
system
B. These
three
classes
of of
PIPTs
are
follows.
Phase A
Phase A
Electron system A
Electron
system A
Lattice
system A
Electron
system A
Lattice
system A
Electron
system B
Lattice
system B
Delayed response
~100 fs -10 ps
Instantaneous response
≤ 10 fs
Electron system B
Phase A
Electron
system B
Lattice
system B
Phase B
Phase B
(a) Class I
(b) Class II
Phase B
(c) Class III
Figure
1. Classification
ofofphotoinduced
phasetransition
transition
dynamics
from
A phase
to B phase.
(a)I: Class
Figure
1. Classification
photoinduced phase
dynamics
from
A phase
to B phase.
(a) Class
I: purely
electronic
phasetransition)
transition)
dominated
electron–electron
interaction;
purely
electronicPIPT
PIPT(photoinduced
(photoinduced phase
dominated
by by
electron–electron
interaction;
(b) Class
II: PIPT
dominated
by both
electron–electron
electron
(spin)–latticeinteractions;
interactions; (c)
(c) Class
(b) Class
II: PIPT
dominated
by both
electron–electron
andand
electron
(spin)–lattice
Classdominated
III: PIPT dominated
by electron
(spin)–lattice
interaction
III: PIPT
by electron
(spin)–lattice
interaction
[2]. [2].
Class I: A PIPT in this class is purely electronic, without any structural changes. In this case, e–l and
Class
I: A PIPTare
innegligibly
this classsmall,
is purely
electronic,
without
any structural
changes.
In this
case, e–l and
s–l
interactions
and this
transition
is dominated
by the change
in electron
itinerancy
s–l interactions
are
negligibly
small,
and
this
transition
is
dominated
by
the
change
in
electron
itinerancy
or localization. A typical example of this transition is a photoinduced Mott-insulator-to-metal transition
or localization.
A
typical
example
of
this
transition
is
a
photoinduced
Mott-insulator-to-metal
transition
as observed in one-dimensional (1D) Mott insulators of a bromine-bridged nickel-chain compound
[Ni(chxn)in
2Br]Br
2 (chxn: cyclohexanediamine)
[4] and an organic
molecular compound,
ET-F2TCNQ
(ET:
as observed
one-dimensional
(1D) Mott insulators
of a bromine-bridged
nickel-chain
compound
[5,6],
bis-(ethylenedithio)-tetrathiafulvalene
and F2TCNQ:
[Ni(chxn)
[4] anddifluorotetracyano-quinodimethane)
an organic molecular compound,
ET-Fand
2 Br]Br2 (chxn: cyclohexanediamine)
2 TCNQ
two-dimensional (2D) Mott insulators of layered
cuprates,difluorotetracyano-quinodimethane)
Nd2CuO4 and La2CuO4 [7,8]. The time [5,6],
(ET: bis-(ethylenedithio)-tetrathiafulvalene
and F2 TCNQ:
scale of this type of(2D)
PIPTMott
is considered
to of
be layered
determined
by the Nd
electron
transfer energy t. In the case
and two-dimensional
insulators
cuprates,
2 CuO4 and La2 CuO4 [7,8]. The time
that t is 0.1–0.5 eV, the time scale of the PIPT is 40–8 fs, that is, on the order of 10 fs.
scale of this type of PIPT is considered to be determined by the electron transfer energy t. In the case
Class II: A PIPT in this class is dominated by both the e–e interaction and e–l (or s–l) interactions.
that t is 0.1–0.5 eV, the time scale of the PIPT is 40–8 fs, that is, on the order of 10 fs.
An initial photoexcited state gives rise to an instability of the electronic system A and converts the
Class II:system
A PIPT
is dominated
by both
theoccurs
e–e interaction
and e–lon
(orthe
s–l)
interactions.
electron
A in
to athis
newclass
electron
system B. This
process
rapidly, probably
time
scale
An initial
photoexcited
state
gives
rise
to
an
instability
of
the
electronic
system
A
and
converts
of t. After that, the lattice system is changed from the original system A to the new system B through the
electron
system
A interaction,
to a new electron
system
B. This
process
occurs rapidly,
probably
the
time scale
of
the e–l
(or s–l)
so that the
electron
system
B is stabilized.
The time
scale foron
the
change
of
the that,
lattice
system
determined
by the from
frequencies
of the system
specific A
phonon
dominating
the the
t. After
the
latticeissystem
is changed
the original
to the modes
new system
B through
structural
change, typically
ranging
from
100 fs Btois10
ps. Some The
of PIPTs
in correlated
e–l (or
s–l) interaction,
so that the
electron
system
stabilized.
time observed
scale for the
change of the
electron
materials
such
as
transition
metal
oxides
and
organic
molecular
compounds
are
classified
lattice system is determined by the frequencies of the specific phonon modes dominating the structural
intotypically
this category.
change,
ranging from 100 fs to 10 ps. Some of PIPTs observed in correlated electron materials
Class III: A PIPT in this class is dominated by the e–l (or s–l) interaction. The lattice system A is
such as transition metal oxides and organic molecular compounds are classified into this category.
destabilized by photoexcited states through the e–l (or s–l) interaction, converting the lattice system
Class III: A PIPT in this class is dominated by the e–l (or s–l) interaction. The lattice system A is
A to the new lattice system B. In this case, the initially produced photoexcited states cannot directly
destabilized
by electron
photoexcited
throughthe
thechange
e–l (orins–l)
converting
theelectron
lattice system
system A to
change the
systemstates
A. Following
theinteraction,
lattice system,
the original
the new
lattice
B. to
Inthe
thisnew
case,
the initially
cannotThat
directly
change
A might
besystem
changed
electron
systemproduced
B throughphotoexcited
the e–l (or s–l)states
interaction.
is, the
the electron
systemchange
A. Following
the change
lattice system,
the original
electron
system A might
photoinduced
of the electron
systemin
is the
determined
by the phonon
frequencies.
Photoinduced
structural
phase
observed
in various
some
correlated
electron
be changed
to the
newtransitions
electron system
B through
the materials
e–l (or s–l)including
interaction.
That
is, the photoinduced
materials
are
classified
into
this
category.
change of the electron system is determined by the phonon frequencies. Photoinduced structural phase
transitions observed in various materials including some correlated electron materials are classified
into this category.
Crystals 2017, 7, 132
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Among the three classes of PIPTs, those in class II in particular show a variety of electron, spin,
and lattice dynamics. A prototypical example of this category of PIPTs is a photoinduced transition
between the neutral phase and the ionic phase observed in TTF-CA (tetrathiafluvalene-p-chloranil).
TTF-CA is one of the most famous organic molecular compounds. At room temperature, it is a
van der Waals neutral crystal. By lowering its temperature or applying pressure, a neutral-to-ionic
(N-to-I) phase transition occurs [9,10]. In the I phase, TTF-CA exhibits dimerization of molecules and
electronic ferroelectricity. As a result, the PIPT between the N and I phases in TTF-CA also shows exotic
dynamical properties. The PIPT in TTF-CA was first reported as the photoinduced I-to-N transition
in 1990 by Koshihara et al. [11]. Since that report, a number of experimental and theoretical studies
about this PIPT have been reported [12–40]. In this paper, we review our studies of the photoinduced
transitions from the N phase to the I phase (the NI transition) and from the I phase to the N phase
(IN transition) in TTF-CA based upon femtosecond-pump-probe spectroscopy. In addition, as a new
topic in the field of PIPTs, we report our recent study using a strong terahertz-electric-field pulse, that
is, the modulation and generation of ferroelectric polarization by a strong terahertz-electric-field pulse
in TTF-CA [41,42].
This review article is constructed as follows. In Section 2, we describe the fundamental electronic
properties of TTF-CA. In Section 3, we briefly introduce the methods of femtosecond-pump-probe
spectroscopy to investigate dynamical aspects of photoinduced NI and IN transitions and of
terahertz-electric-field-induced polarization changes. In Sections 4 and 5, we review photoinduced
NI and IN transitions, respectively. In Sections 6 and 7, respectively, we report the control of the
ferroelectric polarization in the I phase and the generation of the ferroelectric polarization in the N
phase by a strong terahertz-electric-field pulse. This review is summarized in Section 8.
2. Fundamental Properties of TTF-CA
2.1. Neutral-to-Ionic Phase Transition
TTF-CA is a mixed-stack organic molecular compound, which is composed of donor (D) TTF
molecules and acceptor (A) CA molecules. The molecular structures of TTF and CA are presented
in Figure 2a. Figure 2b–d show the crystal structure of TTF-CA. TTF and CA molecules stack along
the a axis, forming a quasi-one-dimensional (1D) electronic structure. At room temperature, TTF-CA
is a neutral van der Waals crystal, as shown in Figure 3a. However, upon lowering its temperature
to Tc = 81 K, it undergoes a phase transition to an ionic crystal via collective electron transfers from
TTF to CA molecules (Figure 3b) [10]. This phase transition is called an N-to-I phase transition or
simply an NI transition. The I phase is stabilized by the energy gain resulting from the long-range
Coulomb attractive interaction, that is, the Madelung potential. Simply put, the lattice contraction
caused by lowering temperature increases the Madelung potential, which converts TTF-CA from the
N phase to the I phase. The electronic state of TTF-CA is characterized by the degree of charge transfer
(CT) ρ (> 0) from A to D molecules [43] and expressed as [ . . . D+ρ A−ρ D+ρ A−ρ D+ρ A−ρ D+ρ A−ρ . . . ].
The value of ρ is not equal to 0 or 1, but equal to approximately 0.3 and 0.6 in the N and I phases,
respectively [44–47]. Such partial values of ρ are caused by the hybridization of the neutral and ionic
states through overlapping of the molecular orbitals of D and A molecules along the a axis. In this
review, however, we sometimes use the more simple expressions, [ . . . D0 A0 D0 A0 D0 A0 D0 A0 . . . ] and
[ . . . D+ A− D+ A− D+ A− D+ A− . . . ], to show the N state and the I state, respectively. It is because such
expressions enable intuitive understandings of the NI transition and related photoinduced phenomena.
In the I phase, each molecule has a spin (S = 1/2). As a result, DA molecules are dimerized because
of the spin-Peierls–like instability, as shown by the ovals in Figure 3b [48–50]. That is, in TTF-CA,
the N state with regular molecular stacks and the I state with dimerized molecular stacks are nearly
degenerate. These two states can be switched by photoirradiation.
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Figure
Figure2.
2.(a)
(a)Molecular
Molecularstructures
structuresof
ofTTF
TTFand
andCA;
CA;(b)
(b)Three-dimensional
Three-dimensionalcrystal
crystalstructure
structureof
ofTTF-CA;
TTF-CA;
Figure
2.
(a)
Molecular
structures
of
TTF
and
CA;
(b)
Three-dimensional
crystal
structure
of
TTF-CA;
(c,d)
Crystal
structure
of
TTF-CA
projected
on
(c)
bc
plane
and
(d)
ab
plane.
Classification
(c,d) Crystal
Crystal structure
structure of
of TTF-CA
Classification of
of
(c,d)
TTF-CA projected
projected on (c) bc plane and (d)
(d) ab
ab plane.
plane. Classification
of
photoinduced
phase
transition
dynamics
from
A
phase
to
B
phase.
photoinduced
phase
transition
dynamics
from
A
phase
to
B
phase.
photoinduced phase transition dynamics from A phase to B phase.
Figure
Figure 3.
3. (a,b)
(a,b)Schematic
Schematicof
ofelectronic
electronicstructures
structuresin
in(a)
(a)the
theneutral
neutralphase
phaseand
and(b)
(b) the
the ionic
ionic phase
phase of
of
Figure
3.
(a,b)
Schematic
of
electronic
structures
in
(a)
the
neutral
phase
and
(b)
the
ionic
phase
of
TTF-CA.
Ovals
in
(b)
show
dimers.
TTF-CA.
Ovals
in
(b)
show
dimers.
TTF-CA. Ovals in (b) show dimers.
In
InFigure
Figure 4a,b,
4a,b,we
weshow
showthe
themolecular
molecularstacking
stackingviewed
viewedalong
alongthe
thedirection
directionperpendicular
perpendicularto
tothe
the
In
Figure
4a,b,
we
show
the
molecular
stacking
viewed
along
the
direction
perpendicular
to
the
aaa axis
at
90
K
in
the
N
phase
and
at
40
K
in
the
I
phase,
respectively
[41,49].
At
40
K,
the
difference
axis at
at 90
90 K in the N phase and at 40 K in the I phase, respectively [41,49]. At
At 40
40 K,
K, the
the difference
difference
axis
between
interdimer
distance
(L
and
the
intradimer
distance
)) is
approximately
0.17
Å,
between the
the interdimer
(L(L
) 11is
approximately
0.17
Å, and
the
between
interdimerdistance
distance(L
(L22)2))and
andthe
theintradimer
intradimerdistance
distance
is
approximately
0.17
Å, and
and
1(L
the
displacement
of
each
D(A)
molecule
along
the
a(−a)
direction,
=
(
−
)/2,
is
approximately
displacement
of each
D(A)
molecule
along
thethe
a(−a(−a)
a) direction,
/2, is approximately
approximately
the
displacement
of each
D(A)
molecule
along
direction,l ==( L(2 −−L1 ))/2,
0.085
of
the
averaged
molecular
distance
(3.60
Å).
the
0.085Å.
Å.This
Thisvalue
valueof
approximately2.4%
2.4%
averaged
molecular
distance
(3.60
Å). Thus,
0.085
Å.
This
value
ofof lis
isisapproximately
approximately
2.4%
ofof
thethe
averaged
molecular
distance
(3.60
Å). Thus,
Thus,
the
molecular
dimerization
in
the
I
phase
is
large,
and
plays
important
roles
on
the
photoinduced
and
the molecular
dimerization
in the
I phase
is large,
and
plays
importantroles
roleson
onthe
thephotoinduced
photoinduced and
and
molecular
dimerization
in the
I phase
is large,
and
plays
important
electric-field-induced
electric-field-induced charge
electric-field-induced
charge and
and molecular
molecular dynamics
dynamics in
in TTF-CA.
TTF-CA.
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Figure4.4.(a,b)
(a,b)Molecular
Moleculararrangements
arrangementsviewed
viewedalong
alongthe
thedirection
directionperpendicular
perpendiculartotothe
thestacking
stackingaxis
axis
Figure
andLL2 2inin(b)
(b)
axis)atat(a)
(a)90
90K
(theneutral
neutralphase)
phase)and
and(b)
(b)40
40K
(theionic
ionicphase)
phase)in
TTF-CA[41].
[41].LLL1 1and
(a(aaxis)
(a)
90
KK(the
(the
neutral
phase)
and
(b)
40
KK(the
(the
ionic
phase)
ininTTF-CA
TTF-CA
[41].
1
2
denote
the
intradimer
and
interdimer
distances,
respectively.
denote the intradimer and interdimer
interdimer distances,
distances, respectively.
respectively.
2.2. Optical Reflectivity Spectra
2.2. Optical Reflectivity Spectra
In this subsection, we review the steady-state optical spectra in TTF-CA, which sensitively reflect
In this subsection, we review the steady-state optical spectra in TTF-CA, which sensitively reflect
the
changes
thedegree
degreeofofcharge
chargetransfer
transferρρacross
acrossthe
theNI
NItransition.
transition.In
Inseveral
severalprevious
previousstudies
studiesofof
the changes ininthe
ρ across the
NI transition.
PIPTsininTTF-CA,
TTF-CA,photoinduced
photoinducedchanges
changes inthe
thereflectivity
reflectivityspectra
spectrawere
wereindeed
indeed usedas
asprobes
probesofof
PIPTs
PIPTs
in TTF-CA, photoinduced
changes in in
the reflectivity
spectra were
indeed usedused
as probes
of both
both
photoinduced
NI
and
IN
transitions.
both photoinduced
NIIN
and
IN transitions.
photoinduced
NI and
transitions.
Figure5a
5ashows
showsthe
thepolarized
polarizedreflectivity
reflectivityspectra
spectraininTTF-CA
TTF-CAwith
withan
anelectric
electricfield
fieldofoflight
lightEE
Figure
parallel(//)
(//)and
andperpendicular
perpendicular (⊥)totothe
the molecularstacking
stacking axisaa[19].
[19]. Thered
red andblue
blue lines show
parallel (//)
and perpendicular(⊥)
(⊥) to themolecular
molecular stackingaxis
axis a [19]. The
The red and
and blue lines show
the
reflectivity
in
the
N
phase
at
90
K
and
in
the
I
phase
at
77
K,
respectively.
The
prominent
peak
the reflectivity in the N phase at 90 K and in the I phase at 77 K,
K, respectively.
respectively. The prominent peak
around0.65
0.65eV
eVobserved
observedfor
forE//a
E//a is attributedtotothe
the CTtransition
transition betweenDDand
and A moleculesalong
along
around
0.65 eV
observed for
E//aisisattributed
attributed to theCT
CT transition between
between D and A+ molecules
−
the aaxis
axis[9,10,51,52].
[9,10,51,52]. Itcorresponds
correspondstotothe
thetransition
transitionthat
thatgenerates
generatesan
anionic
ionic(D
(D
+ A
− ) pair in the N
+
−
A )) pair
the a axis [9,10,51,52]. It corresponds to the transition that generates an ionic (D A
pair in the N
0A0) pair in the I phase. The peaks at 2–2.5 eV and at approximately 3 eV observed
phaseor
oraaneutral
neutral(D
(D
0A
0
0
phase
)
pair
in
the
I
phase.
The
peaks
at
2–2.5
eV
and
at
approximately
3
phase or neutral (D
) pair in the I phase. The peaks at 2–2.5 eV and at approximately eV observed
forE⊥a
E⊥aare
areassigned
assignedtotothe
the intramolecular(IM)
(IM)transitions
transitions of TTF molecules. Weshow
showthe
theschematics
schematics
for E⊥a are assigned to the intramolecular
intramolecular
transitions of TTF molecules. We
We
of CT and IM transitions in Figure 5b,c.The
TheIM
IM transitionatatapproximately
approximately 2.2 eV is a transition of a
of CT and IM transitions in Figure 5b,c.
transition
2.2 eV is a transition of a
TTFcation,
cation,indicated
indicatedby
bythe
theblue
bluecurved
curvedarrow
arrowininFigure
Figure5c.
5c.ItItisisweakly
weaklyobserved
observedeven
evenininthe
the N phase
TTF
TTF cation, indicated by the blue curved arrow in Figure 5c. It is weakly observed even in
the N phase
phase
becauseofofthe
the hybridizationofofneutral
neutral andionic
ionic states.The
The higher-energypeak
peak at approximately33
because
because
of the hybridization
hybridization of neutraland
and ionicstates.
states. Thehigher-energy
higher-energy peakatatapproximately
approximately
eVisisanother
anotherIM
IMtransition
transitionofofTTF
TTFfrom
fromthe
thehighest
highestoccupied
occupiedmolecular
molecularorbital
orbital(HOMO)
(HOMO)totothe
thelowest
lowest
3eVeV
is another
IM transition
of TTF from
the highest
occupied molecular
orbital (HOMO)
to the
unoccupiedmolecular
molecularorbital
orbital(LUMO),
(LUMO),indicated
indicatedby
bythe
thegreen
greencurved
curvedarrows
arrowsininthe
theFigure
Figure5b,c.
5b,c.As
As
unoccupied
lowest
unoccupied molecular
orbital (LUMO),
indicated
by the green
curved arrows
in the Figure
5b,c.
shown
in
Figure
5a,
the
spectral
shapes
of
these
CT
and
IM
transitions
are
changed
between
the
shown
in Figure
5a,5a,
thethe
spectral
shapes
ofofthese
NN
As
shown
in Figure
spectral
shapes
theseCT
CTand
andIM
IMtransitions
transitionsare
arechanged
changed between
between the
the N
and
I
phases;
the
IM
transition
bands
shift
to
the
lower
energy
and
the
CT
transition
band
shift
to
the
and II phases;
the IM
bands shift
shift to
to the
and the
the CT
band shift
shift to
to the
the
and
phases; the
IM transition
transition bands
the lower
lower energy
energy and
CT transition
transition band
higher
energy
with
increasing
ρ
.
Therefore,
we
can
use
the
reflectivity
changes
in
these
two
energy
higher energy
energy with
with increasing
increasing ρ
the reflectivity
reflectivity changes
changes in
these two
two energy
energy
higher
ρ.. Therefore,
Therefore, we
we can
can use
use the
in these
regionstotoprobe
probethe
thetransient
transientchanges
changesininρρby
by aphotoirradiation
photoirradiationand
andby
byterahertz-pulse
terahertz-pulseexcitation.
excitation.
regions
regions
to probe the
transient changes
in ρ by aa photoirradiation
and by
terahertz-pulse excitation.
0.8
0.8
0.6
0.6
Reflectivity
Reflectivity
(a)
(a)
(b)
(b)
EE////aa
77KK
77
90KK
90
000
0
(c)
(c)
Ionic
Ionic
_
+ CA
_
TTF
+
TTF CA
_
+
_
(D+) ) (A
(A) )
(D
CT
CT
transition
transition
0.4
0.4
0.2
0.2
Neutral
Neutral
0 CA0
TTF
0
TTF00 CA
(D0) ) (A
(A0)0)
(D
eV
~~33eV
(×3)
EE⊥⊥aa(×3)
11
22
33
Photonenergy
energy(eV)
(eV)
Photon
44
IM
IM
transition
transition
CT
CT
transition
transition
eV
~~33eV
2.2eV
eV
~~2.2
IM
IM
transition
transition
Figure
5.5.(a)
(a)
Polarized
reflectivity
spectra
at
90
KKin
in
the
neutral
phase
and
at
77K
in
the
ionic
phase
of
Figure5.
(a)Polarized
Polarizedreflectivity
reflectivityspectra
spectraat
at90
90K
inthe
theneutral
neutralphase
phaseand
andat
at77K
77Kin
inthe
theionic
ionicphase
phaseof
of
Figure
TTF-CA
[19];
(b,c)
Schematic
of
optical
transitions
neutralphase
phase(b)
(b)and
andinin
in
the
ionic
phase
(c).
TTF-CA[19];
[19];(b,c)
(b,c)Schematic
Schematicof
ofoptical
opticaltransitions
transitionsininthe
theneutral
neutral
phase
(b)
and
the
ionic
phase
(c).
TTF-CA
the
ionic
phase
(c).
Crystals 2017, 7, 132
Crystals 2017, 7, 132
2.3. Electronic Ferroelectricity
6 of 41
6 of 41
2.3. Electronic
Ferroelectricity
When
a DA stack
is dimerized in the I phase, the inversion symmetry is lost within individual
DA stack
is dimerized
in themoment.
I phase, the
inversion
symmetry
is lost within
individual
stacks andWhen
eachastack
should
have a dipole
X-ray
and neutron
diffraction
studies
on TTF-CA
stacks
and
each
stack
should
have
a
dipole
moment.
