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LASER INDUCED REFRACTIVITY, LIMITING, SWITCHING
OF LASER BEAM AND LASER STRENGTH IMPROVEMENT
OF MATERIALS WITH NANOOBJECTS
Natalia V. Kamanina, Petr Ya. Vasilyev, Vladislav I. Studenov
Vavilov State Optical Institute, 12, Birzhevaya Line, St.-Petersburg, Russia
Abstract. Photophysics properties of organic conjugated systems doped with
nanoobjects as well as the laser and mechanical parameters of the inorganic
compounds covered with nanoobjects and treated with surface electromagnetic waves
are now placed under detailed consideration. As effective nanosensitisers, the
fullerene, nanotubes, nanoparticles, etc. can be considered. The promising nonlinear
optical, photoconductive, and laser-induced dynamic properties can be activated, as
well as the increase in the polarizability can be found under conditions of nanoobjects
sensitization. Some improvement of the laser and mechanical characteristics can be
revealed.
INTRODUCTION
It is well known that the systems with fullerenes and nanotubes have been used with
good advantage in a different area of optoelectronics [1-5]. Fullerene- and nanotubesdoped organic structures have been applied in development of passive and active
optical limiters, laser high-speed switchers, converters, spatial light modulators, new
display elements, etc. High laser and mechanical strength can be revealed when
inorganic matrixes covered with fullerene- or nanotubes have been treated.
In this paper the laser-induced change in the refractive index in fullerene- and
nanotubes-doped conjugated systems due to high frequency Kerr effect has been
considered, the optical limiting of the laser beam has been discussed. As an
additional, the dynamic and laser strength properties improvement has been shown
under nanoobjects (such as nanotubes) covering.
THEORETICAL BACKGROUND AND EXPERIMENTAL CONDITIONS
Let us to consider the Kerr effect in the fullerene- or nanotubes-doped organic
systems, where the lowest nontrivial nonlinearity is the cubic one. The matter
equation of that medium can be the same considered in [6]:
P = 1)E + (3)E3,
(1)
where P is nonlinear polarization of the systems, E is field intensity of the light beam,
1) and (3) is linear and nonlinear optical susceptibilities, respectively. In that
approximation, the refractive index n is defined by the following equation:
D = E +4P = E = n2E,
(2)
that yields
n  1  4P / E .
(3)
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With Eq. (1) and neglecting the nonlinear term, one can obtain:
n  n0 
2 (3) 2
 E ,
n0
(4)
where
n0  1  4π (1) ,
(5)
The light intensity is I = cE2/8. Therefore,
n  n0  n2 I ,
where
(6)
16  2 (3)
n2 
 .
n0 c
(7)
n0 is the linear refractive index and c is the light velocity.
It follows from Eq. (6) that the refractive index depends on the light intensity in the
media with the cubic nonlinearity. This effect causes self-interaction of the light
waves; resulting in self-focusing of a light beam, phase self-modulation of pulses, etc.
n2 is an adequate characteristic of the cubic nonlinearity, for example, the fullereneor nanotubes-doped structures. The mechanism of an anisotropic molecule turn can
result in the Eq. (6) nonlinearity under the effect of the intense polarized light wave.
The process is quite slow in comparison to the electron polarizability of the medium.
Because this mechanism provides the birefringence induced by the dc field (the Kerr
effect), the dependence of the refractive index on the light intensity is the highfrequency Kerr effect, and the Eq. (6) nonlinearity is the Kerr nonlinearity.
The results presented in Table 1 consider the laser-induced change in the refractive
index in the nanoobjects-doped polyimide materials; the data shown in Table 1
provoke the estimation of the cubic optical susceptibility of fullerene- and nanotubesdoped systems with good advantage. These organic -conjugated systems have been
considered as the materials with higher cubic optical susceptibility. Moreover,
nanoobjects-doped 2-cyclooctylamine-5-nitropyridine (COANP), polymer-dispersed
liquid crystals (PDLCs) based on them, etc. should be taken in to account too. It
should be noticed that when these structures have been doped with fullerenes or
nanotubes the efficient charge transfer complex (СTС) formation can be obtained.
