Download ultrafast - University of Surrey

Overview of new
ultrafast research initiative
within ODM Research Group
University of Surrey
Jeremy Allam
School of Physics and
Chemistry
Optoelectronic Devices
and
Materials Research Group
Guildford, Surrey
GU2 7XH, UK
Tel +44 (0)1483 876799
Fax +44 (0)1483 876781
ODM
ultrafast
optical communications
Terabit/second optical
networks are here!
•
•
SOA-based optical switches
for all-optical network
limited above ~100Gbit/s by
intraband carrier dynamics
ODM
ultrafast
ultrafast revolution
ultrafast
opto-electronics
microwave
photonics
electrooptic
sampling
THz
device
physics
NL pulse
propagation
nonlinear
optics
highharmonic
ultrashort (<10fs)
generation
biological /
solid-state
intense (UV, X-ray)
tunable femtosecond
environ(>1TW)
(UV-MIR)
mental
lasers
relativistic
sensing
optical
electron
coherent
spectromotion
scopy
non-stochastic
coherent
breakdown
control controllable
material
ablation
photoprocessing
chemistry
medical
applications
free-space
THz
highenergy
physics
ODM
ultrafast aspects of research at Surrey
(1) bandstructure engineering of material / device dynamics
• theory (beyond effective mass)
• design (optimise device dynamics)
• diagnostics (high-pressure experiments)
(2) advanced experimental and theoretical methods
• femtosecond lasers, OPOs, OPA; FELIX
• broadly-tunable, ultrashort, high-intensity light pulses
• comprehensive, first-principles theoretical models
(3) femtosecond physics in advanced real-world devices
(4) convergence of optics and electronics
• interband
• optical transitions
• high-speed photonics
• ultrafast optics
AND
AND
AND
AND
intraband dynamics
electron transport
microwave electronics
mid/far-infrared spectroscopy
(5) time + frequency domain characterisation
• (ω,τ) optical methods (e.g. FROG) for amplitude, phase dynamics
• electro-optic sampling for THz device / circuit characterisation
• optical pulse shaping for all-optical bit-error-rate measurement?
ODM
ultrafast
research activities
(1) Mid-infrared time-resolved experiments - FELIX
Dr. Ben Murdin
(2) Theory of ultrafast interactions in semiconductors
Dr. Steve Hughes
(3) Ultrafast optical / electronic devices
Prof. Jeremy Allam
1996
1997
1998
1999
2000
2001
2002
1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10 1 4 7 10
BNM
SH
JA
fs experiments
ATI
start
occupation
ODM
ultrafast
research activities 1
(1) Experimental studies of intersub-band transitions
and mid-infrared devices - Dr. Ben Murdin
(i) Mid-infrared lasers
(ii) Time Resolved spectroscopy with FELIX
(iii) New infrared materials: InSbN
ODM
ultrafast

Electrically pumped
semiconductor laser





Conventional interband device
Small, low cost, rugged
Pollution monitoring,
process control etc
applications
Mechanisms preventing
room temperature
operation


Mid-infrared lasers
Elastic collisions between
charges (Auger effect)
valence band absorption
phonon emission (=heat!)
Quantum Cascade device
C
O
Solution = band-structure
engineering


quantisation
strain
ODM
ultrafast Spectroscopy with FELIX
Free Electron Laser for Infrared eXperiments, Utrecht, NL




Tuneable from 4 -250mm!!
1MW peak power!!
Pulses only 6 optical cycles!!!
Example experiments:
 Pure and applied physics
 scattering/recombination
times between charges on
femtosecond scale
 searching for excited states
in new materials like
quantum dots, polymers,
buckyballs
 new fundamental regimes of
ultra-short times and ultrahigh a.c. electric fields
ODM
ultrafast New infrared materials: InSbN

Adding dilute N to III-V semiconductors gives strong
bandgap bowing (energy decreases)
long l emission from wide-gap constituent materials
electron effective mass increases, suppressing Auger


pump-probe using FELIX
shows lifetime in an InSbN
sample with 11mm gap () is
much longer than for
HgCdTe of same gap and
same excitation (—)
[only slightly faster than
InSb (···) and HgCdTe with
7mm gap (- -)]
Normally Auger increases
exponentially with reducing
gap
1.00
D T/T (arb. units)

0.37
290K
0
200
400
probe delay (ps)
600
ODM
ultrafast
research activities 2
(2) Fundamental theory of ultrafast electron-photon
interactions in semiconductors - Dr. Steve Hughes
•
many-body quantum theory of semiconductor optics:
• Rabi flopping, excitonic trapping
• inverted semiconductors
• pulse reshaping in SOAs
• modulation of lasers and SOAs through THz field
•
Few-cycle optical pulse propagation
• beyond slowly-varying envelope approximation
•
Extremely-excited states
• fs optical pulse, THz field, magnetic field
• dynamic Franz-Keldysh effect
• magneto-excitons - dancing with wavepackets
ODM
ultrafast
theory of inverted
semiconductors
previous: • Rate Equation Model (REM)
• phenomenological NL gain and saturation*
• adiabatic light-matter interaction
*via: two-photon absorption (TPA), free-carrer absorption (FCA),
spectral hole burning (SHB), carrier heating (CH)
but experiments on SOAs show:
• coherent effects (phase storage)
• non-transferable NL parameters
so use a first-principles, microscopic approach:
• semiconductor Maxwell-Bloch equations
• diagonal and non-diagonal dephasing
• many-body carrier-carrier interactions
state-of-the-art description of semiconductor gain
future improvements:
• band-structure effects
• non-Markovian dynamics
ODM
Extremely excited ultrafast wavepackets
• newly available sources (Terawatt lasers, ultrashort pulse lasers,
free-electron lasers...) allow extreme excitations
• theoretical treatment requires non-perturbative, many-body
quantum approach...... and reveals new phenomena
optical
excitation
+
THz
field
+
magnetic
field
dancing
wavepackets
ODM
ultrafast
research activities 3
(3) Ultrafast measurements of optical and electronic
devices - Prof. Jeremy Allam
•
optoelectronic devices: lasers and SOAs
• fs pulse propagation in semiconductor LD - ‘solitonic dark pulses’
• key questions:
• How to modulate lasers faster?
• How to increase bandwidth of all-optical switch?
•
ultrafast photodetectors:
• ultrafast photoconductors
• TW-WG PD/PT (with Prof.’s Robertson and Weiss)
• dynamics of impact ionisation (with Sheffield)
•
mid-infrared dynamics
• dynamics dominate CW performance...
• compare interband, type II and intraband (QC) devices
•
ultrafast electronics
• THz electro-optic measurements of devices and circuits
• new concepts for THz electronics
ODM
ultrafast
current
modulation of lasers
electro-absorption
mid-infrared
(Gorfinkel ‘92)
optical pulse
(Elsaesser ‘97)
THz pulse
(Hughes ‘98)
intraband
processes
ODM
ultrafast
optical switch dynamics
desired response
Modulator
Df
MZ
p
t
SMZ
actual response
bandstructure
engineering
of dynamics?
ODM
Ultrafast laser experiments
mid-IR
near-IR
visible
ODM