X-ray
and
neutron
diffraction
studies
on the I
have revealed that the dimeric molecular displacements are three-dimensionally ordered in
TTF-CA
have
revealed
that
the
dimeric
molecular
displacements
are
three-dimensionally
ordered
in
phase [49]. This means that macroscopic polarization can be generated in the I phase.
the
I
phase
[49].
This
means
that
macroscopic
polarization
can
be
generated
in
the
I
phase.
When we consider a displacive-type ferroelectricity in TTF-CA, dimeric molecular displacements
When we consider a displacive-type ferroelectricity in TTF-CA, dimeric molecular
of the ionic molecules, D+ and A− , in opposite
directions produce a ferroelectric polarization Pion
displacements of the ionic molecules, D+ and A−, in opposite directions produce a ferroelectric
along the a axis, as indicated by the grey arrows in Figure 6a. However, detailed X-ray studies and
polarization
along the a axis, as indicated by the grey arrows in Figure 6a. However, detailed
theoretical
simulations
have recently
revealedhave
that recently
the direction
of the
is opposite
X-ray studies
and theoretical
simulations
revealed
thatnet
thepolarization
direction ofPthe
net
to that
of Pion [53–55].
This indicates
polarization
P originates
not from the displacements
polarization
is opposite
to that ofthat the
[53–55].
This indicates
that the polarization
originates
of ionic
(Pion ), but from
the additional
withcharge-transfer
the magnitude of
not molecules
from the displacements
of ionic
molecules (charge-transfer
), but from processes
the additional
processes
with
the
magnitude
of
δ
(~0.2)
within
each
dimer,
as
shown
by
the
curved
arrows
in
δρ (~0.2) within each dimer, as shown by the curved arrows in Figure 6b. It is natural
to consider
Figure
6b. It is natural
considerδρ
that
this additional
δ (~0.2)The
is induced
at the NI
that this
additional
chargetotransfer
(~0.2)
is inducedcharge
at thetransfer
NI transition.
polarization
due to
The polarization
due
these charge-transfer
is denoted by is. called
This type
of
thesetransition.
charge-transfer
processes
is to
denoted
by Pel . This processes
type of ferroelectricity
electronic
ferroelectricity is called electronic ferroelectricity, since the polarization is dominated by changes of
ferroelectricity,
since the polarization is dominated by changes of electron transfers or electron
electron transfers or electron redistributions [56]. The magnitude of the polarization is very large,
redistributions [56]. The2 magnitude of the polarization is very large, reaching 6.3 µC/cm2 , which is 20
reaching 6.3 μC/cm , which is 20 times as large as that estimated by the point charge model [53]. A
timesmolecular
as large ascompound,
that estimated
by the point charge model [53]. A molecular compound, α-ET2 I3 [57,58],
α-ET2I3 [57,58], and a transition metal oxide, LuFe2O4 [59], are also known
to
and ashow
transition
metalferroelectricity.
oxide, LuFe2 OTherefore,
also
knownwe
to show
electronic
4 [59], are in
electronic
TTF-CA,
can expect
that ferroelectricity.
the amplitude ofTherefore,
the
in TTF-CA,
we Pcan
expect
that the inamplitude
of the polarization
could be modulated
in the
polarization
could
be modulated
the subpicosecond
time scale by a P
terahertz-electric-field
pulse
subpicosecond
time scale
by transfers.
a terahertz-electric-field pulse via intermolecular charge transfers.
via intermolecular
charge
Figure
6. (a,b)
Direction
of of
ferroelectric
inthe
thecase
case
displacive-type
ferroelectricity
Figure
6. (a,b)
Direction
ferroelectricpolarization
polarization in
ofof
(a)(a)
displacive-type
ferroelectricity
ion
and
P
el
are
polarizations
originating
from
molecular
and
(b)
electronic-type
ferroelectricity.
P
and (b) electronic-type ferroelectricity. Pion and Pel are polarizations originating from
molecular
displacements
and
from
fractional
intermolecular
charge
transfers,
respectively.
P
a net
displacements and from fractional intermolecular charge transfers, respectively. P is a netis polarization
polarization
(P = Pion + Pel).
(P = P
ion + Pel ).
2.4. Imaging of Ferroelectric Domains
2.4. Imaging of Ferroelectric Domains
In the I phase of TTF-CA, second-harmonic generation (SHG) was observed [16,41,42]. This
In
the Ithat
phase
of TTF-CA,
second-harmonic
generation
(SHG)scale.
wasHowever,
observed
[16,41,42].
suggests
ferroelectric
polarization
is indeed formed
on a macroscopic
to study
This the
suggests
that
polarization
formedand
on lattice
a macroscopic
scale. However,
changes
in ferroelectric
the polarization
as well asisinindeed
the electron
systems induced
by a
photoirradiation
by apolarization
terahertz-electric-field
it electron
is fundamentally
important
to know
the by a
to study
the changesand
in the
as well aspulse,
in the
and lattice
systems
induced
size
and
distribution
of
ferroelectric
domains
in
the
I
phase
in
an
as-grown
single
crystal.
photoirradiation and by a terahertz-electric-field pulse, it is fundamentally important to know the size
Several methods
are known
to visualize
in thesingle
whole crystal.
area of a millimetreand distribution
of ferroelectric
domains
in theferroelectric
I phase indomains
an as-grown
size
crystal.
The
most
popular
method
is
to
use
an
SHG
interference
microscope
[60]. In
method,
Several methods are known to visualize ferroelectric domains in the whole area
of this
a millimetre-size
ferroelectric domains of a sample can be visualized using the interference of an SH light emitted from
crystal. The most popular method is to use an SHG interference microscope [60]. In this method,
the sample with that from a reference in a transmission configuration. However, this method cannot
ferroelectric domains of a sample can be visualized using the interference of an SH light emitted from
be applied to TTF-CA. A single crystal of TTF-CA is thicker than ~100 μm and the transmittance is
the sample
a reference
in and
a transmission
configuration.
However,
this
method
almost with
zero that
in thefrom
near-infrared
(IR)
visible regions,
such that it is
difficult to
detect
SHGcannot
in a be
applied
to
TTF-CA.
A
single
crystal
of
TTF-CA
is
thicker
than
~100
µm
and
the
transmittance
is
almost
transmission configuration. To perform domain imaging in a reflection configuration, Kishida, H. et al.
zero in
the
near-infrared
(IR)
and
visible
regions,
such
that
it
is
difficult
to
detect
SHG
in
a
transmission
adopted the electro-reflectance (ER) spectroscopy [61]. In this method, two electrodes were put on
configuration. To perform domain imaging in a reflection configuration, Kishida, H. et al. adopted the
electro-reflectance (ER) spectroscopy [61]. In this method, two electrodes were put on both sides of a
Crystals 2017, 7, 132
Crystals 2017, 7, 132
7 of 41
7 of 41
crystal (the bc plane) and the reflectivity changes under the presence of a quasi-static electric field were
both sides of a crystal (the bc plane) and the reflectivity changes under the presence of a quasi-static
measured on the ab plane, as shown in Figure 7a. When a polarization P is parallel (antiparallel) to
electric field were measured on the ab plane, as shown in Figure 7a. When a polarization P is parallel
the applied
electric field, P is increased (decreased). If the change in P causes a change in reflectivity
(antiparallel) to the applied electric field, P is increased (decreased). If the change in P causes a change
(R), the
direction
P the
candirection
be determined
the electric-field-induced
R change (∆R).
By measuring
in reflectivityof
(R),
of P canfrom
be determined
from the electric-field-induced
R change
(ΔR).
the spatial
distribution
of
∆R
with
microscopic
spectroscopy,
we
can
construct
ferroelectric
domain
By measuring the spatial distribution of ΔR with microscopic spectroscopy, we can construct
images
over a whole
crystal.
ferroelectric
domain
images over a whole crystal.
7. Polarization-domainimaging
imaging by
by electro-reflectance
electro-reflectance (ER)
spectroscopy
in TTF-CA
[61]. [61].
FigureFigure
7. Polarization-domain
(ER)
spectroscopy
in TTF-CA
(a) Schematic of a polarization-domain imaging method; (b) Polarized reflectivity spectra with E⊥a
(a) Schematic of a polarization-domain imaging method; (b) Polarized reflectivity spectra with E⊥a at
at 90 K (the neutral phase) and at 77 K and 4 K (the ionic phase); (c) A differential reflectivity spectrum
90 K (the neutral phase) and at 77 K and 4 K (the ionic phase); (c) A differential reflectivity spectrum
between 4 K and 77 K, [RI(4 K) − RI(77 K)]/RI(77 K); (d) A spectrum of reflectivity changes ΔR/R
between 4 K and 77 K, [RI (4 K) − RI (77 K)]/RI (77 K); (d) A spectrum of reflectivity changes ∆R/R
induced by external electric fields F (//a) with an amplitude of 1.56 kV/cm; (e) A polarization domain
induced
by external electric fields F (//a) with an amplitude of 1.56 kV/cm; (e) A polarization domain
image shown as a contour map of electric-field-induced reflectivity changes ΔR/R at the photon energy
imageindicated
shown by
as athecontour
map
changes
∆R/R
at the photon
arrow in
(c). of
Redelectric-field-induced
regions (IA) and blue reflectivity
regions (IB) have
opposite
directions
of
energy
indicated by the arrow in (c). Red regions (IA) and blue regions (IB) have opposite directions
polarizations.
of polarizations.
When we apply the ER method to TTF-CA, it is effective to measure the electric-field-induced
reflectivity changes ΔR at which a value of ΔR depends sensitively on the ρ value, since the ρ value
When we apply the ER method to TTF-CA, it is effective to measure the electric-field-induced
should directly reflect the magnitude of the polarization P, as mentioned in the previous subsection.
reflectivity changes ∆R at which a value of ∆R depends sensitively on the ρ value, since the ρ value
In our study, we selected the IM transition of TTF. In Figure 7b, we show the polarized reflectivity
should
directly
reflect
the magnitude
thefor
polarization
as mentioned
previous
subsection.
spectra
in the
IM transition
region ofofTTF
E⊥a at 90 K P,
in the
N phase, andinatthe
77 K
and 4 K in
the I
In ourphase.
study,
we the
selected
IMIR
transition
TTF. In Figure
7b, weρshow
the polarized
From
resultsthe
of the
molecularof
vibrational
spectroscopy,
is estimated
to be 0.58reflectivity
at 4 K
spectra
in0.53
theat
IM
regionand
of 0.32
TTFatfor
a the
at 90
in the
NThese
phase,
at 77 Kbands
and 4shift
K in
and
77transition
K in the I phase,
90EK⊥in
NK
phase
[47].
IMand
transition
to the I
phase.the
From
results
of increasing
the IR molecular
vibrational
spectroscopy,
ρ issame
estimated
to be 0.58the
at 4 K
lowerthe
energy
with
ρ, as mentioned
above.
Panel (c) in the
figure indicates
differential
reflectivity
spectrum
between
4
K
and
77
K,
[R
I
(4
K)
−
R
I
(77
K)]/R
I
(77
K).
This
spectrum
and 0.53 at 77 K in the I phase, and 0.32 at 90 K in the N phase [47]. These IM transition bands shift
to the
reflectivity
change
ρ is increased
0.05 at(c)
77in
K.the same figure indicates the
to thecorresponds
lower energy
with
increasing
ρ, aswhen
mentioned
above.byPanel
In
the
ER
measurements,
an
electric
field
F
with
a
frequency
of
approximately
is applied
differential reflectivity spectrum between 4 K and 77 K, [RI (4 K) − RI (77 K)]/RI (771 kHz
K). This
spectrum
along the a axis, and the difference ΔR in the reflectivity R for F parallel to a (F//a) and F antiparallel
corresponds to the reflectivity change when ρ is increased by 0.05 at 77 K.
to a (F//−a) were measured with a lock-in technique. Such a difference ΔR appears only in a sample
In the ER measurements, an electric field F with a frequency of approximately 1 kHz is applied
without inversion symmetry. In Figure 7d, we show the spectrum of the reflectivity change ΔR/R, the
alongspectral
the a axis,
and
difference
in the reflectivity
for F parallel to a (F//a) and F antiparallel
shape
of the
which
is in good∆R
agreement
with the [RR
I(4 K) − RI(77 K)]/RI(77 K) spectrum shown in
to a (F//
−
a)
were
measured
with
a
lock-in
technique.
Such
a difference
appears only
a sample
Figure 7c, indicating that ρ is changed by the electric field
F. ΔR/R is∆R
proportional
to Finup
to
without inversion symmetry. In Figure 7d, we show the spectrum of the reflectivity change ∆R/R,
the spectral shape of which is in good agreement with the [RI (4 K) − RI (77 K)]/RI (77 K) spectrum
shown in Figure 7c, indicating that ρ is changed by the electric field F. ∆R/R is proportional to F up to
Crystals 2017, 7, 132
8 of 41
F ~1 kV/cm. This means that the reflectivity change is a kind of electro-optic effect and is attributed to
second-order
Crystals 2017,optical
7, 132 nonlinearity in a broad sense. By comparing the magnitude of ∆R/R and
8 of [R
41 I (4 K)
−
4
− RI (77 K)]/RI (77 K), we evaluated the change in ρ to be approximately 10 at 1 kV/cm. The result
F ~ 1 kV/cm.
This that
means
the reflectivity
a kindsuggesting
of electro-optic
and is attributed
in Figure
7d shows
ρ isthat
increased
by thechange
electricis field,
thateffect
F is parallel
to P. When F
to second-order
nonlinearity
in a broad
By comparing
thefor
magnitude
of ΔR/R and
is antiparallel
to P, optical
ρ should
be decreased.
Such sense.
a feature
can be used
the polarization
imaging.
[R
I(4 K) − RI(77 K)]/RI(77 K), we evaluated the change in ρ to be approximately 10−4 at 1 kV/cm. The
It should be noted that TTF-CA had been considered a displacive-type ferroelectric when this study
result in Figure 7d shows that ρ is increased by the electric field, suggesting that F is parallel to P.
was carried out. Therefore, in a paper on the results of this study [61], it was reported that the increase
When F is antiparallel to P, ρ should be decreased. Such a feature can be used for the polarization
(decrease) of ρ induces the increase (decrease) of Pion .
imaging. It should be noted that TTF-CA had been considered a displacive-type ferroelectric when
We study
measured
the positional
dependence
of ∆R/R
3.5 eVofand
77 K[61],
using
an optical
microscope.
this
was carried
out. Therefore,
in a paper
on theatresults
thisat
study
it was
reported
that
In this
measurement,
we
consider
that
the
ab
plane
of
the
crystal,
of
size
400
×
400
µm,
consists of
the increase (decrease) of ρ induces the increase (decrease) of Pion.
225 square
of size
× 27 µm dependence
and recorded
value
of ∆R/R
square.
crystals
Wepieces
measured
the27positional
of the
ΔR/R
at 3.5
eV andinateach
77 K
using In
anmost
optical
microscope.the
In this
we consider
that the
plane
of the crystal,
of size
400 × 400
μm,
we measured,
signmeasurement,
of ∆R/R changes
depending
onabthe
position.
A typical
resulting
polarization
consists
of 225
size 27 map.
× 27 μm
and plot,
recorded
the value that
of ΔR/R
each square.to
In∆R/R
image
is shown
in square
Figure pieces
7e as aofcontour
In this
we assumed
P isinproportional
most
crystals
we
measured,
the
sign
of
ΔR/R
changes
depending
on
the
position.
A
typical
resulting
and applied a spline interpolation to the ∆R/R data. Two kinds of domains with opposite directions of
polarization image
shown
in Figure
a contour
In this
assumed
that by
P is
the polarizations,
P//aisand
P//–a,
coexist7easasindicated
bymap.
the red
and plot,
blue we
regions
denoted
IA and
proportional to ΔR/R and applied a spline interpolation to the ΔR/R data. Two kinds of domains with
IB, respectively. From the measurements of several samples, it was revealed that a typical domain size
opposite directions of the polarizations, P//a and P//–a, coexist as indicated by the red and blue
is 200 × 200 µm or larger. The boundary between the red and blue regions corresponds to a domain
regions denoted by IA and IB, respectively. From the measurements of several samples, it was
wall,revealed
which does
move
when
lower
1 kV/cm.
When
F exceeds
1 the
kV/cm,
the
domain
that a not
typical
domain
sizeF isis200
× 200than
μm or
larger. The
boundary
between
red and
blue
wallregions
starts to
move. This
a coercive
field
at when
77 K.FAccording
to 1the
result
of the
corresponds
to aelectric
domainfield
wall, is
which
does not
move
is lower than
kV/cm.
When
F P–E
characteristic,
the coercive
fieldwall
is ~5
kV/cm
at 50This
K [53].
This
indicates
thatfield
the coercive
field increases
exceeds 1 kV/cm,
the domain
starts
to move.
electric
field
is a coercive
at 77 K. According
the result of
the P–E characteristic, the coercive field is ~5 kV/cm at 50 K [53]. This indicates that
withtodecreasing
temperature.
the coercive field increases with decreasing temperature.
3. Time-Resolved Laser Spectroscopy
3. Time-Resolved Laser Spectroscopy
3.1. Femtosecond-Pump–Probe Spectroscopy and Necessary Time Resolution
3.1. Femtosecond-Pump–Probe Spectroscopy and Necessary Time Resolution
In order to observe a photoinduced transition occurring on an ultrafast time scale, femotosecond
In order tooptical
observespectroscopy
a photoinducedistransition
occurring method.
on an ultrafast
timemethod,
scale, femotosecond
(fs) and
(fs) pump-probe
a most effective
In this
a pump pulse
pump-probe optical spectroscopy is a most effective method. In this method, a pump pulse and a
a probe pulse, the temporal widths of which are ~10–200 fs, are used to excite a solid and to probe
probe pulse, the temporal widths of which are ~10–200 fs, are used to excite a solid and to probe its
its electronic-state changes, respectively. A schematic of this method is shown in Figure 8. In the
electronic-state changes, respectively. A schematic of this method is shown in Figure 8. In the figure,
figure,
show
reflection
configuration,
in which
the change
in optical
reflectivity
of a probe
wewe
show
thethe
reflection
configuration,
in which
the change
in optical
reflectivity
of a probe
light light
induced
by abypump
light
is isdetected.
thedelay
delaytime
time
td the
of the
probe
pulse
relative
induced
a pump
light
detected. We
We control
control the
td of
probe
pulse
relative
to theto the
pump
pulse
by
changing
the
path
length
of
the
pump
pulse
using
a
delay
stage,
and
measure
the time
pump pulse by changing the path length of the pump pulse using a delay stage,
measure the
time characteristics
of the reflectivity
changes.
characteristics
of the reflectivity
changes.
Delay time td
Cryostat
Pump
pulse
Probe
pulse
OPA 2
OPA 1
Ti-S RA
(1.55 eV)
Sample
Detector
Delay Stage
Figure 8. Schematic of optical-pump optical-reflectivity-probe measurement system. Ti-S RA:
Figure
8. Schematic of optical-pump optical-reflectivity-probe measurement system. Ti-S RA:
Ti-sapphire regenerative amplifier; OPA: optical parametric amplifier.
Ti-sapphire regenerative amplifier; OPA: optical parametric amplifier.
To elucidate photoinduced changes in electronic structures, it is fundamentally important to
To elucidate
photoinduced
changes in
structures,
is fundamentally
to
measure
transient
changes of reflectivity
or electronic
absorption spectra
over it
a wide
photon-energyimportant
region.
For
this
purpose,
a
Ti-sapphire
regenerative
amplifier
(RA)
is
widely
used
as
a
main
laser
source.
measure transient changes of reflectivity or absorption spectra over a wide photon-energy region.
Typical
values ofa the
temporal width,
photon energy,
and repetition
rate of a used
Ti-sapphire
RA arelaser
130 fs,
For this
purpose,
Ti-sapphire
regenerative
amplifier
(RA) is widely
as a main
source.
1.55
eV,
and
1
kHz,
respectively.
The
output
of
the
RA
is
divided
into
two
beams,
which
are
used
as
Typical values of the temporal width, photon energy, and repetition rate of a Ti-sapphire RA are
excitation sources of two optical parametric amplifiers (OPAs). An OPA can convert an incident
130 fs, 1.55 eV, and 1 kHz, respectively. The output of the RA is divided into two beams, which
Crystals 2017, 7, 132
9 of 41
are used as excitation sources of two optical parametric amplifiers (OPAs). An OPA can convert
an incident femtosecond pulse with 1.55 eV from the RA to a pulse with almost the same temporal
width and various photon energies (~0.5–1.05 eV) via an optical parametric effect, which is a kind of
second-order nonlinear optical effect. Using the output pulses of the two OPAs and further frequency
conversions of those pulses by several other kinds of second-order nonlinear optical effects, we
can obtain femtosecond laser pulses with a photon energy from 0.1 to 4 eV. The time resolution is
determined by a cross-correlation of the pump and probe pulses. In the case in which the pump and
probe pulses are Gaussian with a temporal width of 130 fs, the time resolution is approximately 180 fs.
Throughout this review article, the excitation photon density xph is defined as the averaged
photon density per pump pulse absorbed within its absorption depth lp (the inverse of the absorption
coefficient). xph is evaluated from the equation xph = Ip 1 − Rp (1 − 1/e)/lp , in which Ip and Rp are
the photon density per unit area and the reflection loss of the pump light, respectively.
Here, we briefly discuss the time resolution necessary to precisely detect photoinduced changes
in electronic states, intramolecular deformations, and molecular displacements in organic molecular
compounds, respectively. It is reasonable to consider that the time scale of electron dynamics would be
dominated by the electron transfer energy t. In organic molecular compounds, t is typically 0.1–0.2 eV,
which corresponds to a characteristic time of 40–20 fs. Molecular deformations occur on the time scale
of intramolecular vibrations. Their frequencies range from 100 to 1000 cm−1 and their characteristic
times range from 300 to 30 fs. In contrast, the frequencies of molecular displacements or, equivalently,
the frequencies of so-called lattice modes range from 20 to 100 cm−1 and their characteristic times range
from 1.5 ps to 300 fs. Taking these values into consideration, the time resolution of 180 fs, which is
obtained in a typical Ti-sapphire RA system, is insufficient to detect electron and molecular dynamics,
and a time resolution equal to or better than 20 fs should be necessary. To obtain an ultrashort laser
pulse with a temporal width shorter than 15 fs, a noncollinear OPA (NOPA) is sometimes used, in which
the temporal width of a pulse can be decreased to 10 fs or shorter. However, such an ultrashort laser
pulse has a large spectral width because of the uncertainty relation, such that the spectral resolution
becomes worse. In our study, for broadband spectroscopy from 0.1 to 4 eV, we used a pump-probe
reflection spectroscopy system with a time resolution of ~180 fs. In cases in which a higher time
resolution was necessary, we used a pump–probe system consisting of two NOPAs, in which the
temporal width of the pump and probe pulses was approximately 15 fs and the time resolution was
approximately 20 fs.