The spectral, mass-spectrometry, photoconductive and quantum chemical simulation
evidences of CTC process have been shown in the papers [7-9]. It should be
mentioned that the 2–4 µm thick films of the polyimide or COANP solution in
tetrachloroethane deposited on glass substrates were investigated. 10 µm thick films
of PDLCs were treated. Fullerene C60 and C70 as well as carbon nanotubes with
concentration of 0.1-5 wt.% was used in order to sensitized the photosensitive
molecules. A holographic grating was recorded by the second harmonic ( = 532 nm)
of a pulsed Nd:YAG–laser with the pulsewidth of 10 ns. Two beams applied to
recording the sinusoidal diffraction grating formed the spot in 5 mm diameter on the
film surface. The write energy density was 0.01–3.5 J cm–2. The spatial frequency was
90-100 mm–1. The films were investigated in self-diffraction mode under Raman-Nath
diffraction conditions. The experimental set-up was the same shown in paper [10]. As
an additional, the holographic experiment at wavelength of 1315 nm has been made to
support the effect.
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Table 1. Laser-induced change in the refractive index in the systems based on
polyimide.
Structures
Pure polyimide
Polyimide+
malachite green dye
Polyimide+C60
Polyimide+C60
Polyimide+C70
Polyimide+C70
Polyimide+nanotubes
Polyimide+C70
Nanoobjects Wavelength, Energy Laser
contents,
nm
density, pulse
wt.%
width,
Jсm-2
ns
0
532
0.6
10-20
0.2
532
0.6
20
Change in
the
refractive
index, n
10-4-10-5
2.8710-4
0.2
0.5
0.2
0.5
0.1
0.1-0.5
4.210-3
4.4710-3
4.6810-3
4.8710-3
5.710-3
10-3
532
532
532
532
532
1315
0.5-0.6
0.5-0.6
0.6
0.6
0.5-0.8
0.2-0.8
10-20
10-20
10-20
10-20
10-20
50
The light-induced refractive index change ni in the thin fullerene- or nanotubesdoped films could be estimated from the experimental data of increase in diffraction
efficiency using the Eq. from [11]:
  I1 / I 0   ni d / 2 2 ,
(8)
where  is the diffraction efficiency, I1 is the intensity of the first diffraction order, I0
is the incident laser beam, d is the film thickness, and  is the laser wavelength.
It should be noticed that the thermal part of delta n in the materials studied is close to
value of 10-5. Thus, the increase in the diffraction efficiency in the current
experiments and hence in the light-induced refractive index change could be
explained by the photorefractive effect stimulated by CTC processes in these
compounds. The electron affinity of fullerenes is ~ 2.65 eV, it is twice as larger as
that of an intramolecular acceptor fragment of polyimide, and it is forth times larger
than that for COANP. Therefore, fullerenes are stronger sensitizers and they dominate
the acceptor fragments of intramolecular complexes. As a result, complexes between
fullerenes and donor fragments are formed enhancing phototransfer of charge in these
systems. The path of the charge transfer changes from the intramolecular donor
fragment of polyimide or COANP not to its acceptor fragment but to fullerene. In this
case the field gradient is formed, that causes the photorefractive effect in these
structures under the laser irradiation.
It should be noticed that the changes of the photorefractive properties correlated with
a long-wave shift of the absorption spectrum and with the occurrence of an additional
absorption band in the near IR range. Moreover, structural changes of the system were
observed. They were associated, for example, for polyimide with a transition of the
polyimide donor fragment from its neutral tetragonal form to the ionized planar one
under the laser irradiation. As a result, electron shells of polyimide and fullerene
overlapped, this effect was conducive to the complex formation between the donor
fragment and fullerene.
In the fullerene-containing polyimide film with different fullerene content, ni
changed from 4.210–3 to 4.8710–3 for nanosecond pulsewidth range. In this case,
the incident laser energy density increased from 0.03 up to 0.5-0.6 J cm–2. It should be
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noticed that ni was not so drastically changed for non-sensitized polyimide.
Moreover, the light-induced refractive index change was more significant on
introducing fullerenes than dyes into the conjugated systems. The same situation has
been observed for COANP matrix. For example, the introduction of 7,7,8,8–
tetracyanoquinodimethane into COANP caused ni of 10–5–10–6 [12] under the Kr+–
laser irradiation with  = 676 nm. These results were less than those for fullerene
sensitization of COANP, namely, ni changed from 3.1610–4 to 6.8910–3 as the
incident laser energy density increased from 0.03 to 0.9 J cm–2.