3.2. Terahertz-Pump Optical-Probe Spectroscopy
A terahertz pulse usually means a nearly monocyclic electromagnetic wave with a central
frequency of approximately 1 THz (a photon energy of ~4 meV, frequency of ~33 cm−1 , and wavelength
of ~300 µm) and with a temporal width of approximately 1 ps. In this article, we use the term ‘terahertz
pulse’ with this meaning. Recent developments of femtosecond laser technology enable us to generate
and detect such a terahertz pulse. There are several methods to generate a terahertz pulse. A typical
method for this is optical rectification, which is schematically shown in Figure 9. As mentioned
in the previous subsection, a typical Ti-sapphire RA generates a pulse with a temporal width of
approximately 130 fs, spectral width of ~7 nm and photon energy of ~14 meV. When such a pulse is
incident to a second-order nonlinear optical crystal, such as ZnTe, a terahertz pulse is emitted through
a differential frequency generation process within the pulse. This process corresponds to an optical
rectification within a light pulse with a fixed frequency and as such, this terahertz pulse generation
method is called an optical rectification method. The spectral width of a terahertz pulse thus obtained
is dominated by the spectral width of the incident pulse.
If a terahertz pulse is used as a pump pulse to control an electronic state of a solid [62–67],
its electric-field amplitude should be enhanced. In order to strengthen the electric-field amplitude
of a terahertz pulse, we should put a strong laser pulse into a second-order nonlinear optical crystal
and simultaneously fulfil the phase matching condition of an incident pulse and a terahertz pulse.
Crystals 2017, 7, 132
10 of 41
For this purpose, a pulse-front tilting method using LiNbO3 was developed [68,69]. Using this method,
a Crystals
strong
terahertz
2017,
7,7,
132
1010
ofof
4141
Crystals
2017,
132 pulse with an electric-field amplitude far beyond 100 kV/cm can be produced.
In our study, we generate a terahertz pulse with an electric-field amplitude as large as 400 kV/cm by
pulse-front
tilting
method
[65]
and
use
ititasas
pump
pulse
totocontrol
the
ininTTF-CA.
InIn
pulse-front
tilting
method
[65]
and
useuse
pump
pulse
control
thepolarization
polarization
TTF-CA.
the
pulse-front
tilting
method
[65]
and
ita aas
a pump
pulse
to control
the polarization
in TTF-CA.
measurements,
we
control
the
delay
time
tdtdof
the
probe
pulse
terahertz-pumpoptical-probe
optical-probe
measurements,
we
control
the
delay
time
the
probe
pulse
Inthe
theterahertz-pump
terahertz-pump
optical-probe
measurements,
we
control
the
delay
time
tof
of
the
probe
pulse
d
relative
terahertz
pump
ofof
the
probe
pulse.
The
experimental
relative
tothe
the
terahertz
pumppulse
pulseby
bychanging
changingthe
thepath
pathlength
length
the
probe
pulse.
The
experimental
relative
totothe
terahertz
pump
by
changing
the
path
length
of
the
probe
pulse.
The
experimental
setup
for
the
terahertz-pump
optical-probe
10.
setup
for
the
terahertz-pumpoptical-probe
optical-probemeasurements
measurementsisis
isshown
showninin
Figure
10.
setup
for
the
terahertz-pump
measurements
shown
inFigure
Figure
10.
THz
THzpulse
pulse
Optical
Opticalpulse
pulse
~5~5THz
THz
~1~1THz
THz
ωω
1 1
2ω
2ω
ωω
1-ω
2 2
1-ω
Frequency
Frequency
ZnTe
ZnTecrystal
crystal
~1~1psps
~100
~100fsfs
Figure
9.9.Schematic
of
aa terahertz-pulse
generation
byananoptical
optical
rectification
(differential
frequency
Figure
ofof
rectification
(differential
frequency
Figure
9.Schematic
Schematic
aterahertz-pulse
terahertz-pulsegeneration
generationby
optical
rectification
(differential
frequency
generation)
process
in
aa ZnTe
crystal.
generation)
process
inin
crystal.
generation)
process
aZnTe
ZnTe
crystal.
Delay
Delaytime
timetd td
Cryostat
Cryostat
Pump
Pump
pulse
pulse
Probe
Probe
pulse
pulse
Delay
DelayStage
Stage
OPA
OPA2 2
THz
THzpulse
pulse
generation
generation
Ti-S
Ti-SRA
RA
(1.55
(1.55eV)
eV)
Sample
Sample
Detector
Detector
Figure
10.
Schematic
ofof
terahertz-pump
optical-reflectivity-probe
measurement
system.
AA
terahertz-pump
Figure
10.
Schematic
terahertz-pump
optical-reflectivity-probe
measurement
system.
terahertz-pump
Figure
10.
Schematic
of terahertz-pump
optical-reflectivity-probe
measurement
system.
pulse
3 3crystal.
pulseisisgenerated
generatedbybya apulse-front-tilting
pulse-front-tiltingmethod
methodinina aLiNbO
LiNbO
crystal.Ti-S
Ti-SRA:
RA:Ti-sapphire
Ti-sapphire
A terahertz-pump pulse is generated by a pulse-front-tilting method in a LiNbO3 crystal.
Ti-S RA:
regenerative
regenerativeamplifier;
amplifier;OPA:
OPA:optical
opticalparametric
parametricamplifier.
amplifier.
Ti-sapphire regenerative amplifier; OPA: optical parametric amplifier.
4.4.Photoinduced
PhotoinducedNeutral
NeutraltotoIonic
IonicPhase
PhaseTransition
TransitionininTTF-CA
TTF-CA
4. Photoinduced Neutral to Ionic Phase Transition in TTF-CA
4.1.
4.1.Photoinduced
PhotoinducedNeutral-to-Ionic
Neutral-to-IonicTransition
Transition
4.1. Photoinduced Neutral-to-Ionic Transition
InInthis
thissubsection,
subsection,we
wereport
reportthe
thedynamics
dynamicsofofthe
thephotoinduced
photoinducedN-to-I
N-to-Itransition
transitioninvestigated
investigatedbyby
In this subsection, we
report the dynamics the
of the photoinduced
N-to-I pump
transition investigated
by
pump-probe
pump-probereflection
reflectionspectroscopy,
spectroscopy,ininwhich
which thetemporal
temporalwidth
widthofofeach
each pumpand
andprobe
probepulse
pulse
pump-probe
reflection
spectroscopy,
in
which
the
temporal
width
of
each
pump
and
probe
pulse
was
was
wasapproximately
approximately130
130fs,fs,and
andthe
thetime
timeresolution
resolutionwas
wasapproximately
approximately180
180fsfs[19].
[19].
approximately
130
fs,
and
the
time
resolution
was
approximately
180
fs
[19].
Figure
11a
shows
the
transient
change
(ΔR/R)
in
the
polarized
reflectivity
(R)
Figure 11a shows the transient change (ΔR/R) in the polarized reflectivity (R)spectrum
spectrumininthe
theIM
IM
Figure
11a
shows
the
transient
change
(∆R/R)
in
the
polarized
reflectivity
(R)
spectrum
transition
transitionband
bandofofTTF
TTFfor
forE⊥a
E⊥aatat9090KKjust
justafter
afterthe
theresonant
resonantexcitation
excitationofofthe
theCT
CTband
bandatat0.65
0.65eV
eV in
the
IM
transition
band
of
TTF
for
E
⊥
a
at
90
K
just
after
the
resonant
excitation
of
the
CT
band
with
(t(t
d d
~ ~0.2
withEex
E//a
ex//a
0.2ps).
ps).The
Theexcitation
excitationphoton
photondensity
densityxph
xphisisapproximately
approximately0.15
0.15photon
photon(ph)
(ph)per
perDA
DA
atpair.
0.65The
eVspectral
with
Eex
//a (tofdofΔR/R
~ΔR/R
0.2atat
ps).
excitation
photon
density xph
isFigure
approximately
0.15
shapes
from
tdtd= =0–10
InInFigure
11b,
pair.
The
spectral
shapes
fromThe
0–10pspsare
arealmost
almostunchanged.
unchanged.
11b,the
thesolid
solid
photon
per
DA pair.
The spectral
shapes[ of
∆R/R
from
t/
=
0–10
ps
are
almost
unchanged.
(90K),
(4K)−−at (90K)
(90K)
(90K),
[ (4K)
calculated
from
the
line
the
reflectivity
spectrum,
/
calculated
from
the
lineisis(ph)
thedifferential
differential
reflectivity
spectrum,
d
(90K)].
(4K)]
(90K)].
Insteady-state
Figure
11b,reflectivity
the
solid line
is the
differential
reflectivity
spectrum,
K) −atRat90
spectra
inin
the
and
K,K,K
[ )]
[ RNI (N4phase
(90
steady-state
reflectivity
spectra
theI phase
I phaseat
at4 4K,K,[ [ (4K)]
andthe
the
phase
[ /R
N90
N (90 K),
The
shape
ofofΔR/R
dtd= =0–10
below
almost
shape
ofof
Thespectral
spectral
shape
ΔR/Rfor
fortreflectivity
0–10pspsspectra
below2.3
2.3eV
eVisIisphase
almost
equal
shape
calculated
from
the steady-state
in
the
atequal
4 K, to
[ RtoIthe
4 Kspectral
] and the
N phase
(the
)spectral
[ [ (4K)
(4K)−− (90K)
(90K)/ / (90K)
(90K). .Above
(4K)−− (90K)
(90K)/ /
Above2.3
2.3eV,
eV,the
theexperimental
experimentalΔR/R
ΔR/Rand
and[ [ (4K)
(90K)
(90K)spectra
spectraare
aredifferent
differentfrom
fromeach
eachother.
other.Such
Suchdifferences
differencesare
arerelated
relatedtotothe
thefact
factthat
thatthe
the
Crystals 2017, 7, 132
11 of 41
at 90 K, [ RN (90 K)]. The spectral shape of ∆R/R for td = 0–10 ps below 2.3 eV is almost equal to
the spectral shape of [ RI (4 K) − RN (90 K)]/RN (90 K). Above 2.3 eV, the experimental ∆R/R and
[ RI (4 K) − RN (90 K)]/RN (90 K) spectra are different from each other. Such differences are related to
Crystals 2017, 7, 132
11 of 41
the fact that the number of photoinduced I states decreases with increasing distance from the crystal
surface,
depending
on the number
absorbedwith
photons
of thedistance
pump light.
In other
number
of photoinduced
I statesofdecreases
increasing
from the
crystalwords,
surface,this is
because
the penetration
depth
of the pump
pulse
(~500
Å)light.
is smaller
that
probe
depending
on the number
of absorbed
photons
of the
pump
In otherthan
words,
thisofis the
because
thepulse
penetration
depth and
of thequantitatively
pump pulse (~500
is smaller
of the probe
pulse we
(~1500
Å). To the
(~1500
Å). To exactly
treatÅ)such
effectsthan
on that
reflectivity
changes,
calculated
andassuming
quantitatively
such effects
onphotogenerated
reflectivity changes,
we calculated
spectra, from
∆R/Rexactly
spectra,
that treat
the volume
of the
I states
depends the
on ΔR/R
the distance
assuming
that
the
volume
of
the
photogenerated
I
states
depends
on
the
distance
from
the
crystal
the crystal surface and is proportional to the absorbed photon number at each position, and
that the
surface and is proportional to the absorbed photon number at each position, and that the complex
complex refractive index at each position is the weighted average of the complex refractive indexes of
refractive index at each position is the weighted average of the complex refractive indexes of the N
the N state and the photogenerated I state. Here, we also assume that the refractive indexes of the N
state and the photogenerated I state. Here, we also assume that the refractive indexes of the N state
state and
and the
thephotogenerated
photogenerated
I states
equal
to those
in the
steady
K 4and
4 K, respectively.
I states
areare
equal
to those
in the
steady
statestate
at 90at
K 90
and
K, respectively.
The ∆R/R
spectra
calculated
using
the
multilayer
model,
shown
in
Figure
11b
[70],
areexcellent
in excellent
The ΔR/R spectra calculated using the multilayer model, shown in Figure 11b [70], are in
agreement
withwith
the the
experimental
∆R/R
Fromthis
thisanalysis,
analysis,
evaluate
the volume
agreement
experimental
ΔR/Rspectra.
spectra. From
wewe
cancan
evaluate
the volume
ratio ratio
of theofphotogenerated
I states
atatthe
asaafunction
function
time,
which
is shown
in Figure
the photogenerated
I states
thecrystal
crystal surface
surface as
ofof
time,
which
is shown
in Figure
11b. 11b.
(a) Experiment
0.04
0.03
ΔR/R
∆R/R
(b) Simulation
0.2
ps
0 ps
ps
11 ps
ps
33 ps
10 ps
10
50 nm 15.6 %
50 nm 9.5 %
50 nm 6.2 %
50 nm 3.3 %
0.02
0.01
0
-0.01
1.8
90 K
0.65eV pump
2
2.2
RI(4K) − RN(90K)
×0.18
RN(90K)
2.4
2.6
2
2.2
2.4
2.6
Photon Energy (eV)
Figure
11. Results
of optical-pumpoptical-reflectivity-probe
optical-reflectivity-probe spectroscopy
with
the the
timetime
resolution
of
Figure
11. Results
of optical-pump
spectroscopy
with
resolution
of
in N
thephase
N phase
of TTF-CA
[19].(a)
(a)Spectra
Spectra of
changes
(ΔR/R)
around
~180 ~180
fs in fs
the
of TTF-CA
[19].
of photoinduced
photoinducedreflectivity
reflectivity
changes
(∆R/R)
around
the intramolecular
transition
bandofofTTF
TTFat
at 90
90 K
K reflecting
reflecting photoinduced
N to
Electric
the intramolecular
transition
band
photoinduced
N Itotransition.
I transition.
Electric
field of a probe pulse E is perpendicular to a (E⊥a). Photon energy of a pump pulse is 0.65 eV and an
field of a probe pulse E is perpendicular to a (E⊥a). Photon energy of a pump pulse is 0.65 eV and
excitation photon density is 0.15 ph/DA pair; (b) Spectra of ΔR/R calculated with the multilayer model.
an excitation photon density is 0.15 ph/DA pair; (b) Spectra of ∆R/R calculated with the multilayer
The solid line shows a differential reflectivity spectrum between 90 K and 4 K, [RI(4 K) − RN(90K)]/RN(90K).
model. The solid line shows a differential reflectivity spectrum between 90 K and 4 K, [RI (4 K) −
RN (90K)]/R
N (90K).
From the
magnitudes of the ΔR/R signals just after the photoirradiation, we deduce that
approximately 10 D0A0 pairs are converted to D+A− pairs by one photon. That is, an I domain
+A− pairs is rapidly generated from a CT excited state initially
consisting
approximately
10 D∆R/R
From
the of
magnitudes
of the
signals just after the photoirradiation, we deduce that
0
0
produced
by
a
photoexcitation.
A
formation to
process
an I domain
schematically
in
approximately 10 D A pairs are converted
D+ A−of pairs
by oneisphoton.
Thatillustrated
is, an I domain
Figure 12(aI,II). Such an I domain
formation
is based upon the fact that the long-range Coulomb
+
−
consisting of approximately 10 D A pairs is rapidly generated from a CT excited state initially
attractive interaction decreases the formation energy of the I domain, which is much lower than the
produced by a photoexcitation. A formation process of an I+ domain
is schematically illustrated in
formation energy of a CT excited state, that is, an isolated D A− pair [22,26,28,71]. This causes the
Figure
12(aI,II).
Such
an
I
domain
formation
is
based
upon
the
fact
that
the formation.
long-rangeSuch
Coulomb
collective CT processes illustrated in Figure 12(aI,II) and the resultant I domain
a
attractive
interaction
formation
energy
of the I domain,
which
than
collective
nature ofdecreases
I states is the
the most
important
characteristic
of TTF-CA
and is
themuch
reasonlower
why it
is the
formation
energy
of aofCT
excited electron
state, that
is, an
isolated
D+ A−feature
pair [22,26,28,71].
This causes
regarded
as a kind
correlated
system.
The
macroscopic
of the photoinduced
N-to- the
collective
CT processes
illustrated
I transition
is discussed
in Sectionin
4.5.Figure 12(aI,II) and the resultant I domain formation. Such a
collective nature of I states is the most important characteristic of TTF-CA and the reason why it is
regarded as a kind of correlated electron system. The macroscopic feature of the photoinduced N-to-I
transition is discussed in Section 4.5.
Crystals 2017, 7, 132
12 of 41
Crystals 2017, 7, 132
12 of 41
(a) Photo-induced N→I transition
(b) Photo-induced I→N transition
CT excitation
CT excitation
D+A– D+A– D+A– D+A– D0A0 D+A– D+A– D+A–
D0 A0 D0 A0 D0 A0 D0 A0 D0 A– D+ A0 D0 A0 D0 A0
(I)
D 0 A0 D 0 A0 D + A– D 0 A0 D 0 A0 D 0 A 0 D 0 A 0 D 0 A0
(I)
D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A– D+A0 D0 A0
1D ionic-domain
generation
D+A– D+A– D+A– D+A– D+A– D+A– D0A0 D+A–
1D neutral-domain
generation
∼ 20 fs
D 0 A 0 D 0 A − D + A –D + A –D + A − D + A –D + A –D + A 0
(II)
D+ A– D+A–D+ A–D+ A–D+ A– D+A–D0 A0 D0 A0
∼ 300 fs
(II)
D+A– D+A– D+A– D+A– D+A– D+A– D0 A0 D0 A0
Release of the dimerization
in the neutral domains
~10 ps
(III)
(IV)
D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D+A– D+A–
D+A– D+A– D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0
Generation of the
semimacroscopic domains
~ 20 ps
D 0 A0 D 0 A0 D 0 A0 D 0 A0 D 0 A 0 D 0 A 0 D 0 A0 D 0 A0
D 0 A0 D 0 A0 D 0 A0 D 0 A0 D 0 A 0 D 0 A 0 D 0 A0 D 0 A0
D 0 A0 D 0 A0 D 0 A0 D 0 A0 D 0 A 0 D 0 A 0 D 0 A0 D 0 A0
∼ 300 fs
D+A– D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D+A–
D 0 A 0 D 0 A − D + A − D + A –D + A – D + A − D + A –D + A 0
Annihilation of the
1D ionic domins
D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D+A– D+A–
D+A– D+A– D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0
D 0 A 0 D 0 A − D + A –D + A –D + A − D + A –D + A –D + A 0
(III)
∼ 20 fs
D+A– D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D+A–
D 0 A 0 D 0 A − D + A − D + A –D + A –D + A − D + A –D + A 0
Dimerization through
spin-Peierls mechanism
D+A– D+A– D0A0 D+A– D+A– D+A– D+A– D+A–
(IV)
D 0 A0 D 0 A0 D 0 A0 D 0 A0 D 0 A 0 D 0 A 0 D 0 A0 D 0 A0
D 0 A0 D 0 A0 D 0 A0 D 0 A0 D 0 A 0 D 0 A 0 D 0 A0 D 0 A0
D 0 A0 D 0 A0 D 0 A0 D 0 A0 D 0 A 0 D 0 A 0 D 0 A0 D 0 A0
Annihilation of the
semimacroscopic domains
≥ 1 μs
D+A– D+A– D+A– D+A– D+A– D+A– D+A– D+A–
(V)
D+A– D+A– D+A– D+A– D+A– D+A– D+A– D+A–
D+A– D+A– D+A– D+A– D+A– D+A– D+A– D+A–
Figure
12. Schematics
(a,b) Schematics
(a) photoinduced
transition
and
(b) photoinduced
Figure
12. (a,b)
of (a) of
photoinduced
N-to-IN-to-I
transition
and (b)
photoinduced
I-to-NI-to-N
transition
0 ) and DD
+ 0(A
transition
(A−0)) represent
and D+ (A−neutral
) represent
and ionic
donor (acceptor)
molecules,
in TTF-CA.
D0 in
(ATTF-CA.
andneutral
ionic donor
(acceptor)
molecules,
respectively.
respectively.
Underlines
show Underlines
dimers. show dimers.
4.2. Ultrafast Dynamics of Ionic Domain Formation
4.2. Ultrafast Dynamics of Ionic Domain Formation
From the pump-probe experiments with an approximate time resolution of 180 fs discussed in
From
the pump-probe
experiments
with
an photoinduced
approximate N-to-I
time resolution
of revealed.
180 fs discussed
the previous
subsection, an
overall picture
of the
transition was
The
in thenext
previous
subsection,
overall
picture
of the is
photoinduced
transition
was revealed.
important
subject is an
to clarify
how
an I domain
produced fromN-to-I
an initial
photoexcited
state
The next
subject isIntothe
clarify
how and
an Istabilization
domain is produced
from
initial molecular
photoexcited
and important
how it is stabilized.
formation
processes of
an Ian
domain,
deformations
molecular In
displacements
are expected
to play certain
roles in addition
the long-range
state and
how it isand
stabilized.
the formation
and stabilization
processes
of an I to
domain,
molecular
Coulomb
attractive
interaction.
In
order
to
precisely
detect
the
charge
and
molecular
dynamics
after
deformations and molecular displacements are expected to play certain roles in addition
to the
photoirradiation,
a
time
resolution
much
better
than
180
fs
should
be
necessary,
as
discussed
in
long-range Coulomb attractive interaction. In order to precisely detect the charge and molecular
Section 3.1. Considering these facts, we used a pump–probe system based upon two NOPAs [30].
dynamics after photoirradiation, a time resolution much better than 180 fs should be necessary,
In Figure 13a, we show again the steady-state polarized reflectivity (R) spectra. The dashed-dotted
as discussed in Section 3.1. Considering these facts, we used a pump–probe system based upon two
line shows the differential reflectivity (ΔR/R) spectrum between 90 K and 77 K, which is the spectral
NOPAs
[30].expected when the N state is converted to the I state. The shaded areas in the same figure
change
In
Figure
13a, weofshow
againand
theprobe
steady-state
polarized
reflectivity
(R) spectra.
show
the spectra
the pump
pulses obtained
from
two NOPAs.
Similar toThe
the dashed-dotted
case of the
line shows
the differential
reflectivity
(∆R/R)
spectrum
between
90 K
77 K, which
is the spectral
pump–probe
experiments
with the 180
fs time
resolution
discussed
in and
the previous
subsection,
we
set expected
the pumpwhen
photon
at the
CT transition
band
and the
photon
energy
the IM
change
theenergy
N state
is converted
to the
I state.
Theprobe
shaded
areas
in theatsame
figure
transition
band
of
TTF.
By
measuring
the
reflectivity
changes
ΔR/R
of
the
probe
pulse,
we
can
obtain
show the spectra of the pump and probe pulses obtained from two NOPAs. Similar to the case of
information about
the ultrafast
dynamics
thetime
photoinduced
transition
withprevious
a time resolution
the pump–probe
experiments
with
the 180offs
resolutionN-to-I
discussed
in the
subsection,
of
approximately
20
fs.
we set the pump photon energy at the CT transition band and the probe photon energy at the IM
transition band of TTF. By measuring the reflectivity changes ∆R/R of the probe pulse, we can obtain
information about the ultrafast dynamics of the photoinduced N-to-I transition with a time resolution
of approximately 20 fs.
Crystals 2017, 7, 132
13 of 41
13 of 41
(a)
RN(90 K)
60
RI − RN
RN
E // a
0
102R
40
RI(77 K)
20
0
ΔR/R (x 10-2)
102∆R/R
2
Photon Energy (eV)
10
5
0
Probe
(x 3)
1
(b)
0
8
6
4
2
0
-8
RN(90 K)
E⊥a
Pump
0
8
102∆R/R
Crystals 2017, 7, 132
3
τd
0 fs
20 fs
40 fs
-0.05
0
0.05
Delay Time (ps)
1
2
3
Delay Time (ps)
Figure 13. Results of optical-pump optical-reflectivity-probe spectroscopy with the time resolution of
Figure 13. Results of optical-pump optical-reflectivity-probe spectroscopy with the time resolution
~20 fs in the N phase of TTF-CA [30]. (a) Polarized reflectivity spectra at 90 K (solid lines) and at 77 K
of ~20 fs in the N phase of TTF-CA [30]. (a) Polarized reflectivity spectra at 90 K (solid lines) and at
(the broken line). A differential reflectivity spectrum, [RI(77 K) − RN(90K)]/RN(90K), is shown by the
77 K (the broken line). A differential reflectivity spectrum, [RI (77 K) − RN (90K)]/RN (90K), is shown by
dashed-dotted line. Shaded areas show spectra of pump and probe pulses; (b) Time evolution of
the dashed-dotted
line. ΔR/R.
Shaded
show spectra
of pump
and aprobe
pulses; (b)profile
Time evolution
reflectivity changes
Theareas
gray shaded
area in the
inset shows
cross-correlation
of pump of
reflectivity
changes
∆R/R.
The
gray
shaded
area
in
the
inset
shows
a
cross-correlation
profile
of pump
and probe pulses. Solid lines in the inset are simulated time characteristics with different time
and probe
pulses.
Solid
lines
in
the
inset
are
simulated
time
characteristics
with
different
time
constant
constant (rise time) τd of ΔR/R signals.
(rise time) τ d of ∆R/R signals.
In Figure 13b, we show with open circles the time characteristic of ΔR/R measured with
=
0.07ph/ pair at 90 K. By comparing the magnitude of the signal to [ (77K) − (90K) /
In Figure 13b, we show with open circles the time characteristic of ∆R/R measured
with x =
(90K), we deduce that an I state generated by one photon consists of ~10 D+A− pairs, which isph
0.07 ph/DA pair at 90 K. By comparing the magnitude of the signal to [RI (77 K) − RN (90 K)]/RN (90 K),
consistent with the results of the pump–probe experiments with the
180-fs time resolution.
− pairs, which is consistent with the
we deduceThe
thatformation
an I state dynamics
generatedofbyanone
photonare
consists
of ~10
D+ A
I domain
reflected
by the
initial
time characteristic of ΔR/R.
resultsTherefore,
of the pump–probe
experiments
with
the
180-fs
time
resolution.
we investigated the rise time τd of ΔR/R. In the inset of Figure 13b, we show the expanded
The
dynamics
an I domain
reflected
by the
initial
time the
characteristic
of ∆R/R.
timeformation
characteristic
of ΔR/R of
around
the time are
origin.
The shaded
area
indicates
cross-correlation
profilewefor
the pump the
and rise
probe
which represents
time resolution
(~20 fs)
our
Therefore,
investigated
timepulses,
τ d of ∆R/R.
In the insetthe
of Figure
13b, we show
theofexpanded
measurement system.
Thearound
solid lines
simulated
time characteristics
for τdthe
= 0,cross-correlation
20, and 40 fs,
time characteristic
of ∆R/R
theshow
timethe
origin.
The shaded
area indicates
a riseand
time
of ΔR/R
signals.
comparison
theresolution
experimental
time
profilewhich
for theispump
probe
pulses,
whichArepresents
theoftime
(~20and
fs) ofsimulated
our measurement
characteristics
reveals
that
the
time
constant
for
the
I-domain
formation
is
almost
equal
to
or
slightly
system. The solid lines show the simulated time characteristics for τ d = 0, 20, and 40 fs, which is
smaller than 20 fs. The time scale of the CT processes estimated from the magnitude of t (~0.2 eV) [72]
a rise time of ∆R/R signals. A comparison of the experimental and simulated time characteristics
is ~20 fs in TTF-CA. It is therefore reasonable to consider that the primary I domain is formed via
reveals that the time constant for the I-domain formation is almost equal to or slightly smaller than
purely electronic processes, without structural changes.
20 fs. The time scale of the CT processes estimated from the magnitude of t (~0.2 eV) [72] is ~20 fs in
TTF-CA.
is therefore
reasonable
to consider
thatDisplacements
the primaryand
I domain
is formed via purely electronic
4.3. It
Stabilization
of an
Ionic Domain
by Molecular
Deformations
processes, without structural changes.
After the initial increase of ΔR/R, complicated oscillatory structures are observed. Such
oscillatory structures are sometimes observed in pump–probe experiments on PIPTs and reflect
4.3. Stabilization of an Ionic Domain by Molecular Displacements and Deformations
structural dynamics during and after PIPTs [73–87].
After the initial increase of ∆R/R, complicated oscillatory structures are observed. Such oscillatory
structures are sometimes observed in pump–probe experiments on PIPTs and reflect structural
dynamics during and after PIPTs [73–87].
Crystals 2017, 7, 132
14 of 41
By subtracting
Crystals
2017, 7, 132 the gradual background change of ∆R/R from the original signal, we
14extracted
of 41
the oscillatory component, ∆ROSC /R, as shown by the open circles in Figure 14a. We analyzed this
By using
subtracting
the gradual
background
change of
ΔR/R
from
the original
signal, we extracted
component
the sum
of five damped
oscillators
(i =
1–5),
expressed
as follows.
the oscillatory component, ΔROSC/R, as shown by the open circles in Figure 14a. We analyzed this
component using the sum of
five
damped oscillators (i = 1–5), expressed
t as follows.
∆R
OSC
R∆
= ∑ − Ai cos(ωi t + φi ) exp −
=i
−
cos(
+
τ
) exp(− ) i
(1)
(1)
Here, Ai , ωi , φi , and τi are the amplitude, frequency, phase, and decay time, respectively, of
Here, , , ,and
are the amplitude, frequency, phase, and decay time, respectively, of the
the oscillation i. The fitting curve is shown by the red line in Figure 14a, which reproduces well the
oscillation i. The fitting curve is shown by the red line in Figure 14a, which reproduces well the
experimental oscillatory profile. Each oscillation component is also shown in the lower part of the
experimental oscillatory profile. Each oscillation component is also−1shown in the lower part of the
samesame
figure.
Their
frequencies
320,438,
438,740,
740,and
and
. From
the frequency
values,
−1. From
figure.
Their
frequenciesare
are 53,
53, 320,
957957
cmcm
the frequency
values, we
can we
−1 is
can divide
these
five
oscillations
into
two
categories.
The
low-frequency
oscillation
with
53
divide these five oscillations into two categories. The low-frequency oscillation with 53 cm−1 is assignedcm
to
assigned
to mode
a lattice
mode with
associated
with the
molecular
dimerization
assignment
a lattice
associated
the molecular
dimerization
[19,24,30,33].
This [19,24,30,33].
assignment wasThis
supported
was supported
by the
polarized
Raman spectroscopy
The other
withhigher
the frequencies
by the polarized
Raman
spectroscopy
[41]. The other[41].
oscillations
withoscillations
the frequencies
than
−
1
−1 are attributable
300
cm
to
the
intramolecular
vibration
modes
associated
with
shrinkages
(or
higher than 300 cm are attributable to the intramolecular vibration modes associated with shrinkages
expansions)
and
deformations
of
molecules.
(or expansions) and deformations of molecules.
Figure
analyses
of the oscillatory
component
obtained
in optical-pump
optical-reflectivity-probe
Figure
14.14. Fitting
Fitting
analyses
of the
oscillatory
component
obtained
in optical-pump
spectroscopy
with
the
time
resolution
of
~20
fs
in
the
N
phase
of
TTF-CA
[30].
(a)
Oscillatory
optical-reflectivity-probe spectroscopy with the time resolution of ~20 fs in the
N phase of
component ΔROSC/R (circles) which is extracted from ΔR/R (Figure 13b). The red solid line is a fitting
TTF-CA [30]. (a) Oscillatory component ∆ROSC /R (circles) which is extracted from ∆R/R (Figure 13b).
curve, which is obtained by summing up below five oscillation components and convoluting it with
The red solid line is a fitting curve, which is obtained by summing up below five oscillation
the instrumental function; (b) The lattice mode of dimeric molecular displacements which is the origin
components and
convoluting it with the instrumental function; (b) The lattice mode of dimeric
of the 53 cm−1 oscillation mode. Oscillation of electron transfer −Δρ1 indicated by yellow arrows is
molecular
displacements
which is the origin of the 53 cm−1 oscillation mode. Oscillation of electron
induced by this lattice mode; (c) Intramolecular ag modes which are origins of high-frequency
transfer
−∆ρ1 components;
indicated by
arrows
induced by this lattice
mode;
(c) Intramolecular
oscillatory
(d) yellow
Schematic
of theiselectron-intramolecular
vibration
(EIMV)
coupling.
ag modes
which aare
origins
ofelectron
high-frequency
components;
(d) arrows.
Schematic of the
Intramolecular
g modes
induce
transfers −Δoscillatory
ρ3 and −Δρ5 indicated
by yellow
electron-intramolecular vibration (EIMV) coupling. Intramolecular ag modes induce electron transfers
−∆ρ3 and −∆ρ5 indicated by yellow arrows.
Crystals 2017, 7, 132
15 of 41
The generation of the 53 cm−1 oscillation can be explained in the following way. The formation
of an initial I domain occurs within 20 fs. During this process, the molecular positions never change,
since the time scale of the molecular displacements is much slower than 20 fs. In the I domain, each
molecule has spin S = 1/2, such that it involves the spin-Peierls instability and the DA molecules in
each I domain are dimerized. The transient dimeric molecular displacements give rise to a coherent
oscillation corresponding to the dimerization. The dimeric molecular displacements in the I domain
enhance the Coulomb attractive interaction within each dimer, which enhances the degree of CT ρ.
Therefore, the coherent oscillation of the dimeric molecular displacements also modulates the Coulomb
attractive interaction between the neighbouring D and A molecules, which causes the modulation ∆ρ1
of ρ. Such a modulation of ρ is schematically shown in Figure 14b. As a result, this coherent oscillation
is clearly observed as the oscillatory structure in ∆R/R.
By comparing the higher-frequency oscillations with the results of the Raman spectroscopy and
theoretical calculations [44,88–91], we can relate those oscillations with 320, 438, 740, and 957 cm−1 to
the totally symmetric (ag ) modes of the intramolecular vibrations, CA ag ν5 , TTF ag ν6 , TTF ag ν5 , and
CA ag ν3 , respectively. Each oscillation mode is schematically illustrated in Figure 14c. The reason
why these intramolecular ag modes give rise to the oscillations of ρ and are observed as the coherent
oscillations in ∆R/R can be explained by the electron-intramolecular vibration (EIMV) coupling [92,93].
An initially produced I domain is stabilized not only by the dimeric molecular displacements but also
by the intramolecular deformations. When an I domain is produced by collective charge transfers, each
D+ and A− molecule would be stabilized by atomic displacements within the molecule or, equivalently,
by molecular deformations. This is quite reasonable, since the bond lengths within a molecule should
largely change depending on the valence of the molecule. Such changes in molecular deformations
would be expressed by the combinations of ag modes. For the ag modes of TTF, the molecular
deformations related to those modes change the molecular orbital energy, as shown by the blue
arrows in the upper part of Figure 14d. These changes in the molecular orbital energy induce the
additional intermolecular charge transfers, ∆ρi . Such molecular deformations also give rise to coherent
oscillations of molecular structures and ρ. Similar modulations of ρ can be induced by the ag modes of
CA. Note that oscillations of ρ are detected in the IM transition band of TTF. The modulation of ρ in
TTF by the vibrations in CA is unambiguous evidence for the intermolecular charge-transfer process
driven by EIMV coupling. As seen in Figure 14a, the sum of the amplitudes of the coherent oscillations
associated with the ag modes exceeds ~30% of the background ∆R/R signals originating from the
initial formation of I domains via the collective charge-transfer processes. This means that ionic (D+
and A− ) molecules or I states are largely stabilized by molecular deformations.
4.4. Charge and Molecular Dynamics Deduced from Oscillation Analyses
To obtain more detailed information of the molecular dynamics, we analyzed the oscillatory
component in Figure 14a with wavelet analysis. Using this method, we can obtain time-dependent
Fourier power spectra, from which we can discuss the time dependence of the oscillation frequencies.
In the right panel of Figure 15a, we show the results of the wavelet analysis, in which time-dependent
Fourier power spectra are shown as a contour map. The left panel of Figure 15a and its inset are
the Fourier power spectrum of the whole oscillatory signal up to td = 3 ps. Similar to the results of
the fitting analyses discussed above, we can see five oscillations: one dimeric molecular oscillation
and four intramolecular vibrations. Scrutiny of the contour map in the right panel reveals that the
oscillation frequencies of the four intramolecular vibrations vary with time (see white broken lines in
the right panel and its inset).
The time dependences of the frequencies of the four intramolecular vibrations extracted from the
contour map in Figure 15a are plotted in Figure 15b. The frequency of CA ν5 is almost unchanged,
while those of CA ν3 , TTF ν5 , and TTF ν6 are periodically modulated. In particular, the frequency
modulations are large in the CA ν3 and TTF ν5 modes. The interval between the two neighboring
dips in the time evolutions of the frequencies in Figure 15b is common to both oscillations and
Crystals 2017, 7, 132
16 of 41
Crystals 2017, 7,
132 fs, which is almost equal to the period of the coherent oscillation of the
16 of
41
approximately
600
dimeric
−
1
molecular displacements with the frequency of 53 cm . This indicates that the modulations of the
molecular displacements with the frequency of 53cm . This indicates that the modulations of the
frequencies
in the
ν3 and
TTF
ν5νmodes
are related to the modulations of ρ induced by the coherent
frequencies
inCA
the CA
ν3 and
TTF
5 modes are related to the modulations of ρ induced by the coherent
oscillation
of
the
dimeric
molecular
displacements.
In general,
general,a afrequency
frequency
intramolecular
oscillation of the dimeric molecular displacements. In
of of
an an
intramolecular
vibration
mode
depends
on
the
molecular
ionicity
ρ.
vibration mode depends on the molecular ionicity ρ.
Figure
15. 15. Wavelet
componentobtained
obtainedin in
optical-pump
Figure
Wavelet analyses
analyses of
of the
the oscillatory
oscillatory component
optical-pump
optical-reflectivity-probe
spectroscopy
with
the
time
resolution
of
~20
fs
in
the
N
phase
of
TTF-CA
optical-reflectivity-probe spectroscopy with the time resolution of ~20 fs in the N phase of TTF-CA
[30]. [30].
(a) Left
panel:
Fourier
transform
of
the
oscillatory
component
(circles)
and
the
fitting
function
(solid
(a) Left panel: Fourier transform of the oscillatory component (circles) and the fitting function (solid
lines)lines)
shown
in Figure
14a.
panel:Wavelet
Wavelet
transform
ofoscillatory
the oscillatory
component.
Broken
shown
in Figure
14a.Right
Right panel:
transform
of the
component.
Broken lines
dependence
of of
oscillation
frequencies
of of
lines show
show peaks
peaks inin transient
transientFourier
Fourierspectra;
spectra;(b)(b)Time
Time
dependence
oscillation
frequencies
intramolecular
a
g
modes.
Broken
lines
show
fitting
curves
obtained
by
assuming
an
exponential
shift
intramolecular ag modes. Broken lines show fitting curves obtained by assuming an exponential shift
a periodic
oscillation.
Solidlines
linesshow
show results
results of
(4);(4);
(c) Changes
in in
and aand
periodic
oscillation.
Solid
of the
the analyses
analysesbybyequation
equation
(c) Changes
molecular structures in the neutral and ionic phases. TTF and CA molecules are plane in the neutral
molecular structures in the neutral and ionic phases. TTF and CA molecules are plane in the neutral
phase and bent in the ionic phase; (d) Bending modes of TTF and CA.
phase and bent in the ionic phase; (d) Bending modes of TTF and CA.
When we compare the frequency modulations of the CA ν3 and TTF ν5 modes in Figure 15b more
When
wewe
compare
thethe
frequency
of the
ν3 and
TTF ν5the
modes
in Figure
carefully,
notice that
phases ofmodulations
those modulations
areCA
different
between
two modes.
The 15b
dottedwe
lines
in Figure
theof
delay
times
td of 0.6 ps are
and different
1.2 ps, which
correspond
to the
more blue
carefully,
notice
that 15b
the show
phases
those
modulations
between
the two
modes.
positions
with
the
phase
ω
1
t
=
2π
and
4π
for
the
case
in
which
a
cosine-type
oscillation
−cos(ω
1
t)
with
The blue dotted lines in Figure 15b show the delay times td of 0.6 ps and 1.2 ps, which correspond to
the frequency
53cmω t is= assumed.
two a
dips
in the timeoscillation
evolution of
the
the positions
with the =
phase
2π and 4πThe
forpositions
the caseof
inthe
which
cosine-type
−cos(ω
1
1 t)
frequency for the TTF ν5 mode
accord with the two blue lines. On the other hand, those for the CA ν3
−
1
with the frequency ω1 = 53 cm is assumed. The positions of the two dips in the time evolution of
mode shift by ~130 fs or ~0.45 π relative to the two blue lines. In addition, the frequencies of the
the frequency for the TTF ν5 mode accord with the two blue lines. On the other hand, those for the CA
coherent oscillations apart from the CA ν5 mode shift to the lower frequencies with time. The time
ν3 mode
shift by
fs or ~0.45
π relative
to the
twoν5blue
addition,
frequencies
of the
evolutions
of ~130
the frequency
changes
in the CA
ν3, TTF
, and lines.
TTF ν6 In
modes
can be the
almost
reproduced
coherent
oscillations
apart from
the CA
shift tofunction
the lower
frequencies
time. The time
by the
sum of a damped
oscillator
andν5anmode
exponential
corresponding
towith
a low-frequency
evolutions
of shown
the frequency
changes
inin
the
CA ν15b.
ν5 ,frequency
and TTF ω
ν6Ci(0)
modes
beorigin
almost
shift, as
by the broken
lines
Figure
Each
at thecan
time
is reproduced
set at a
3 , TTF
by thevalue
sumof
ofaaplane
damped
oscillator
an while
exponential
functionωcorresponding
to aatlow-frequency
molecule
with and
ρ = 0.7,
each frequency
Ci(∞) at 1.5 ps is set
a value in the I shift,
phasebyofthe
TTF-CA
[44,90].
this analysis,
thefrequency
time constant
of the
frequency
shift was
as shown
broken
lines From
in Figure
15b. Each
ω Ci (0)
at the
time origin
is setevaluated
at a value of
to
be
~0.35
ps.
As
mentioned
above,
an
I
domain
is
generated
within
20
fs
after
photoirradiation;
a plane molecule with ρ = 0.7, while each frequency ω Ci (∞) at 1.5 ps is set at a value in the I phase
therefore,
the observed
frequency
muchconstant
slower than
20 frequency
fs cannot beshift
explained
by purelyto be
of TTF-CA
[44,90].
From this
analysis,shifts
the time
of the
was evaluated
~0.35 ps. As mentioned above, an I domain is generated within 20 fs after photoirradiation; therefore,
Crystals 2017, 7, 132
17 of 41
the observed frequency shifts much slower than 20 fs cannot be explained by purely electronic processes.
The slow frequency shifts suggest that the charge distributions in TTF and CA molecules are changed
with the time constant of ~0.35 ps and they are dominated by additional slow molecular deformations,
which may cause the phase shifts of the frequency modulations in the coherent oscillations.
With these results in mind, we carefully checked the molecular structures of TTF and CA and
found significant differences between the molecular structures of the N and I phases; as shown in
Figure 15c, TTF and CA molecules are planar in the N phase, while they are bent in the I phase.
We show in Figure 15d the corresponding bending modes of TTF and CA molecules, which were
obtained from theoretical calculations [88,89]. The frequencies (periods) of those bending modes
are 58 cm−1 (0.58 ps) in TTF and 65 cm−1 (0.52 ps) in CA. These periods of the bending modes
are close to the time constant 0.35 ps of the frequency shift. Accordingly, we can consider that the
additional molecular deformations responsible for the frequency shifts of coherent oscillations are
molecular bendings.
To ascertain the validity this interpretation, we performed a simulation using a simple model.
First, we assumed that the dimeric molecular displacement Q1 (t) oscillates and converges to a certain
value Q1 (∞), as expressed by the following formula.
Q1 (t) = Q1 (∞)(1 − exp(−t/τ1 ) cos(ω1 t))
(2)
Next, we assumed that a molecular bending QBj (t) is driven by the dimeric molecular
displacement. This assumption is reasonable, because the bendings of TTF and CA molecules within a
dimer, shown in Figure 15d, strengthen the dimerization. In this case, it is natural to consider that an
external force originates from the dimeric molecular displacement Q1 (t) and the equation of motion
for QBj (t) can be expressed as
..
.
2
QBj + γBj QBj + ωBj
QBj = FBj (1 − exp(−t/τ1 ) cos(ω1 t))
(3)
Here, j = 1 and 2 indicate the bending modes of TTF and CA, respectively. ω Bj and γBj are the
frequency and the damping of each bending mode, respectively. FBj is an external force when the
dimeric molecular displacement reaches Q1 (∞), after a long time has elapsed. The time characteristic
of an intramolecular vibration frequency is expressed as
t
ωCi (t) = −Ωi exp −
τ1
"
QBj (t)
cos(ω1 t) + ωCi (0) 1 −
QBj (∞)
!
QBj (t)
+ ωCi (∞)
QBj (∞)
#
(4)
Here, the first term shows the direct frequency modulation by the change of ρ induced by the
dimeric molecular displacements. Ωi is the amplitude of the frequency modulation. The second term
is the frequency change by the charge redistribution within a molecule induced by the molecular
bending. i denotes each intramolecular vibration mode. Using this equation, we calculated the time
characteristics of the frequencies of the coherent oscillations corresponding to the CA ν3 and TTF ν5
modes, and show them by the solid lines in Figure 15b. As the oscillation frequencies ω Bj of the bending
modes in TTF and CA, we used the theoretical values calculated in each isolated molecule [88,89],
since no values had been obtained in a solid. The calculated curves reproduced both the frequency
changes and the phase shifts of the frequency modulations well, although the fitting parameters are
only the damping of the bending mode γBj and the amplitude of the frequency modulation Ωi . Such
a success of the simulation suggests that the used ω Bj values are not so different from their actual
values in solids. The obtained values of γBj are approximately 0.2 eV in both TTF and CA molecules.
Such a large value of the damping gives rise to a monotonic frequency shift and then a monotonic
charge redistribution, as observed in Figure 15b. As seen in Figure 14a, the decay times of the coherent
oscillations of the intramolecular ag modes differ from each other. A coherent oscillation with a large
frequency shift is strongly coupled to the bending mode and decays rapidly. This is explained by the
Crystals 2017, 7, 132
18 of 41
fact that the strong coupling to the bending mode with large energy dissipation shortens the decay
time of the oscillation.
In Figure 16, we show the ultrafast I domain formation and stabilization via the molecular
dynamics revealed from the pump–probe reflection spectroscopy with the time resolution of 20 fs.
Just Crystals
after a2017,
photoirradiation,
an I domain is generated via purely electronic processes,18as
shown
7, 132
of 41
in (i). Subsequently, the I domain is stabilized by molecular deformations (ii) and dimerizations (iii).
Subsequently,
the I domain
stabilized
by molecular
deformations
(ii) and
dimerizations
These
structural changes
andisthe
subsequent
oscillations
give rise
to the
changes in(iii).
theThese
degree of
structural
changes
and
the
subsequent
oscillations
give
rise
to
the
changes
in
the
degree
of
CT
ρ in
CT ρ in each molecule within the I domain and the subsequent oscillations in ρ. The dimerization
each molecule within the I domain and the subsequent oscillations in ρ. The dimerization produces
produces subsequent molecular bendings (iv), which cause the charge redistribution in each molecule
subsequent molecular bendings (iv), which cause the charge redistribution in each molecule in the I
in the I domain and the resultant complicated frequency changes of the coherent oscillations. Such
domain and the resultant complicated frequency changes of the coherent oscillations. Such complex
complex
dynamics
in photoinduced
NI transitions
an ultrafast
timescale
also been
discussed
dynamics
in photoinduced
NI transitions
on anon
ultrafast
timescale
have have
also been
discussed
theoretically
[38].
theoretically [38].
Figure
Schematic
photoinduced N-to-I
N-to-I transition
in TTF-CA
[30].[30].
BlueBlue
curved
arrows
Figure
16. 16.
Schematic
ofof
photoinduced
transitiondynamics
dynamics
in TTF-CA
curved
arrows
indicate
electron
transfers.
indicate electron transfers.
4.5. Macroscopic Feature of Photoinduced Neutral-to-Ionic Transition
4.5. Macroscopic Feature of Photoinduced Neutral-to-Ionic Transition
Here, we discuss the macroscopic feature of the photoinduced N-to-I transition. Figure 17 shows
Here, we discuss the macroscopic feature of the photoinduced N-to-I transition. Figure 17
the time evolutions of normalized ΔR/R at ~2.2 eV at 90 K and 260 K measured in the NOPA-based
shows
the time system
evolutions
of normalized
∆R/R
at ~2.2
eV0.14
at ph/DA
90 K and
260
K measured
pump–probe
[33]. The
excitation photon
density
xph is
pair or
0.014
ph/DA pair.in the
NOPA-based
pump–probe
system [33].
The excitation
photon
density
xph is 0.14(tdph/DA
pair or
The inset shows
the xph dependence
of the magnitudes
of ΔR/R
just after
photoirradiation
= 0.15 ps).
0.014Byph/DA
pair.
The inset shows
the xwith
ofreflectivity,
the magnitudes
of−∆R/R
just/ after
(90K)
[ (77K)
comparing
the magnitudes
of the signals
the differential
ph dependence
(90K), we can
the conversion
efficiency
the N-to-I transition,
which is
approximately
photoirradiation
(td estimate
= 0.15 ps).
By comparing
the of
magnitudes
of the signals
with
the differential
+A− pairs per photon at both 90 K and 260 K. The value of [
(77K)
(90K)
10
D
−
/
reflectivity, [ RI (77 K) − RN (90 K)]/RN (90 K), we can estimate the conversion efficiency(90K)
of theisN-to-I
+
−
approximately
0.11.
The
saturation
value
of
ΔR/R
also
reaches
0.11
at
both
90
K
and
260
K.
transition, which is approximately 10 D A pairs per photon at both 90 K and 260 K. TheThis
value of
indicates
that90
most
of
the crystal
within
the absorption
depth
ofsaturation
the probe pulse
is converted
to thereaches
I
R
77
K
−
R
K
/R
90
K
is
approximately
0.11.
The
value
of
∆R/R
also
[ I(
)
)] N (
)
N(
state. Therefore, we can regard the observed photoinduced N-to-I transition as a photoinduced ‘phase
0.11 at both 90 K and 260 K. This indicates that most of the crystal within the absorption depth of the
transition’.
probe pulse
is converted to the I state. Therefore, we can regard the observed photoinduced N-to-I
In the case of the strong excitation (xph = 0.14 ph/DA pair), in which the NI transition is almost
transition
as athe
photoinduced
‘phase
transition’.
complete,
photogenerated
I states
decay rapidly on a time scale of several picoseconds at both 90 K
In
the
case
of
the
strong
excitation
(xph =(ferroelectric)
0.14 ph/DAIpair),
which
the This
NI transition
almost
and 260 K. This suggests that the meta-stable
state isinnot
formed.
feature canisbe
complete,
the
photogenerated
I
states
decay
rapidly
on
a
time
scale
of
several
picoseconds
at
explained in the following way. In the I phase below 81 K, a ferroelectric domain is very large, both
200 suggests
μm, as mentioned
in Section 2.4.(ferroelectric)
Therefore, we Ishould
regard
the I phase
as feature
a
90 Ktypically
and 260200
K. ×This
that the meta-stable
state is
not formed.
This
three-dimensionally
ferroelectric
a case
in which
is irradiated
with is
a very
can be
explained in theordered
following
way. Inphase.
the I In
phase
below
81 K,TTF-CA
a ferroelectric
domain
femtosecond
laser×pulse
in theasNmentioned
phase, the in
microscopic
I domains
are initially
generated
as the I
large,
typically 200
200 µm,
Section 2.4.
Therefore,
we should
regard
mentioned above. We consider that, during the formation process of an I domain, initial molecular
phase as a three-dimensionally ordered ferroelectric phase. In a case in which TTF-CA is irradiated
deformations or displacements would determine the direction of the dipole moment within each
domain. In other words, the two kinds of I domains, [ … D0A0D+A−D+A−D+A−D+A−D0A0 … ] or
[…A0D0A−D+A−D+A−D+A−D+A0D0…], are randomly generated in the initial processes, probably on the
Crystals 2017, 7, 132
19 of 41
with a femtosecond laser pulse in the N phase, the microscopic I domains are initially generated as
mentioned above. We consider that, during the formation process of an I domain, initial molecular
deformations or displacements would determine the direction of the dipole moment within each
domain. In other words, the two kinds of I domains, [ . . . D0 A0 D+ A− D+ A− D+ A− D+ A− D0 A0 . . . ]
Crystals 2017, 7, 132
19 of 41
or [ . . . A0 D0 A− D+ A− D+ A− D+ A− D+ A0 D0 . . . ], are randomly generated in the initial processes,
probably
the of
time
scale of a half-period
the fastest intramolecular
deformation,
which
close to
timeon
scale
a half-period
of the fastestofintramolecular
deformation, which
is close to
20 fs.isAfter
20 fs. After
those
I
domains
are
stabilized
by
the
molecular
deformations
and
displacements,
a
dipole
those I domains are stabilized by the molecular deformations and displacements, a dipole moment
moment
of each
I domain
is difficult
to reverse.
As a aresult,
a three-dimensionally
ordered ferroelectric
of each
I domain
is difficult
to reverse.
As a result,
three-dimensionally
ordered ferroelectric
I state
bebe
stabilized.
ThisThis
feature
is schematically
illustrated
in Figure
I stateisisdifficult
difficulttoto
stabilized.
feature
is schematically
illustrated
in12(aI–III).
Figure 12(aI–III).
1.5
xph= 0.014 ph/DA
1
10
102ΔR/R
Normalized ΔR/R
xph=0.14 ph/DA
260 K
5
0
0
0.5
90 K
0.05
0.1
xph (ph/DA)
90 K
260 K
0
0
2
4
6
Delay time (ps)
8
10
Figure 17. Normalized time evolutions of ΔR/R signals at different temperatures and excitation
Figure 17. Normalized time evolutions of ∆R/R signals at different temperatures and excitation
densities in the ionic phase of TTF-CA [33]. Spectra of pump and probe pulses are shown in Figure 13a.
densities in the ionic phase of TTF-CA [33]. Spectra of pump and probe pulses are shown in Figure 13a.
The green line: 90 K and 0.014 ph/DA. The blue line: 90 K and 0.14 ph/DA. The orange line: 260 K and
The green line: 90 K and 0.014 ph/DA. The blue line: 90 K and 0.14 ph/DA. The orange line: 260 K and
0.014 ph/DA. The red line: 260 K and 0.14 ph/DA. The inset shows excitation photon density (xph)
0.014 ph/DA.
Theofred
line:
260
K andcircles)
0.14 ph/DA.
inset
shows excitation photon density (xph )
dependence
ΔR/R
at 90
K (solid
and 260 KThe
(open
circles).
dependence of ∆R/R at 90 K (solid circles) and 260 K (open circles).
More detailed studies about the excitation photon density dependence of the NI transition
efficiency
and studies
dynamicsabout
revealed
the following
facts density
[33]. With
increase inofthe
of
More detailed
the excitation
photon
dependence
thenumber
NI transition
photogenerated I states, their decay time is prolonged. This tendency is observed at 260 K, as shown
efficiency and dynamics revealed the following facts [33]. With increase in the number of
in Figure 17. This can be attributed to the fact that, with the increase in the number of photogenerated
photogenerated I states, their decay time is prolonged. This tendency is observed at 260 K, as shown in
I states, the Coulomb attractive interactions among I domains increase, making themselves more
Figurestable
17. This
be attributed
to the
fact At
that,
with
increase
the number ofI states
photogenerated
I
and can
prolonging
their decay
times.
90 K,
thethe
decay
time ofinphotogenerated
is longer
states,than
the Coulomb
attractive
interactions
among
I
domains
increase,
making
themselves
more
stable
that at 260 K, but the difference in their decay times for the weak excitation case (0.014 ph/DA
and prolonging
their decay
times.
90 K,
the decay
time
of photogenerated
I states
is longer than
pair) and strong
excitation
caseAt(0.14
ph/DA
pair) is
rather
small. At 90 K, the
spin-Peierls-like
that atinstability
260 K, but
the difference
in their decay
timesisfor
the weak
excitationtocase
(0.014
ph/DA
working
in the photogenerated
I states
enhanced
as compared
the case
at 260
K andpair)
the
dimeric
molecular
displacements
within
the
photogenerated
I
domain
are
increased,
which
and strong excitation case (0.14 ph/DA pair) is rather small. At 90 K, the spin-Peierls-like instability
prolongs
decay time of Iphotogenerated
I domains,
even if the
I domain
isolated.
the
working
in the the
photogenerated
states is enhanced
as compared
to the
case atis260
K andThus,
the dimeric
prolongation
of
the
decay
time
at
90
K
relative
to
that
at
260
K
originates
from
the
increase
in
molecular displacements within the photogenerated I domain are increased, which prolongs thethe
decay
dimeric molecular displacements. At 90 K, this effect overcomes the increase in the decay time with
time of photogenerated I domains, even if the I domain is isolated. Thus, the prolongation of the
increasing number of I domains via the Coulomb attractive interaction.
decay time at 90 K relative to that at 260 K originates from the increase in the dimeric molecular
displacements.
At 90 K,
this effect overcomes
the increase
in the decay time with increasing number of
5. Photoinduced
Ionic-to-Neutral
Phase Transition
in TTF-CA
I domains via the Coulomb attractive interaction.
5.1. Photoinduced Ionic-to-Neutral Transition
5. Photoinduced Ionic-to-Neutral Phase Transition in TTF-CA
In this subsection, we review the photoinduced I-to-N transition [17,19] and qualitatively compare
its
transition
dynamics to thoseTransition
of the photoinduced N-to-I transition discussed in Section 4.
5.1. Photoinduced Ionic-to-Neutral
The transient changes in the reflectivity spectra in the region of the IM transition of TTF have
Inalso
thisbeen
subsection,
we review
thebyphotoinduced
I-to-N transitionreflection
[17,19] and
qualitatively
investigated
in detail
the femtosecond-pump–probe
spectroscopy
withcompare
the
time
resolution
of
~180
fs
[17,19].
The
dynamics
revealed
by
these
studies
are
schematically
shown
its transition dynamics to those of the photoinduced N-to-I transition discussed in Section 4. in
Figure 12b. When TTF-CA is irradiated with a femtosecond laser pulse in the I phase, N states are
generated. The efficiency of N-state-generation per photon is evaluated to be approximately 8 D0A0
Crystals 2017, 7, 132
20 of 41
The transient changes in the reflectivity spectra in the region of the IM transition of TTF have
also been investigated in detail by the femtosecond-pump–probe reflection spectroscopy with the
time resolution of ~180 fs [17,19]. The dynamics revealed by these studies are schematically shown
in Figure 12b. When TTF-CA is irradiated with a femtosecond laser pulse in the I phase, N states are
generated. The efficiency of N-state-generation per photon is evaluated to be approximately 8 D0 A0 pairs
at 4 K and approximately 24 D0 A0 pairs at 77 K, just below the NI transition temperature Tc = 81 K. Since
the energy of the 1D N domain, [ . . . D+ A− D+ A− D0 A0 D0 A0 D0 A0 D0 A0 D+ A− D+ A− . . . ], consisting of
8 (or 24) D0 A0 pairs, is much lower than that of 8 (or 24) isolated D0 A0 pairs. It is, therefore, reasonable
to consider that 1D N domains are formed. The 1D N domain formation is schematically illustrated in
Figure 12(bII). The increase in the domain size at 77 K relative to that at 4 K occurs because the energy
difference between the I and N states decreases as the temperature increases to Tc . A 1D N domain
is formed within 20 fs, similar to the case of the 1D I-domain formation in the N phase. The ultrafast
dynamics of the initial N-domain formation is discussed in the following subsection.
At 4 K, the photogenerated N domains disappear with a time constant of approximately 300 ps
when the density of N domains is low. When the excitation photon density is increased and the density
of photogenerated N domains is enhanced, the surrounding or residual I states become unstable
and are converted to N states, as shown in Figure 12(bIV). As a result, a macroscopic N region is
generated. The number of N states finally produced is several times larger than that of the N states
initially generated. The time constant τ m of this multiplication process of the N states is approximately
20 ps, which is very slow as compared to the formation time (~20 fs) of a microscopic N domain.
τ m is considered a characteristic time for the changes in the lattice constants, that is, the increases in
the volume, which are necessary for the stabilization of macroscopic N states. Note that the lattice
constants in the N phase are longer than those in the I phase. Accordingly, τ m is considered to be
dominated by the time scale of acoustic phonons or the corresponding sound velocity. The macroscopic
N states thus produced are fairly stable and their decay time is very long, typically being on the time
scale of microseconds, although this depends on the excitation photon density and temperature.
Such a multiplication process of N domains and a formation of macroscopic and stable N states
in the photoinduced I-to-N transition are in contrast to unstable and short-lived I states in the
photoinduced N-to-I transition (see Figure 12a). We can relate this difference to the fact that the
I-to-N transition is a destruction of the three-dimensional (3D) ferroelectric order. A destruction
process of a 3D ordered state by photoirradiation occurs as a domino phenomenon. On the other hand,
the photoinduced N-to-I transition is a reverse process, that is, a formation process of a 3D ordered
state, so that it is difficult to induce by photoirradiation, as mentioned in Section 4.5.
5.2. Ultrafast Dynamics of Neutral Domain Formation and Multiplication
In the photoinduced I-to-N transition, 1D N domains are initially produced. To investigate
the ultrafast dynamics of this 1D N-domain formation, we also performed pump-probe reflection
measurements with a time resolution of ~20 fs using the two-NOPA system. In this subsection,
we report the results of these studies.
In Figure 18a, we show the polarized reflectivity spectra with E//a at 4 K as well as with E⊥a at
4 K and 90 K. We also show the spectra of pump and probe pulses by the shaded areas, which are the
same as those used in the experiments for the photoinduced N-to-I transition discussed in Section 4.2.
The N-domain formation was investigated by probing the IM transition of TTF. In the same figure, we
show the differential reflectivity spectrum, [ RN (90 K) − RI (4 K)]/RI (4 K), by the dashed-dotted line,
which is the spectral change expected when the I state is converted to the N state. The starting point is
in the I phase, so that the decrease in the reflectivity for the probe pulse indicates the photogeneration
of N states.
In Figure 18b, we show the time evolutions of the photoinduced reflectivity changes ∆R/R
at 10 K for the strong excitation case (xph = 0.12 ph/DA pair) (I, II) and the weak excitation case
(xph = 0.03 ph/DA pair) (III, IV). In both cases, the initial decrease in the reflectivity is very fast. Their
Crystals 2017, 7, 132
21 of 41
time constant are both estimated to be approximately 20 fs, which is comparable to the time scale of
the transfer energy t. This suggests that a 1D N domain is formed via purely electronic processes,
or equivalently, collective charge transfers, similar to the case of the photoinduced 1D I-domain
formation in the N phase.
We measured the time evolutions of ∆R/R at various excitation densities xph at 10 K. In Figure 19a,
Crystals
7, 132values at td = 0.1 ps with squares as a function of xph , which reflects the number
21 of of
41
we
plot2017,
∆R/R
photogenerated I states. ∆R/R is roughly proportional to xph for xph < 0.07 ph/DA pair. Assuming a
linear
relation for
forxxph <
< 0.07 ph/DA pair and the value of [ (90K) − (4K) / (4K), we estimated
linear relation
ph 0.07 ph/DA pair and the value of [ RN (90 K) − RI (4 K)] /RI (4 K), we estimated
0 pairs, which is
the
size
of
the
1D
N
domain
generated
per photon
to be approximately
7 D0A
the size of the 1D N domain
generated
per photon
to be approximately
7 D0 A0 pairs,
which
is consistent
0
0
0
0
consistent
with(8the
(8 DofAthepairs)
of the pump–probe
with
the timeofresolution
with
the result
D result
A pairs)
pump–probe
experimentsexperiments
with the time
resolution
180 fs at 4 of
K
180
fs
at
4
K
(see
Section
5.1).
The
saturation
of
the
signal
at
approximately
x
ph ~ 0.08 ph/DA is due to
(see Section 5.1). The saturation of the signal at approximately xph ~0.08 ph/DA is due to the space
the
space
filling
of the photogenerated
filling
of the
photogenerated
N states. N states.
80
102R
60
RN − RI
RI
RI(4 K)
E // a
0
40
20
8
E⊥a
(x 3)
Pump
0
0
Probe
1
2
Photon Energy (eV)
(b)
-8
RN(90 K)
RI(4 K)
102∆R/R
(a)
3
(II)
(I)
Normalized ∆R/R
xph=0.12 ph/DA
(IV)
(III)
xph= 0.03 ph/DA
0
0.2
0.4
0.6
0
1
Delay Time (ps)
2
3
Figure 18. Results
with
thethe
time
resolution
of
Resultsof
ofoptical-pump
optical-pumpoptical-reflectivity-probe
optical-reflectivity-probespectroscopy
spectroscopy
with
time
resolution
~20
fs in
I phase
of TTF-CA.
(a) Polarized
reflectivity
spectra
at 4 at
K (solid
lines)lines)
and and
at 90atK90
(the
of ~20
fs the
in the
I phase
of TTF-CA.
(a) Polarized
reflectivity
spectra
4 K (solid
K
(the
broken
A differential
reflectivity
spectrum,
K) −I(4RK),
K)]/R
K),the
is dashed-dotted
shown by the
broken
line). line).
A differential
reflectivity
spectrum,
[RN(90 K)[R
−N
R(90
I(4 K)]/R
shown
I (4 is
I (4by
dashed-dotted
line.
Shaded
areas
show and
spectra
of pulses;
pump (b)
andTime
probe
pulses; (b)
Time evolutions
of
line.
Shaded areas
show
spectra
of pump
probe
evolutions
of reflectivity
changes
reflectivity
changes
aroundexcitation
2.2 eV bywith
a strong
with
= 0.12
and
0.12 ph/DA
[(I)xph
and
(II)] ph/DA
and by [(I)
a weak
(ΔR/R)
at around
2.2(∆R/R)
eV by at
a strong
xph =excitation
(II)]
and bywith
a weak
with [(III)
xph =and
0.03 (IV)].
ph/DA
[(III) lines
and (IV)].
in (II) waveforms
and (IV) show
= 0.03 ph/DA
Green
in (II)Green
and lines
(IV) show
of
excitation
xph excitation
−
1
−1
waveforms
of ħω
−cos(ωt)
. h̄ω = 55 cm .
−cos(ωt) with
= 55 cmwith
Crystals 2017, 7, 132
22 of 41
Crystals 2017, 7, 132
22 of 41
(b)
5
3
0.5
2
1
∆R/R (arb. unit)
4
-102∆R/R
−cos
1
A2 (arb. unit)
(a)
+cos
fit
xph=0.03 ph/DA
0
0
0.05
0.1
xph (ph/DA)
(c)
0
0.15
0
1
2
3
Delay Time (ps)
・・・ D+A– D+A– D+A– D+A– D+A– D+A– D+A– ・・・
・・・ D+A– D+A– D+A– D+A– D+A– D+A– D+A– ・・・
・・・ D+A– D+A– D+A– D0+A0– D+A– D+A– D+A– ・・・
・・・ D+A– D+A– D+A– D+A– D+A– D+A– D+A– ・・・
・・・ D+A– D+A– D+A– D+A– D+A– D+A– D+A– ・・・
・・・ D+A– D+A– D+A– D+A– D+A– D+A– D+A– ・・・
・・・ D+A– D+A– D+A– D+A– D+A– D+A– D+A– ・・・
・・・ D+A– D+0A–0 D+0A–0 D+0A–0 D+0A–0 D+0A–0 D+A– ・・・
・・・ D+A– D+A– D+A– D+A– D+A– D+A– D+A– ・・・
・・・ D+A– D+A– D+A– D+A– D+A– D+A– D+A– ・・・
Figure 19. Analyses of the results of optical-pump optical-reflectivity-probe spectroscopy with the
Figure
19. Analyses of the results of optical-pump optical-reflectivity-probe spectroscopy with the
time resolution of ~20 fs in the I phase of TTF-CA. (a) Excitation photon density (xph) dependence of
time resolution of ~20 fs in the I phase of TTF-CA. (a) Excitation photon density (xph ) dependence of
reflectivity changes ΔR/R at around 2.2 eV (squares) and initial amplitudes of +cosine-type oscillations
reflectivity changes ∆R/R at around 2.2 eV (squares) and initial amplitudes of +cosine-type oscillations
(circles) shown by the orange line in (b); (b) Normalized time evolutions of ΔR/R (the blue line). The
(circles)
by athe
orange
line
incomponents
(b); (b) Normalized
time
evolutions
(theorange
blue line).
red shown
line shows
fitting
curve.
Two
of the fitting
curve
are shownof
by∆R/R
green and
The red
line
shows
a
fitting
curve.
Two
components
of
the
fitting
curve
are
shown
by
green
and
orange
lines; (c) Schematic illustrations of coherent oscillations. −cosine-type oscillation and +cosine-type
lines;oscillation
(c) Schematic
illustrations
of
coherent
oscillations.
−
cosine-type
oscillation
and
+cosine-type
are generated in a neutral domain and in a surrounding ionic region, respectively.
oscillation are generated in a neutral domain and in a surrounding ionic region, respectively.
5.3. Collective Molecular Dynamics
5.3. Collective
Molecular
Dynamics
As shown
in Figure
18b, after the initial rapid decrease in ΔR/R reflecting the photoinduced 1D
N-domain formation, complicated oscillatory structures appear, which are attributable to the
As shown in Figure 18b, after the initial rapid decrease in ∆R/R reflecting the photoinduced 1D
molecular deformations and molecular displacements, similar to the case of the photoinduced 1D
N-domain formation, complicated oscillatory structures appear, which are attributable to the molecular
I-domain formation in the N phase [30]. Here, we focus on the slow oscillations with the period of
deformations
and 0.6
molecular
displacements,
similartoto
the case
the photoinduced
1Dshown
I-domain
approximately
fs. This oscillation
is proportional
−cos(ωt)
withofa frequency
of 55 cm−1, as
formation
in
the
N
phase
[30].
Here,
we
focus
on
the
slow
oscillations
with
the
period
of
approximately
by the green lines in Figure 18(bII,IV).
0.6 fs. ThisOwing
oscillation
proportional
−cos(ωt)towith
frequency
55 cm−1in
, as
shown
by the green
to theissimilarity
of thistooscillation
thatawith
53 cm−1ofobserved
the
photoinduced
lines N-to-I
in Figure
18(bII,IV).
transition,
it is natural to consider that this oscillation is related to the dissolution of the dimeric
molecular
1Doscillation
N domains.toSince
1D N 53
domain
formed very
fs) via
Owing
to displacements
the similaritywithin
of this
thatawith
cm−1isobserved
in fast
the (~20
photoinduced
the
collective
charge
back
transfer
processes,
dimeric
molecular
displacements
do
not
change
at dimeric
all
N-to-I transition, it is natural to consider that this oscillation is related to the dissolution of the
during
this
process.
In
the
1D
N
domain,
each
molecule
has
no
spin;
therefore,
the
dimeric
molecular
molecular displacements within 1D N domains. Since a 1D N domain is formed very fast (~20 fs) via
displacements should disappear, which gives rise to the coherent oscillation of the optical mode
the collective charge back transfer processes, dimeric molecular displacements do not change at all
corresponding to the dimerization. This oscillation is expressed by −cos(ωt), such that the reflectivity
during this process. In the 1D N domain, each molecule has no spin; therefore, the dimeric molecular
starts to increase from the time origin. This means that the degree of CT ρ initially increases by the
displacements
disappear,
which
gives riseConsidering
to the coherent
oscillation
the optical
decrease inshould
the dimeric
molecular
displacements.
that the
oscillationofoccurs
in the Nmode
corresponding
to the in
dimerization.
This
oscillation
is expressed
−cos(ωt),
such
the reflectivity
state, this change
ρ is attributable
to the
increase of
the effectiveby
transfer
energy
t bythat
the dissolution
startsoftothe
increase
the time
origin. This
means
CT
ρ initially
increases
by the
dimericfrom
molecular
displacements.
This
meansthat
thatthe
thedegree
effect ofof
the
increase
in the
interdimer
transfer
energy
is more
important
than that of the
decrease in the
transfer
energy.
decrease
in the
dimeric
molecular
displacements.
Considering
thatintradimer
the oscillation
occurs
in the N state,
In in
theρweak
excitation case,
initial dynamics
shown in
Figureenergy
18(bIV) tcannot
reproducedof the
this change
is attributable
to thethe
increase
of the effective
transfer
by thebe
dissolution
only
by
the
minus-cosine-type
oscillation.
The
phase
of
the
oscillation
seems
to
be
reversed
onlytransfer
at
dimeric molecular displacements. This means that the effect of the increase in the interdimer
t
d < 0.7 ps. This behavior suggests that in the weak excitation case, the oscillatory component
energy is more important than that of the decrease in the intradimer transfer energy.
proportional to cos(ωt) exists in addition to the long-lived oscillation proportional to −cos(ωt), and
In the weak excitation case, the initial dynamics shown in Figure 18(bIV) cannot be reproduced
only by the minus-cosine-type oscillation. The phase of the oscillation seems to be reversed only
at td < 0.7 ps. This behavior suggests that in the weak excitation case, the oscillatory component
proportional to cos(ωt) exists in addition to the long-lived oscillation proportional to −cos(ωt), and that
Crystals 2017, 7, 132
23 of 41
the decay time of the former is very short. Considering these two kinds of damped oscillators, we
adopt the fitting function F(t)
F (t) = − A1 exp(−t/τ1 ) cos(ωt + φ1 ) + A2 exp(−t/τ2 ) cos(ωt + φ2 ) : (τ1 > τ2 )
(5)
where the first and second terms correspond to the long-lived oscillation and short-lived oscillations,
respectively. By using this function, the oscillatory component for the weak excitation case can
be well reproduced, as shown by the red line in Figure 19b. In this fitting, we adopted a simple
exponential-decay function to express the background signal. Two oscillatory components are also
shown in the same figure. The obtained parameters are ω = 55 cm−1 , τ 1 >> 10 ps, and τ 2 = 0.22 ps.
We carried out similar analyses on the oscillatory components at several excitation photon
densities xph and extracted the xph dependence of the amplitudes A2 of the short-lived oscillation,
which is plotted using circles in Figure 19a. A2 is saturated at xph ~ 0.02 ph/DA pair and rather
decreased at xph > 0.05 ph/DA pair. We explain such peculiar behaviors by attributing the short-lived
oscillation to the oscillation in the I state. After the initial formation of a 1D N domain, the I state
surrounding the N domain destabilized and the dimeric molecular displacements in the I state would
also be slightly decreased, as shown in Figure 19c. The I state is a 3D ordered ferroelectric state.
Therefore, the decrease in the dimerization would be induced over a wide spatial region through the
interchain interactions. Assuming a space filling of the oscillating region [35] and using the saturation
value of xph = 0.02 ph/DA pair, the oscillating region is estimated to be ~50 DA pairs per photon. This
short-lived oscillation is of the plus-cosine type and the reflectivity starts to decrease at the time origin,
as seen in Figure 19b. This indicates that the decrease in the dimerization gives rise to the decrease in
ρ. Such a decrease in ρ by the decrease in the dimeric molecular displacements is attributable to the
decrease in the intradimer Coulomb attractive interaction. This interpretation is consistent with that of
the coherent oscillation in the photogenerated I domains discussed in Section 4.3.
6. Control of Ferroelectric Polarization by Terahertz Electric Field in the Ionic Phase of TTF-CA
6.1. Rapid Modulation of Ferroelectric Polarization
Important applications of ferroelectrics in the field of optics are based upon the Pockels effect and
second-order optical nonlinearity. By using the Pockels effect, we can rotate the polarization of light
via an anisotropic change in refractive index by an electric field. Second-order optical nonlinearity
is indispensable for SHG and various other kinds of light frequency conversion. If the amplitude or
direction of a macroscopic polarization in ferroelectrics could be rapidly changed, advanced control of
light, which is difficult in conventional nonlinear optical materials, would be achieved. As discussed
in Section 2.3, TTF-CA is a typical electronic ferroelectric, in which the ferroelectric polarization is
produced by intermolecular collective electron transfers from D to A molecules. Therefore, we can
expect that its ferroelectric polarization can be controlled on the subpicosecond time scale by an
external stimulus, as well as via the collective intermolecular electron transfers. When TTF-CA in the I
phase is irradiated with a femtosecond laser pulse at above the CT transition peak, the I-to-N transition
occurs and the SHG is suppressed [16]. In this phenomenon, however, the recovery of the original
polarization takes 300 ps or longer [19]. For the rapid control of the ferroelectric polarization, it is
useful to use an electric-field component of light. In this section, we review the study aiming to control
the ferroelectric polarization in TTF-CA by a nearly monocyclic terahertz electric-field pulse [41].
6.2. Terahertz-Pulse-Pump Second-Harmonic-Generation-Probe Measurements in the Ionic Phase
In order to detect a change in a macroscopic polarization P, it is effective to use SHG. As mentioned
in Section 2.4, a ferroelectric domain in the I phase is large, typically 200 × 200 µm in size. This
domain size is comparable to or larger than the spot size of a probe pulse (diameter of 100–200 µm)
in pump-probe experiments. Therefore, we can detect changes in the macroscopic polarization by
probing the change in SHG intensity. The terahertz-pulse-pump SHG-probe measurement on TTF-CA
Crystals 2017, 7, 132
24 of 41
2017, 7, 132
of 41
was Crystals
performed
in a reflection configuration, the schematic of which is shown in Figure 20a. 24
The
electric
fields
of
both
the
pump
and
probe
pulses
are
parallel
to
the
a
axis.
The
red
line
in
Figure
20b
shows
shown in Figure 20a. The electric fields of both the pump and probe pulses are parallel to the a axis.
the electric
waveform
[ETHz
of aelectric
terahertz
The amplitude
terahertz
electric
(t)]the
( )] ofofa the
terahertz
pulse.
The field
The red field
line in
Figure 20b
shows
fieldpulse.
waveform
[
is approximately
36
kV/cm.
The
Fourier
power
spectrum
of
the
terahertz
electric
field
is
in
amplitude of the terahertz electric field is approximately 36 kV/cm. The Fourier power spectrumshown
of
−1 ).
−1
Figure
20c.
The
peak
frequency
is
~0.75
THz
(~25
cm
the terahertz electric field is shown in Figure 20c. The peak frequency is ~0.75 THz (~25 cm ).
Figure
Resultsofof terahertz-pump
terahertz-pump SHG-probe
measurements
in the
I phase
of
Figure
20. 20.Results
SHG-probe
measurements
in ferroelectric
the ferroelectric
I phase
of
TTF-CA [41]. (a) Schematics of terahertz-pump SHG-probe measurement in the ionic phase; (b) a
TTF-CA [41]. (a) Schematics of terahertz-pump SHG-probe measurement in the ionic phase;
Fourier power spectrum of a terahertz electric-field pulse shown by the red line in; (c) time evolutions
(b) a Fourier power spectrum of a terahertz electric-field pulse shown by the red line in; (c) time
) at 65 K, ∆
( )/
, by a terahertz electric field (blue
of changes in an SH intensity (
evolutions of changes in an SH intensity ( ISHG−65K ) at 65 K, ∆ISHG (t)/ISHG−65K , by a terahertz electric
circles), with the electric-field amplitude of (c) 36 kV/cm; and (d) 415 kV/cm. Red lines show electricfieldfield
(blue
circles), with the electric-field(amplitude
of (c) 36 kV/cm; and (d) 415 kV/cm. Red lines
waveforms of terahertz pulses (
)).
show electric-field waveforms of terahertz pulses (ETHz (t)).
When the probe light with the photon energy of 1.3 eV was incident to a single crystal of TTF-CA
atWhen
65 K, the
the probe
SH light
with
thethe
photon
energy
of 2.6
observed.
Both
incident
probe
light
with
photon
energy
of eV
1.3was
eV was
incident
to the
a single
crystal
of light
TTF-CA at
and the SH light were polarized parallel to the a axis. It was ascertained that the intensity of the SH
65 K, the SH light with the photon energy of 2.6 eV was observed. Both the incident probe light and the
light,
, was proportional to the square of the intensity of the incident probe light. This
SH light were polarized parallel to the a axis. It was ascertained that the intensity of the SH light, ISHG ,
confirmation is important to avoid the saturation of SH light intensity and to execute accurate
was measurements.
proportional toInthe
square of the intensity of the incident probe light. This confirmation is important
Figure 20c, we show the time characteristic of the terahertz-electric-field-induced
to avoid
the[∆saturation
of
to
execute
accurate
measurements.
In Figure
20c, we
( )/
change
] inSH
thelight
SHGintensity
intensity and
(
) by
open circles,
which
is in good agreement
with
show
the
time
characteristic
of
the
terahertz-electric-field-induced
change
[∆I
t
/I
]
in
the
(
)
( )] of the terahertz pulse (the red line). This demonstrates
SHG
SHGthat the SHG
the electric-field waveform [
intensity
(ISHG ) by
open circles,
is inby
good
with the electric-field
( ), as ∆waveform
( )/ [E∝THz (t)]
macroscopic
polarization
P is which
modulated
the agreement
terahertz electric-field
( ).
of the terahertz
pulse (the red line). This demonstrates that the macroscopic polarization P is modulated
by the terahertz electric-field ETHz (t), as ∆ISHG (t)/ISHG ∝ ETHz (t).
6.3. Terahertz-Pulse-Pump Optical-Reflectivity-Probe Measurements in the Ionic Phase
6.3. Terahertz-Pulse-Pump
Optical-Reflectivity-Probe
in the Ionic Phase
As mentioned in Section
2.3, the origin for theMeasurements
ferroelectric polarization
in the I phase is the
increase
in ρ across
the NI transition
at 81 K.for
Therefore,
we expectpolarization
that the electric-field-induced
As mentioned
in Section
2.3, the origin
the ferroelectric
P in the I phase is the
change in
may be attributed to the change in  via a partial charge transfer between D and A
increase in ρ across the NI transition at 81 K. Therefore, we expect that the electric-field-induced change
molecules. To ascertain this, we investigated the electric-field-induced change in  , using the
in P may be attributed to the change in ρ via a partial charge transfer between D and A molecules.
reflectivity change of the IM transition band of TTF. This is the terahertz version of the ER
To ascertain this, we investigated the electric-field-induced change in ρ, using the reflectivity change
of the IM transition band of TTF. This is the terahertz version of the ER spectroscopy reported in
2017,
CrystalsCrystals
2017, 7,
1327, 132
25 of 41
25 of 41
spectroscopy reported in Section 2.4 [61]. Figure 21a shows the schematic of the terahertz-pump
optical-reflectivity-probe
The probe of
energy
is set at 2.2 eV. The
polarization of the
Section
2.4 [61]. Figure 21a spectroscopy.
shows the schematic
the terahertz-pump
optical-reflectivity-probe
probe pulse
perpendicular
theata 2.2
axiseV.
(E⊥a)
its temporal
is approximately
130 fs. By to
spectroscopy.
Theisprobe
energy istoset
Theand
polarization
ofwidth
the probe
pulse is perpendicular
measuring
the
reflectivity
change
∆
/
in
this
region,
we
can
observe
the
electric-field-induced
the a axis (E⊥a) and its temporal width is approximately 130 fs. By measuring the reflectivity change
∆ in ρ.
∆R/Rchanges
in this region,
we can observe the electric-field-induced changes ∆ρ in ρ.
21. Results
of terahertz-pumpoptical-reflectivity-probe
optical-reflectivity-probe measurements
in the
ferroelectric
I
FigureFigure
21. Results
of terahertz-pump
measurements
in the
ferroelectric
I
phase of TTF-CA [41]. (a) Schematic of terahertz-pump optical-reflectivity-probe measurement; (b)
phase of TTF-CA [41]. (a) Schematic of terahertz-pump optical-reflectivity-probe measurement; (b)
an electric-field waveform of a terahertz pulse (ETHz); (c) time evolution of reflectivity changes ΔR/R
an electric-field waveform of a terahertz pulse (ETHz ); (c) time evolution of reflectivity changes ∆R/R
at 78 K in the ionic phase (circles) and the normalized ETHz(t) (the solid line) in TTF-CA [41]; (d) probe
at 78 K in the ionic phase (circles) and the normalized ETHz (t) (the solid line) in TTF-CA [41]; (d)
energy dependence of ΔR/R at the delay time td = 0 ps and at 78 K (yellow circles). The solid line shows
probethe
energy
dependence of ∆R/R at the delay time t = 0 ps and at 78 K (yellow circles). The solid
first derivative (dR/dE) of a reflectivity spectrum dat 77 K with respect to the photon energy E,
line shows
the
firstthe
derivative
(dR/dE)
a reflectivity
spectrum
respectcomponent
to the photon
ρ; 77
(e)Kanwith
oscillatory
which shows
reflectivity
change of
due
to a slight increase
in at
energy
E,
which
shows
the
reflectivity
change
due
to
a
slight
increase
in
ρ;
(e)
an
oscillatory
component
ΔROSC(t)/R (circles) and fitting curve (solid line); (f–h) time-dependent Fourier power spectra (wavelet
∆ROSCtransforms)
(t)/R (circles)
and
fitting
curve
(solid
line);
(f–h)
time-dependent
Fourier
power
spectra
(wavelet
of (b), ETHz(t) (c), ΔR/R(t), and (e) ΔROSC/R(t), respectively.
transforms) of (b), ETHz (t) (c), ∆R/R(t), and (e) ∆ROSC /R(t), respectively.
In Figure 21c, we show by open circles the time evolution of ∆ ( )/ at 2.2 eV measured at 78
K. The reflectivity increase (decrease) at this energy indicates the increase (decrease) in ρ. The red
In Figure 21c, we show by open circles the time evolution of ∆R(t)/R at 2.2 eV measured at 78 K.
circles in Figure 21b and the red line in Figure 21c show the electric-field waveform,
( ). The
The reflectivity
increase
this ∆energy
(decrease)
ρ. The (red
circles
latter is shown
on a (decrease)
normalizedat
scale.
( )/ indicates
at aroundthe
theincrease
time origin
accords in
with
). In
in Figure
21b
and
redthe
line
in Figure
21c show the
waveform,
latter is
). The
THz ( tare
Figure
21d,
wethe
show
probe
energy dependence
of electric-field
∆ (0)/ by yellow
circles, E
which
in good
shown
on a normalized
∆R(t)/R
at around
the reflectivity
time origin(the
accords
with E
Figure
(t). Inshows
agreement
with the scale.
first energy
derivative
of the
blue line).
The
a 21d,
THzlatter
we show
the probe
energy
dependence
∆Rof
/RIM
bytransition
yellow circles,
are incorresponds
good agreement
(0)the
reflectivity
change
by the
lower energyofshift
band ofwhich
TTF, which
to
an increase
in ρ. This
indicatesofthat
is increased by
theblue
electric
fieldThe
of the
terahertz
pulse.
In Figure 22a,
with the
first energy
derivative
theρ reflectivity
(the
line).
latter
shows
a reflectivity
change
we
plot
∆
(
)/
in
the
range
of
t
d
=
−1.5
to
1.5
ps
as
a
function
of
(
).
This
plot
clearly
by the lower energy shift of the IM transition band of TTF, which corresponds to an increaseshows
in ρ. This
that that
∆ / ρ is
and
ρ are proportional
to field
( )ofand
the change
follows22a,
the change
the
indicates
increased
by the electric
thethat
terahertz
pulse.inInρ Figure
we plotof∆R
(t)/R
electric field with no time delay. These results suggest that the electric-field-induced change in ρ
in the range of td = −1.5 to 1.5 ps as a function of ETHz (t). This plot clearly shows that ∆R/R and ρ are
occurs via electronic processes without structural changes and is attributable to the intermolecular
proportional to ETHz (t) and that the change in ρ follows the change of the electric field with no time
fractional CTs within each dimeric DA pair.
delay. These results suggest that the electric-field-induced change in ρ occurs via electronic processes
without structural changes and is attributable to the intermolecular fractional CTs within each dimeric
DA pair.
Crystals 2017, 7, 132
26 of 41
Crystals 2017, 7, 132
26 of 41
(a)
(b)
Crystals 2017, 7, 132
3
ΔR/R (Δρ) － ETHz
−20
t (ps)
0
-1 ΔR/R
-1
probe
-2
-1 0 1 -20
1
t (ps)
02
-1
1
ETHz
t (ps)
103 ΔR/R
ΔR/R (Δρ) － ETHz
103 Δρ
1
3
0
2
-1 ΔR/R
1 probe
-2
-1 0 1
0 t (ps)
1
THz pump
0
20
ETHz (kV/cm)
0
−5
(b)
-1
5
40 kV/cm
Polarization: P P – ETHz
−20
103 Δρ
103 ΔR/R
2
(a) 1
Polarization: P
26 of 41
2
−5
P–E
P–E
5
40 field
kV/cm
Electric
E or ETHz
P – ETHz
Electric field
E or ETHz
40
ETHz
0
22. Comparison-1of ferroelectric polarization
P and terahertz electric field ETHz in TTF-CA [41].
THz pump
Figure 22.Figure
Comparison
of ferroelectric polarization
P and terahertz electric field ETHz in TTF-CA [41].
(a) Interrelation in time evolutions
change ΔR/R (Δρ) and ETHz at 65 K; (b) Schematic of a
-20
0 of a reflectivity
20
40
(a) Interrelation
in
time
evolutions
of
a
reflectivity
change
∆R/R (∆ρ) and ETHz at
65 K; (b) Schematic
ETHz
(kV/cm)
polarization–electric field curve (P–E curve) and a polarization–terahertz-electric-field
curve (P–ETHz
of a polarization–electric
field
curve
(P–E
curve)
and
a
polarization–terahertz-electric-field
curve
curve).
Figure 22. Comparison of ferroelectric polarization P and terahertz electric field ETHz in TTF-CA [41].
(P–ETHz curve).
(a) Interrelation in time evolutions of a reflectivity change ΔR/R (Δρ) and ETHz at 65 K; (b) Schematic of a
In Figure 23, we show schematically the observed response to the terahertz electric field ((a→b)
polarization–electric field curve (P–E curve) and a polarization–terahertz-electric-field curve (P–ETHz
and (a→c)). In Figure 21c, ∆ ( )/ at the time origin is positive, indicating that ∆ and ΔP are
curve).
In Figure
23,This
we means
showthat
schematically
the observed
response
the terahertz
electric(0)
field ((a→b)
positive.
the original polarization
P is parallel
to theto
terahertz
electric field
at the
time
origin
as
shown
in
Figure
23b.
We
performed
similar
experiments
on
several
samples.
In
and (a→c)).
InInFigure
21c,
∆R
t
/R
at
the
time
origin
is
positive,
indicating
that
∆ρ
and
∆P
are positive.
(
)
Figure 23, we show schematically the observed response to the terahertz electric field ((a→b)
some
of the In
samples,
∆ ( )/
was
negative
at origin
the time
origin, indicating
that
∆∆ and
ΔP are
(
)/
and
(a→c)).
Figure
21c,
∆
at
the
time
is
positive,
indicating
that
and
are
This means that the original polarization P is parallel to the terahertz electric field ETHz (0) at the time
negative.
In those
samples,
P is
anti-parallel
to
as shown
in Figure
23c.electric field
(0)
positive. This
means
that the
original
polarization
P(0),
is parallel
to the
terahertz
origin as shown in Figure 23b. We performed similar experiments on several samples. In some of the
at the time origin as shown in Figure 23b. We performed similar experiments on several samples. In
samples, ∆R
/Rthe
was
negative at the time origin, indicating that ∆ρ and ∆P are negative. In those
(t)of
some
samples, ∆ ( )/ was negative at the time origin, indicating that ∆ and  are
samples, Pnegative.
is anti-parallel
to ETHz
), as shown
(0), as 23c.
In those samples,
P (is0anti-parallel
to in Figure
shown in Figure 23c.
Figure 23. Schematics of observed responses to a terahertz electric-field pulse in the ionic phase of
TTF-CA [41]. (a) Ionic phase with ferroelectric polarization P; (b,c) Direct modulation of charge
transfers and polarization by a terahertz electric field with ETHz(0) directed to the left (b) and right (c),
respectively. Amplitude of ferroelectric polarization increases in the case of (b) and decreases in (c);
Figure 23. Schematics of observed responses to a terahertz electric-field pulse in the ionic phase of
Additional modulation
of charge
transferto
and
via dimeric molecular
Figure 23.(d,e)
Schematics
of observed
responses
a polarization
terahertz electric-field
pulseoscillations;
in the ionic phase
TTF-CA [41]. (a) Ionic phase with ferroelectric polarization P; (b,c) Direct modulation of charge
increases
and (e)phase
decreases
dimeric
molecular displacements,
charge
transfers,
and
polarization. of charge
of TTF-CA(d)
[41].
(a)
Ionic
with
ferroelectric
polarization
P;
(b,c)
Direct
modulation
transfers and polarization by a terahertz electric field with ETHz(0) directed to the left (b) and right (c),
Straight green arrows represent dimeric molecular displacements; Curved arrows in (b–d) indicate
respectively.
Amplitude
ferroelectricelectric
polarization
increases
the(0)
casedirected
of (b) andto
decreases
(c); and right
transfers and
polarization
by aof terahertz
field
with EinTHz
the leftin(b)
induced charge transfers.
(d,e)
Additional
modulation
of
charge
transfer
and
polarization
via
dimeric
molecular
oscillations;
(c), respectively. Amplitude of ferroelectric polarization increases in the case of (b) and decreases in
(d) increases and (e) decreases dimeric molecular displacements, charge transfers, and polarization.
(c); (d,e) Additional modulation of charge transfer and polarization via dimeric molecular oscillations;
Straight green arrows represent dimeric molecular displacements; Curved arrows in (b–d) indicate
(d) increases
and charge
(e) decreases
induced
transfers. dimeric molecular displacements, charge transfers, and polarization.
Straight green arrows represent dimeric molecular displacements; Curved arrows in (b–d) indicate
induced charge transfers.
6.4. Magnitudes of Polarization Modulation
In this subsection, we discuss the magnitude of the electric-field-induced change ∆ρ in the
degree of CT ρ and ∆P in the polarization P. From the magnitude of ∆R(0)/R, ∆ρ is evaluated
Crystals 2017, 7, 132
27 of 41
to be ~2.5 × 10−3 at ETHz (0) = 38 kV/cm. ∆P can also be calculated from the electric-field change
∆ISHG (0)/ISHG in the SHG intensity ISHG . Assuming that the second-order nonlinear susceptibility χ(2)
is proportional to the magnitude of P, we obtain ∆ISHG /ISHG = 2∆P/P using the relation, ISHG ∝ P2 .
From the value of ∆ISHG (0)/ISHG , ∆P(0)/P is estimated to be ~0.75% at ETHz (0) = 36 kV/cm. P is
produced by the collective charge transfers, δρ ∼ 0.2, from A to D molecules, which occur across Tc .
Therefore, we can also calculate ∆P(0)/P from the magnitude of ∆ρ obtained by the terahertz-pump
optical-reflectivity-probe measurements. In this case, ∆P(0)/P is evaluated to be ~1.25% at ETHz (0)
= 38 kV/cm, which is consistent with the value obtained from the SHG measurements (~0.75% at
ETHz (0) = 36 kV/cm).
Next, we compare the observed response to the terahertz electric field with the polarization
electric-field (P–E) characteristic previously reported [53]. The P–E characteristic is schematically
shown by the orange line in Figure 22b. The coercive field EC is approximately 5 kV/cm at 50 K. By an
application of a static electric field larger than EC , the polarization is reversed probably via domain-wall
motions. The response to the terahertz electric field is essentially different from that to the static electric
field. The ∆R/R − ETHz curve in Figure 22a can also be regarded as the ∆ρ − ETHz curve (see the right
vertical scale). Considering that the magnitude of the charge transfer, δρ ~ 0.2, across the NI transition
determines the magnitude of the original polarization Ps , P is proportional to (δρ + ∆ρ). Namely,
the (δρ + ∆ρ) − ETHz curve corresponds to the P − ETHz curve. This P − ETHz curve is schematically
shown by the blue line in Figure 22b. As a motion of a ferroelectric domain wall is slow and its time scale
is in the millisecond range [53], it never moves by a subpicosecond change in an electric field within a
terahertz pulse. This means that we can apply a large electric field only to the electronic system without
inducing any motions of ferroelectric domain walls and, therefore, we can cause a large and ultrafast
polarization modulation through electronic-state changes. Such a feature has also been theoretically
demonstrated by H. Gomi, A. Takahashi, et al. [94]. In fact, the recent terahertz-pump SHG-probe
measurement with the higher electric field (ETHz (0) = 415 kV/cm) revealed that the increase in SHG,
∆ISHG (t)/ISHG , reaches 14%, as shown in Figure 20d. In this case, the increase in the polarization,
∆P(t)/P, reaches 7%. Thus, we have successfully achieved ultrafast and large polarization modulation
in TTF-CA by using a strong terahertz electric-field pulse.
6.5. Coherent Oscillation Generated by Electric-Field-Induced Polarization Modulation
In this subsection, we discuss the reflectivity change ∆R/R for td > 0.5 ps. ∆R/R in this time
domain cannot be reproduced by ETHz (t) alone, as seen in Figure 21c. An additional oscillatory
component seems to exist in ∆R/R. By subtracting the normalized terahertz waveform [ETHz (t)]
from ∆R/R, we extracted the oscillatory component ∆ROSC /R and showed it with open circles in
Figure 21e. The period of the oscillation is approximately 0.6 ps. We also performed a wavelet analysis
and obtained the time dependence of the Fourier power spectra of the terahertz waveform ETHz (t),
the reflectivity change ∆R/R(t), and the oscillatory component ∆ROSC /R(t), which are shown as the
contour maps in Figure 21f–h, respectively. ETHz (t) ranges from 5 to 60 cm-1 and is localized around
the time origin. Conversely, ∆ROSC (t)/R shows a monochromatic component at 54 cm−1 , which is
observed even at td > 10 ps. The same component is also clearly observed in ∆R(t)/R. This oscillation
mode with 54 cm−1 modulates the degree of charge transfer ρ, so that it is reasonable to assign it to
the lattice mode associated with the dimerization. The similar coherent oscillation of this mode is
observed in the photoinduced N-to-I transition, as detailed in Section 4 [19].
Detailed studies of infrared (IR) vibrational spectroscopy on TTF-CA have been reported by A.
Girlando et al. [95]. According to them, several IR-active lattice modes were observed below 100 cm−1
in the I phase. In the terahertz-pump optical-reflectivity-probe experiment, we observed only a mode
with a frequency of 54 cm−1 as a coherent oscillation. This result supports our interpretation that this
mode corresponds to the dimerization and coupled strongly with intermolecular charge transfers.
Next, we discuss the generation mechanism of this coherent oscillation more in detail. A possible
mechanism is the direct generation of the oscillation by the terahertz electric field. However,
Crystals 2017, 7, 132
28 of 41
this possibility is excluded, because the directions of the field-induced displacements of D and A
molecules are opposite to those expected for displacive-type ferroelectricity, as mentioned in Section 2.3
(see Figure 6). Note that the first cycle of the oscillatory change in ρ in Figure 21e is in good agreement
with the initial rapid change in ρ (Figure 21c) directly driven by the terahertz electric field. This
suggests that ETHz (0) is parallel to P (Figure 23b) as seen in Figure 22a and that D (A) molecules move
antiparallel (parallel) to the original polarization P, as shown in Figure 23d.
The most plausible mechanism for the generation of the coherent oscillation is the modulation of
the spin-Peierls dimerization via the rapid modulation of ρ. When the direction of ETHz (0) is parallel
to the ferroelectric polarization P (Figure 23b), ρ should be increased first to ρ + ∆ρ(t) via electronic
processes, such that P is increased as mentioned above. The increase in ρ induces an increase in the
spin moment of each molecule, as illustrated by the red arrows in Figure 23b. Then, the spin-Peierls
mechanism works more effectively, and the molecular dimerization is strengthened (Figure 23d).
The resultant increase in the molecular displacements further increases the degree of charge transfer
from ρ + ∆ρ to ρ + ∆ρ + ∆ρ0 by the increase in the Coulomb attractive interaction within each dimer.
Thus, the terahertz field gives rise to a forced molecular oscillation of the spin-Peierls mode via the
change in ρ. Then, an additional oscillation in the degree of charge transfer with amplitude ∆ρ0 and in
the polarization with amplitude ∆P’ occurs, synchronized with the molecular oscillation, as (d) ↔ (e)
in Figure 23. When the direction of ETHz (0) is antiparallel to the original polarization P, as shown in
Figure 23c, ρ should first decrease and then oscillate coherently as (e) ↔ (d) in Figure 23, triggered by
the decrease in the dimeric molecular displacements (Figure 23e).
In this mechanism, the time evolution of ∆ROSC (t)/R is expressed by the following formula,
which is the convolution of ETHz (t) and a sine-type damped oscillator.
∆ROSC (t)/R ∝
Z t
−∞
ETHz (τ )e−(t−τ )/τ0 sin[Ω(t − τ )]dτ
(6)
Here, τ0 and Ω are the decay time and frequency of the oscillation, respectively. One may regard this
formula as the forced oscillation of the optical mode driven directly by the terahertz electric field. However,
we can show that the oscillation driven by the electric-field-induced modulation of ρ is also expressed by
the same formula [41]. By using this formula, the time evolution of ∆ROSC (t)/R was reproduced well with
values of τ0 ~ 8.7 ps and Ω ~ 54 cm−1 , as shown by the solid line in Figure 21e. From these results, we
conclude that the coherent oscillation of the degree of charge transfer ρ originates from the modulation of
the spin-Peierls dimerizations due to the rapid change in ρ, which is induced by the terahertz electric field.
It is valuable to quantitatively evaluate the terahertz-field-induced change in the dimeric
molecular displacement. This is possible by assuming that the change of the displacement along
the a axis is proportional to the additional increase in the degree of charge transfer, which is reflected
by the amplitude of ∆ROSC (t)/R. At 40 K, the dimeric molecular displacement l is 0.085 Å, as
mentioned in Section 2.1. Comparing the initial amplitude of the oscillation ∆ROSC /R ∼ 5×10−4
with the normalized reflectivity change between 77 K (ionic phase) and 90 K (neutral phase),
[ RI (77 K) − RN (90 K)]/RN (90 K) ∼ 0.1, the change of the displacement ∆l/l induced by the terahertz
electric field was evaluated to be 5 × 10−3 at 38 kV/cm.
7. Generation of Ferroelectric Polarization by Terahertz Electric-Field Pulse in the Neutral Phase
of TTF-CA
7.1. Rapid Generation of Large Polarization
A large change in a macroscopic polarization in a solid will cause large changes in refractive
index and absorption coefficient. If we generate a large polarization in a paraelectric material in an
impulsive manner, we will be able to realize an ultrafast optical switching via instantaneous changes
in refractive index or absorption coefficient. It has been one of the most important subjects in modern
optical technology for a long time to accomplish such an ultrafast optical switching without real
Crystals 2017, 7, 132
29 of 41
Crystals 2017, 7,Electronic-type
132
29 of 41a control of
carrier excitations.
dielectrics will be good target materials to pursue such
polarization for the achievement of ultrafast optical switching. In this section, we review the recent
optical technology for a long time to accomplish such an ultrafast optical switching without real
study, which
aimed
at theElectronic-type
ultrafast generation
large
polarization
by atoterahertz
electric
field in the N
carrier
excitations.
dielectricsof
will
be good
target materials
pursue such
a control
phase of TTF-CA
[42].for the achievement of ultrafast optical switching. In this section, we review the recent
of polarization
study, which aimed at the ultrafast generation of large polarization by a terahertz electric field in the
N phase of TTF-CA Second-Harmonic-Generation-Probe
[42].
7.2. Terahertz-Pulse-Pump
Measurements in the Neutral Phase
7.2. Terahertz-Pulse-Pump
in the Neutral
Phase
To detect
the polarizationSecond-Harmonic-Generation-Probe
generation, we performedMeasurements
a terahertz-pump
SHG-probe
experiment,
which is schematically
shown
in
Figure
24a.
In
the
N
phase,
TTF-CA
has
inversion
symmetry
To detect the polarization generation, we performed a terahertz-pump SHG-probe experiment, and does
which
shown terahertz-electric-field-induced
in Figure 24a. In the N phase, TTF-CASHG,
has inversion
symmetry
andexperiment
not show SHG.isToschematically
precisely detect
we carried
out the
does
not
show
SHG.
To
precisely
detect
terahertz-electric-field-induced
SHG,
we
carried
out
the
in the following way. First, the sample was cooled down to 65 K and brought to the ferroelectric
I
experiment in the following way. First, the sample was cooled down to 65 K and brought to the
phase. Atferroelectric
65 K, the probe
light
(1.3
eV)
with
E//a
was
incident
to
the
crystal
and
the
intensity
of
the
I phase. At 65 K, the probe light (1.3 eV) with E//a was incident to the crystal and the
SH light (2.6
eV),ofISHG
, was
measured ,in
the
reflection
configuration.
The SHThe
light
was polarized
−65K
intensity
the SH
light
(2.6 eV),
was
measured
in the
reflection configuration.
SH light
parallelthe
to the
a axis. Next, the
temperature
was
to the
90 Ksample
and the sample
was
parallel towas
thepolarized
a axis. Next,
temperature
was
increased
toincreased
90 K and
was returned
to the
theexecuted
N phase. At
90 terahertz-pump
K, we executed the SHG-probe
terahertz-pump
SHG-probe measurements
N phase. returned
At 90 K,towe
the
measurements
with the with
same intensity
the same intensity of the incident probe light as that used at 65 K. In this procedure, we can
of the incident
probe light as that used at 65 K. In this procedure, we can
quantitatively compare the
quantitatively compare the terahertz-field-induced SH intensity ∆
( ) in the N phase with
terahertz-field-induced
SH
intensity
∆I
t
in
the
N
phase
with
I
(
)
SHG−90K
SHG−65K in the I phase.
in the I phase.
Figure 24. Results of terahertz-pump SHG-probe and optical-reflectivity-probe measurements in the
Figure 24. Results of terahertz-pump SHG-probe and optical-reflectivity-probe measurements in the
paraelectric N phase (90 K) of TTF-CA [42]. (a) Schematics of terahertz-pump SHG-probe
paraelectric
N phase (90
K)neutral
of TTF-CA
(a) Schematics
of terahertz-pump
measurement
measurement
in the
phase; [42].
(b) Electric-field
waveform
of a terahertz pulseSHG-probe
(ETHz); (c) Time
evolution
of an(b)
SH intensity
at 90 K by
a terahertz of
electric-field
pulsepulse
shown(E
inTHz
(b); );
(d)(c)
Fourier
in the neutral
phase;
Electric-field
waveform
a terahertz
Timepower
evolution of an
spectrum
electric-field
pulse shownpulse
in (b); shown
(e) The time
evolution
of reflectivity
change
SH intensity
at 90ofKa terahertz
by a terahertz
electric-field
in (b);
(d) Fourier
power
spectrum of a
at 1.3 eV and at 90 K. Shaded area in (c,e) shows a waveform of the square of a terahertz electric field
terahertz electric-field pulse shown in (b); (e) The time evolution of reflectivity change at 1.3 eV and at
(
( )) .
90 K. Shaded area in (c,e) shows a waveform of the square of a terahertz electric field ( ETHz (t))2 .
(0)
When the maximum electric field of the terahertz pulse,
(0), was not large [
(0) to approximately 400 kV/cm, we
40kV/cm], we could not find SHG signals. By increasing
When the maximum electric field of the terahertz pulse, ETHz (0), was not large [ETHz (0) <
40 kV/cm], we could not find SHG signals. By increasing ETHz (0) to approximately 400 kV/cm, we
successfully detected the SHG signal at 90 K, which is shown in Figure 24c. The electric-field waveform
and the Fourier power spectrum of the terahertz pulse are shown in Figure 24b,d, respectively.
The maximum of the electric field, ETHz (0), is 415 kV/cm. The observation of the SHG demonstrates
that the macroscopic polarization ∆P is generated by the terahertz electric field.
Crystals 2017, 7, 132
30 of 41
As shown in Figure 24c, ∆ISHG (t) exhibits a pulsed response at around the time origin and
is almost in agreement with the square of the terahertz electric field ( ETHz (t))2 (the shaded area).
A quadratic dependence of ∆ISHG (0) on ETHz (0) is reasonable, since inversion symmetry exists
in the N phase. The initial SH intensity at 90 K, ∆ISHG−90K (0), is approximately 2.9% of the SH
intensity, ISHG−65K , measured at 65 K. Using the relation, ∆ISHG (0) ∝ (∆P(0))2 , we can deduce that
the electric-field-induced polarization ∆P(0) reaches 17% of the polarization PI = 6.3 µC/cm2 at
65 K in the I phase. The pulsed response of ∆ISHG (t) at around the time origin leads us to expect
that the electric-field-induced charge transfers are responsible for the initial polarization generation.
For td > 0.3 ps, however, the time evolution of ∆ISHG deviates from the waveform of ( ETHz (t))2 and
an oscillatory structure appears, which is discussed in the next subsection.
7.3. Terahertz-Pulse-Pump Optical-Reflectivity-Probe Measurements in the Neutral Phase
To ascertain the charge-transfer mechanism of the pulsed response of ∆ISHG (t) around
the time origin mentioned in the previous subsection, we carried out a terahertz-pulse-pump
optical-reflectivity-probe experiment, focusing on the CT band in the near-IR region. The reflectivity
spectra of the CT band at 77 K in the I phase and at 90 K in the N phase, and the differential reflectivity
spectrum, [ RI (77 K) − RN (90 K)], are shown in Figure 25a,b, respectively. The spectral shape of the CT
band is sensitive to ρ, similar to the IM transition bands of TTF; the reflectivity at 0.6–1.35 eV increases
with increasing ρ. In this experiment, we first set the probe energy at 1.3 eV, which was used in the SHG
probe experiment. Using the same probe photon energy is appropriate to directly compare the time
evolutions of ∆R(t) and ∆ISHG (t). In Figure 24e, we show the time evolution of the reflectivity change
∆R(t)/R at 90 K. The increase in reflectivity indicates that ρ is increased by the terahertz electric field.
The shaded area shows the square of the terahertz electric field, ( ETHz (t))2 . The observed ∆R(t)/R at
around the time origin completely follows ( ETHz (t))2 , as well as ∆ISHG (t). This result indicates that the
electric-field-induced change in ρ occurs with no delay and that the fractional charge transfers [∆ρ(t)]
from A to D molecules are responsible for the polarization generation ∆P(t). Such a mechanism of the
polarization generation by the terahertz electric field is similar to that of the spontaneous polarization
generation across the NI transition. For td > 0.3 ps, the time evolution of ∆R(t)/R deviates from the
waveform of ( ETHz (t))2 and an oscillatory structure appears, similar to ∆ISHG (t).
To clarify the characteristic time evolutions of ∆ISHG (t) and ∆R(t)/R, we investigated the
temperature dependence of ∆R(t)/R, which is shown in Figure 25c. The pulsed response around the
time origin appears in common at all the temperatures from 260 K to 82 K. Its intensity gradually
increases as the temperature approaches Tc . The second peak characterizing the oscillation shows a
more prominent temperature dependence. In Figure 25d, the magnitudes of the first peak [∆R(0)/R]
and the second peak are shown by circles and triangles, respectively. ∆R(0)/R is enhanced with
decreasing temperature. Simultaneously, the second peak in ∆R(t)/R (colored regions) characterizing
the oscillation not only increases, but also shifts to the longer times. These results suggest that the
instantaneous change in ρ by the terahertz electric field is coupled with the specific lattice mode and
the oscillation frequency of the mode shifts to lower frequency as the temperature approaches Tc .
The detailed analyses of the time evolutions of ∆R(t)/R revealed that the oscillation frequency of the
mode decreases from 33 cm−1 at 170 K to 18 cm−1 at 81 K (just above Tc ) and is much smaller than
the dimerization-mode frequency (~54 cm−1 ) related to the NI transition [42]. Such a feature of the
oscillation cannot be explained by a simple critical behavior associated with the dimerization transition.
In fact, it was ascertained from the Raman studies that the frequency of the dimerization mode does
not depend strongly on temperature, which is in contrast to the observed oscillation. The coherent
oscillation due to the dimerization mode (~54 cm−1 ) was indeed observed in our experiments, but
its oscillation amplitude was very small and does not contribute to the ∆R(t)/R signals in the time
domain shown in Figure 25c. The details of those results are reported in Morimoto, T. et al. [42].
Crystals 2017, 7, 132
31 of 41
signals in the time domain shown in Figure 25c. The details of those results are reported in Morimoto,
31 of 41
T. et al. [42].
Crystals 2017, 7, 132
(c)
1.3 eV probe
ETHz (0)
= 415 kV/cm
E // a
0.6
77 K
(Ionic)
0.4
82 K
90 K
90 K
(Neutral)
0.2
100 K
1.5
1
0.1
0.5
103ΔR (0)
R77K – R90K
102 ∆R/R
0
(b) 0.2
120 K
130 K
10
5
0
0
110 K
0
0.6
0.8
1
1.2
1.4
1.6
140 K
150 K
170 K
-1
0
Photon Energy (eV)
(d)
2
10
15
102∆R(t = 0)/R
1
Delay Time (ps)
10
8
lonic
Neutral
6
4
5
0
0
102∆R(t)/R
Reflectivity
(a)
(a) 0.8
2
50
100
150
200
250
0
Temperature (K)
Figure 25. Temperature dependence of transient optical-reflectivity change by a terahertz electric field
Figure 25. Temperature dependence of transient optical-reflectivity change by a terahertz electric field
in the paraelectric N phase of TTF-CA [42]. (a) Polarized reflectivity spectra in the CT transition
in the paraelectric N phase of TTF-CA [42]. (a) Polarized reflectivity spectra in the CT transition region
region at 90 K (the neutral phase) and at 77 K (the ionic phase); (b) Probe energy dependence
at 90 K (the neutral phase) and at 77 K (the ionic phase); (b) Probe energy dependence of terahertzof terahertz-electric-field-induced reflectivity changes at the time origin ∆R(0)/R (yellow circles).
electric-field-induced reflectivity changes at the time origin ∆ (0)/ (yellow circles). The blue line shows
The blue line shows the differential reflectivity spectrum, [RI (77 K) − RN (90 K)]; (c) Time evolutions
the differential reflectivity spectrum, [RI(77 K) − RN(90 K)]; (c) Time evolutions of terahertz-field-induced
of terahertz-field-induced reflectivity changes ∆R(t)/R at various temperatures. Shaded areas show
reflectivity changes ∆ ( )/ at various temperatures. Shaded areas show second peaks; (d)
second peaks; (d) Temperature dependence of reflectivity changes ∆R(t)/R at the first peak (the time
Temperature dependence of reflectivity changes ∆ ( )/ at the first peak (the time origin) (circles)
origin) (circles) and the second peak (triangles).
and the second peak (triangles).
To
anomalous
temperature
dependence
of theofsecond
peak inpeak
∆R(t)in
/R,∆ we
consider
To interpret
interpretthe
the
anomalous
temperature
dependence
the second
( )/
, we
microscopic
1D I domains,
as shown as
in shown
Figure 26a,b.
Previous
and transport
consider microscopic
1D I domains,
in Figure
26a,b. optical,
Previousdielectric,
optical, dielectric,
and
measurements
suggest that
such Ithat
domains
generated
in the even
N phase
because
of the
valence
transport measurements
suggest
such are
I domains
are even
generated
in the
N phase
because
of
instability
(or
the
instability
of
the
NI
transition),
and
fluctuate
in
time
and
space
[96–101].
To
accurately
the valence instability (or the instability of the NI transition), and fluctuate in time and space [96–101].
evaluate
the number
such
microscopic
domains, we
performed
molecular
To accurately
evaluateofthe
number
of such Imicroscopic
I domains,
weinfrared
performed
infraredvibrational
molecular
spectroscopy
very
carefully
and
revealed
that
approximately
20%
of
molecules
are
ionized
and
existare
as
vibrational spectroscopy very carefully and revealed that approximately 20% of molecules
Iionized
domains
atexist
90 K as
(just
above Tcat) [42].
and
I domains
90 K (just above Tc) [42].
Taking
of 1D
microscopic
I domains
into account,
we explain
observed
Taking the
thepresence
presence
of 1D
microscopic
I domains
into account,
we the
explain
the responses
observed
to
the
terahertz
electric
field
in
the
following
way.
We
simply
consider
two
I
domains,
I
+ and I− ,
responses to the terahertz electric field in the following way. We simply consider two I domains,
with
directionsdirections
of dipole-moments,
+µ and −+
µ, respectively
(Figure 26b).
The moment
µ
and opposite
, with opposite
of dipole-moments,
and − , respectively
(Figure
26b). The
is
large, since
I domain
consists
of approximately
10 DA pairs10[30,101].
In[30,101].
the absence
ofabsence
electric
moment
is an
large,
since an
I domain
consists of approximately
DA pairs
In the
field,
+µ and
−µ+cancel.
applying
right-handed
field, a fractional
transfer
in
of electric
field,
and When
− cancel.
Whena applying
a right-handed
field, a charge
fractional
chargeoccurs
transfer
each
DA
(decreasing)
ρI by ∆ρ within
a finite
dipolea
( I− ) domain.
) domain.
occurs
inpair,
eachincreasing
DA pair, increasing
(decreasing)
Although
bythe
∆ I+within
the ( Although
moment
is
generated,
the
changes
in
ρ
in
the
two
I
domains
(
±
∆ρ)
cancel,
which
does
not
agree
with
finite dipole moment is generated, the changes in ρ in the two I domains (±∆ ) cancel, which does
the
that is, the
reflectivity
change
showing
the electric-field-induced
increase in ρ.
not experimental
agree with theresult;
experimental
result;
that is, the
reflectivity
change
showing the electric-field-induced
increase in ρ.
Crystals 2017, 7, 132
Crystals 2017, 7, 132
32 of 41
32 of 41
(a)
・・・ A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 ・・・
・・・ A0 D0 A0 D0 A–D+ A–D+ A–D+ A–D+ A0 D0 A0 D0 A0 D0 A0 ・・・
・・・ A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 ・・・
・・・ A0 D0 A0 D0 A0 D0 A0 D+A– D+A– D+A– D+A– D0 A0 D0 A0 ・・・
・・・ A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 ・・・
(b)
I+ domain
ρI
ρN
A D A D A D
A D
A D
A D
I− domain
ρI
ρN
ρN
A D A D
D A D A D A
D A
D A
ρN
D A
D A D A
(b)
N DA
pairs
Dipole moment +μ
(c)
ρN
ρI + ∆ρ
A D A D A D
A D
ρN
A D
A D
ρN
A D A D
N DA
pairs
Dipole moment −μ
D A D A D A
ρI ∆ρ
D A
D A
ρN
D A
D A D A
(b)
Domain
+∆N
expansion
Charge
transfer
Domain
shrinkage
+∆ρ
∆ρ
ETHz
ETHz
(e)
Charge back
transfer
−μ − ∆μ
+μ + ∆μ
(d)
∆N
A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0
A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0
A0 D0 A–D+ A–D+ A–D+ A–D+ A–D+ A–D+ A0 D0 A0 D0 A0
A0 D0 A0 D0 A0 D0 A–D+ A–D+ A0 D0 A0 D0 A0 D0 A0 D0 A0
A0
A0
A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0
A0 D0 A0 D0 A0 D0 A0 D0 A0 D+A– D+A– D0 A0 D0 A0 D0 A0
A0 D0 A0 D0 A0 D+A– D+A– D+A– D+A– D+A– D+A– D0 A0
A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0
A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0 D0 A0
D0
A0
D0
A0
D0
A0
D0
A0
D0
A0
D0
A0
D0
A0
+μ
−μ
D0
A0
D0
−μ − ∆μ
+μ + ∆μ
P
ETHz
Steady State
Large macroscopic
polarization
by the THz wave irradiation
Figure
Figure 26. Schematics
Schematics of
of observed
observed responses
responses to
to aa terahertz
terahertz electric-field
electric-field pulse
pulse in
in the
the paraelectric
paraelectric
neutral
[42].
(a,b)(a,b)
1D microscopic
ionic ionic
domains
fluctuating
in the neutral
neutral phase
phaseofofTTF-CA
TTF-CA
[42].
1D microscopic
domains
fluctuating
in the region.
neutral
has a dipole
momentmoment
+μ and+µan
I− an
domain
has ahas
dipole
moment
−μ; −
(c)
An
I+ domain
region.
An I+ domain
has a dipole
and
I − domain
a dipole
moment
µ;
Terahertz-electric-field-induced
changes
of
1D
ionic
domains.
A
terahertz
electric
field
with
the
(c) Terahertz-electric-field-induced changes of 1D ionic domains. A terahertz electric field with the
right-hand
an II++ (I −
right-hand direction
direction increases
increases (decreases)
(decreases) the size of an
domain via
via intermolecular
intermolecular charge
charge
−)) domain
transfers
Δρ(−Δρ)
transfers at
at domain
domain boundaries
boundaries (red
(red curved
curved arrows)
arrows) and
and changes
changes ρρ by ∆ρ(
−∆ρ) within
within the
the ionic
ionic
domain
domain (yellow
(yellow curved
curved arrows);
arrows); (d)
(d) Coherent
Coherent oscillations
oscillations of
of NIDW
NIDW pairs.
pairs. (e)
(e) A
A large
large macroscopic
macroscopic
polarization produced
produced by microscopic changes of 1D ionic domains.
polarization
The
can be
be explained
explained in
in terms
The observed
observedfield-induced
field-inducedincrease
increaseininthe
thevalues
valuesofof ρ and
and µ can
terms of
of the
the
motions
of
microscopic
domain
walls
between
the
N
and
I
states,
which
are
called
neutral–ionic
motions of microscopic domain walls between the N and I states, which are called neutral–ionic
domain
domain walls
walls (NIDWs)
(NIDWs) [71].
[71]. When
When the
the N
N and
and II states
statesare
are almost
almost degenerate,
degenerate, an
an II domain
domain is
is regarded
regarded
as
as an
an excitation
excitation of
of an
an NIDW
NIDW pair.
pair. Just
Just above
above Tc,, TTF-CA
TTF-CA shows
shows anomalous
anomalous enhancements
enhancements of
of the
the
dielectric
response
and
nonlinear
electronic
current,
which
are
interpreted
as
being
due
to
motions
dielectric response and nonlinear electronic current, which are interpreted as being due to motions of
of
NIDWs
[96,98].
If Ithe
domain,
polarized
parallel
(0), expands
the
domain,
NIDWs
[96,98].
If the
polarized
parallel
to ETHzto(0), expands
and the and
I− domain,
polarized
+ domain,
polarized
antiparallel
(0),
shrinksinasFigure
shown26c,d,
in Figure
26c,d,
the of
total
values
of
and
antiparallel
to ETHz (0)to
, shrinks
as shown
the total
values
ρ and
µ increase.
Such
increase.
Such
microscopic
changes
in
in
a
number
of
I
domains
generate
the
large
macroscopic
microscopic changes in µ in a number of I domains generate the large macroscopic polarization—the
polarization—the right panel of Figure 26e. Indeed, the ∆
( ) and ∆ ( )/ responses around the
time origin accurately follow the changes in (
( )) on the subpicosecond time scale, as shown by
Crystals 2017, 7, 132
33 of 41
right panel of Figure 26e. Indeed, the ∆ISHG (t) and ∆R(t)/R responses around the time origin
accurately follow the changes in ( ETHz (t))2 on the subpicosecond time scale, as shown by the shaded
areas in Figure 24c,e, suggesting that the initial NIDW motions are purely electronic processes brought
about by intermolecular charge transfers.
Based on the qualitative interpretations of the results presented above, we can derive a formula
for ∆R(t)/R. When the electric field is applied, the number of DA pairs included in the I domain,
n (the domain size), changes as n → n + ∆n (or n → n − ∆n ) and ρ changes as ρ → ρI + ∆ρ (or
ρ → ρI − ∆ρ ) in the I+ (or I− ) domain (Figure 26b,c). Assuming that ∆R is proportional to the change
in the total value of ρ within I domains, we obtain the following relation.
∆R ∝
1
[(n + ∆n)(ρI + ∆ρ) + (n − ∆n)(ρI − ∆ρ) − 2ρn] = ∆ρ∆n
2
(7)
The enhancement of ∆R(0)/R as the temperature approaches Tc (Figure 25c,d) can be attributed
to the increase in the number of 1D I domains and/or to the increase in ∆n originating from the
enhancement of the valence instability. Both ∆ρ(0) and ∆n(0) are proportional to ETHz (0), resulting in
∆R(t)/R ∝ ( ETHz (t))2 around the time origin, as observed in the experiments.
The subsequent oscillatory responses observed in ∆ISHG (t) and ∆R(t)/R can be attributed to
the breathing oscillation of NIDW pairs as shown in Figure 26d [102]. After the initial rapid NIDW
motions by charge-transfer processes, the altered I domains are stabilized by molecular displacements
within a few picoseconds. In the photoinduced N-to-I transition in TTF-CA, such transient molecular
displacements are observed, as discussed in Section 4. When the terahertz electric field disappears,
the I domain returns to the original state; the expanded I domain shrinks and the shrunken I
domain expands, giving rise to the coherent breathing oscillation of the NIDW pair. The molecular
displacements slow down the dynamics of the NIDW pairs and decrease the breathing oscillation
frequency into the terahertz region.
7.4. Dynamical Aspects of Electric-Field-Induced Polarization Generation
Based upon the qualitative interpretation discussed in the previous subsection, we constructed a
phenomenological model of the NIDW dynamics and carried out quantitative analyses of the time
evolutions of ∆ISHG (t) and ∆R(t)/R, which were reported in detail [42]. According to these analyses,
the complicated time characteristics of ∆ISHG (t) and ∆R(t)/R can be almost reproduced. In this
subsection, we summarize the main results of these analyses and discuss the important physical
aspects of the electric-field-induced polarization generation.
The main results of the analyses of ∆ISHG (t) and ∆R(t)/R are as follows.
(1)
(2)
(3)
The large polarization generation by the terahertz electric field around the time origin originates
from the expansions and shrinkages of microscopic I domains via the charge transfer processes
shown by the red arrows in Figure 26c, as well as the intermolecular fractional charge transfers
within the original I domains shown by the yellow arrows in the same figure. These charge
transfer processes are electronic and the responses in ∆ISHG (t) and ∆R(t)/R around the time
origin are very fast.
We estimated the change in the degree of charge transfer ∆ρ within the DA molecules in an I
domain and the change in the size of a microscopic I domain ∆n/n by the terahertz electric field
to be ∆ρ ∼ 0.085 ( ∆ρ/ρN ∼ 0.27) and ∆n/n ∼ 0.43, respectively, at ETHz (0) = 415 kV/cm and
90 K. That is, the terahertz electric-field pulse increases ρ by ~27% and the I-domain size by ~43%.
This causes the formation of the large macroscopic polarization reaching 17% of the ferroelectric
polarization in the ferroelectric I phase.
The expanded or shrunken I domains are stabilized by molecular displacements and also
probably by molecular deformations. In their recovery processes after the electric field disappears,
the coherent breathing oscillations of NIDW pairs occur and their frequency is in the terahertz
Crystals 2017, 7, 132
34 of 41
region. The frequency of the oscillation changes from 32 cm−1 at 170 K to 19 cm−1 at 82 K. Such
a softening can be explained by the fact that the energy difference between the N and I states
decreases as the temperature approaches Tc .
In the N phase of TTF-CA, Far-IR vibrational spectroscopy was performed [103–107]. According
to that, several lattice modes are observed below 100 cm−1 and show softening as the temperature
approaches Tc . Characteristic time evolution of ∆R(t)/R induced by a terahertz electric-field pulse and
its temperature dependence could not be explained by simple IR-active lattice mode [42]. Assuming
NIDW motions as well as instantaneous charge transfers induced by a terahertz electric field, we
succeeded in effectively reproducing the time evolutions not only of ∆R(t)/R but also ∆ISHG (t).
We would like to emphasize two important aspects of the observed phenomena. One concerns
the viewpoint of optical nonlinearity. The observed ultrafast response around the time origin can
be considered as a kind of third-order optical nonlinearity. However, the observed phenomenon is
considerably different from conventional third-order optical nonlinearity characterized by coherent
electronic processes, since the former includes intermolecular charge transfers. As a result, a large
response to the electric field occurs. Although the dynamics include such real charge-transfer processes,
the response occurs very fast. Thus, we expect that a mechanism of this phenomenon, that is, the
ultrafast valence change and polarization generation by the terahertz electric field associated with the
instability of the NI transition will be applicable to ultrafast optical switching in the future. The other
concerns the dynamics of NIDWs. We succeeded in the real-time detection of the breathing motions
of NIDW pairs for the first time. This is crucial, since the NIDW dynamics dominates dielectric and
transport properties in the donor–acceptor-type molecular compounds. Real-time detection of such
microscopic dynamics of the NIDWs is difficult to achieve by other methods.
8. Summary
In this paper, we review the photoinduced and electric-field-induced electronic state changes
in a mixed-stack organic molecular compound, TTF-CA, investigated by femtosecond-pump–probe
reflection spectroscopy and SHG measurements. TTF-CA shows an NI transition at 81 K, which
originates from the long-range Coulomb attractive interaction. This mechanism gives a collective
nature of both ionic and neutral states and gives rise to a variety of exotic photoinduced and
electric-field-induced phenomena.
In Section 2, we report the nature of electronic ferroelectricity in the I phase and the imaging
of ferroelectric domains using electro-reflectance spectroscopy. In Section 3, we introduce the
experimental methods of transient optical reflectivity and SHG spectroscopy.
In Sections 4 and 5, we review the photoinduced N-to-I and I-to-N transitions, respectively.
In the N-to-I transition induced by the resonant excitation of the CT band, it was demonstrated that
microscopic 1D I domains consisting of approximately 10 DA pairs are initially produced within
~20 fs via purely electronic processes, and subsequently, several kinds of coherent oscillations with
subpicosecond periods are observed in the photoinduced reflectivity changes. They are reasonably
assigned to the dynamical dimeric displacements of molecules associated with the spin-Peierls
instability and the intramolecular deformations. We would like to emphasize that several molecular
degrees of freedom cooperatively stabilize the photogenerated microscopic I domains and that their
detailed dynamics can be observed by using pump–probe spectroscopy with high time resolution
(~20 fs). These I domains decay within several picoseconds and the three-dimensionally ordered I
state is never formed. In the I-to-N transition induced by the resonant excitation of the CT band,
the microscopic 1D N domains are also initially produced within ~20 fs via purely electronic processes,
which are stabilized by the release of dimeric molecular displacements and molecular deformations.
They are converted to a semi-macroscopic N state within 20 ps through their cooperative multiplication,
which has a very long lifetime. The difference between the N-to-I and I-to-N transitions can be
explained by the fact that the I-to-N transition is a destruction of the three-dimensional ferroelectric
order by photoirradiation, which can occur as a domino phenomenon, while the N-to-I transition is
Crystals 2017, 7, 132
35 of 41
a reverse process; that is, it is a formation process of a 3D ordered state, which is difficult to induce
by photoirradiation.
In Section 6, we review the control of ferroelectric polarization by a terahertz electric field in
the I phase. The observed polarization modulation originates from the modulation of the degree of
charge transfer ρ within each DA dimer, and is characteristic of electronic ferroelectrics. The increase
in ρ increases a spin moment on each molecule, which enhances the spin-Peierls instability and the
dimeric molecular displacements. Such an enhancement of dimeric molecular displacements gives
rise to the additional increases in ρ by the increase in the Coulomb attractive interaction within
each dimer through the electron–lattice interaction. Thus, by using a terahertz electric-field pulse,
we can not only achieve ultrafast polarization modulation, but also distinguish the effects of the
spin–lattice and electron–lattice interactions involved in materials. Such a polarization modulation in
electronic ferroelectrics has also been successfully performed in a 2D ET-based molecular compound,
α-(ET)2 I3 [58].
In Section 7, we review the large polarization generation in the paraelectric N phase by a strong
terahertz pulse. The magnitude of the electric-field-induced polarization reaches 17% of the ferroelectric
polarization in the I phase, which can be regarded as macroscopic polarization generation. We also
succeeded in the real-time observations of the motions of neutral–ionic domain walls (NIDWs) for
the first time. This is crucial because the NIDWs dominate the anomalous dielectric responses and
nonlinear transport near the NI transition, as well as photoresponses in TTF-CA. The analyses of the
NIDW dynamics are not discussed in detail in this review article, but have been reported in our recent
paper [42].
Finally, we would like to comment on a couple of future subjects. One concerns the formation
dynamics of a 1D I domain or a 1D N domain. An initial photoexcited state is a Bloch state spread
over a crystal. It is not correct to express the initial photoexcited state as a D+ A− pair in the N phase or
D0 A0 pair in the I phase, as shown in Figure 12(aI,bI), respectively. The formation of such a D+ A− pair
or D0 A0 pair is realistic when the initial photoexcited state is localized by some relaxation processes.
Accordingly, it is an important subject, from both theoretical and experimental viewpoints, to clarify
the formation process of a 1D I domain or a 1D N domain from the initial photoexcited state spread
over a crystal.
Another subject concerns the formation of a more ordered state from a less ordered state, or
equivalently, the formation of a lower-symmetry state from a higher-symmetry state by light. In
most of the previously reported photoinduced phase transitions, a lower-symmetry phase is broken
or melted by light. It is usually difficult to form a lower-symmetry state by light. This fact is also
shown in the photoinduced N-to-I transition discussed in Section 4. To overcome this problem, it is
necessary to use a strong cooperative interaction in a material, which can grow a lower-symmetry
state from a photoexcited state and stabilize that order. As such an attempt, we recently investigated
the photo-responses in 4,40 -dimethyltetrathiafulvalene(DMTTF)-2,6-dichlorodbromo-p-benzoquinone
(2,6QBr2 Cl2 ) [35], which is also a mixed-stack organic molecular compound in the N phase and shows
quantum paraelectricity at low temperatures. When this compound is irradiated with a femtosecond
laser pulse, microscopic I domains are also generated, as in TTF-CA. We found that a large coherent
oscillation related to electronic ferroelectricity is generated over a wide region in a crystal, although
a macroscopic ferroelectric order is not formed. We consider that each I domain with a large dipole
moment restores the ferroelectric nature suppressed by quantum lattice fluctuations [35]. If we could
align the dipole moments of the initially photogenerated microscopic 1D I domains, a wide region of a
crystal might be converted to a transient ferroelectric state. In the studies using a terahertz pulse as a
pump pulse to control electronic states, the enhancement of an electric-field amplitude might realize
an ultrafast polarization reversal in the ferroelectric phase and an ultrafast paraelectric-to-ferroelectric
transition using collective charge transfer processes.
Acknowledgments: The studies presented in this review have been done in collaboration with a number of
researchers and graduate students in H. Okamoto’s laboratory in the University of Tokyo. We would like to
Crystals 2017, 7, 132
36 of 41
thank H. Uemura, H. Yamakawa, S. Tanaka, Y. Ishige, T. Hamamoto, K. Kimura, H. Takamatsu, K. Fujinuma,
T. Terashige, H. Yada, N. Kida, (Univ. of Tokyo), H. Kishida (Nagoya Univ.), S. Iwai (Tohoku Univ.), Y. Tokura
(RIKEN), H. Matsuzaki, and S. Horiuchi (AIST). We also thank S. Brazovski and N. Kirova (CNRS Orsey) for
enlightening discussions. This work was partly supported by Grants-in-Aid for Scientific Research from the Japan
Society for the Promotion of Science (JSPS) (Project Numbers 20110005, 25247049, and 15H06130) and CREST,
Japan Science and Technology Agency. Takeshi Morimoto was supported by the JSPS through the Program for
Leading Graduate School (MERIT).
Author Contributions: All of the authors contributed to writing this review paper.
Conflicts of Interest: The authors declare no conflict of interest.
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