The spectral shift in the pyridine compounds was also more with the fullerene
introduction [13]. It was reasonable that the light-induced refractive index change
influenced the nonlinear absorption as a whole. The general tendency implied that the
introduction of only 0.2 wt.% of C60 into photorefractive polymers increased their
diffraction efficiency 10 and more times.
The light-induced refractive index change established proposes larger nonlinear
refractive index n2 and larger third order nonlinear optical susceptibility (3).
Determine these values from Eqs. (9) and (10):
n
n2  i ,
(9)
I
nnc
 (3)  2 02 .
(10)
16
For example, for the polyimide film with 0.2 wt.% of C70, and delta n of 4.6810-3
(see Table 1) the nonlinear refraction n2 is 0.7810–10 cm2 W–1 and (3) is 2.6410–
9
esu at the incident laser energy density of 0.6 J cm–2. For the COANP film with 5
wt% of C70, n2 and χ(3) are 0.77×10−10 cm2W−1 and 2.4×10−9 esu, respectively, at the
incident energy density of 0.9 J cm−2 and ni = 6.89×10−3.Thus, the nonlinear
refraction n2 and third order susceptibility (3) for conjugated organic strictures doped
with nanoobjects (estimated from holographic recording data) could be respectively:
~10-7 cm2kW-1 and ~10-9 esu for thin films of the fullerene- or nanotubes-doped
organic structures; ~10-6 cm2kW-1 and ~10-8 esu for the fullerene- or nanotubes-doped
polymer-dispersed liquid crystals. The value of third order susceptibility (3)
estimated above is in good coinciding with that patented for nanotubes in paper [14];
the authors of this patent testified that (3)=8.510-8 esu for nanotubes systems.
Moreover, it should be noticed that these nonlinear optical parameters are close to
those for silicon (10−10 cm2 W−1 and 10−8 esu, respectively). Thus, for nonlinear
optical aims the inorganic structures can be replaced with organic ones with good
advantage. The results obtained suggest broad potentialities for application of
fullerene- or nanotubes-doped materials not only to hologram recording, but to optical
limiting too. The basic results of the optical limiting properties of the fullerene- and
nanotubes-doped materials for the visible and IR spectral range have been shown in
paper [15]. The energy loses due to diffraction on the reversible photorefractive
grating have been considered as an additional mechanism.
Let us to consider the pronouns organic conjugated structures in display technique to
increase the speed of liquid crystal (LC) electrooptical switchers and display
elements. Really, due to drastic increase in (3), thus in local volume polarizability
[16], the LC element with fullerene- or nanotubes СTС reveals better switching
parameters than the one without nanoobjects. Some switching characteristics of LC
mesophase with fullerene-doped СTС and main mechanism explained the
accelerating effect have been shown in paper [16]. It should be mentioned that for
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typical nematic liquid crystals, such as 5CB or TN LC, the time-on of the electrooptic
response fell in the range of 8-16 ms. After the self-arrangement under condition of
СTC doping in the fullerene-doped structure, the time-on of the electrooptic response
can be less than the 0.5 ms that is by one order of magnitude shorter. Moreover,
namely nanoobjects-doped LC mesophase placed between conducting indium-tin
oxide (ITO) layers treated with surface electromagnetic wave (SEW) reveals good
high speed operation when direct alignment layers can be absent. In this case higher
transparency of LC display element owing to absence of additional alignment layers
can be found. The SEW source was a quasi-CW gap CO2 laser generating p-polarized
radiation with a wavelength of 10.6 micrometers and a power of 30 W. The skin layer
thickness for this radiation was ~0.05 micrometers. Due to larger number of CC
bond in the nanoobjects we have observed the high laser strength of the ITO contacts
with nanotubes and studied the laser strength of the ITO layers with and without
nanoobjects placing. The results are shown in the Table 3.
Table 3. Improvement of the laser strength properties of ITO contacts.
Type of the layers
Pure ITO
ITO with SEW
ITO covered with
nanotubes
ITO covered with
nanotubes than
treated with SEW
Laser energy
density,
Jcm-2
0.35-0.5
1.025-1.05
0.4-0.7
Laser energy
density destroyed
layer, Jcm-2
0.65-0.67
1.25
0.75
Number of pulse
before destroy
0.94-1.25
1.5-1.56
10 at 1.5 Jcm-2
10 at 0.66 Jcm-2
7-10 at 1.25 Jcm-2
10 at 0.75 Jcm-2
Analyzing the Table 3 results, one can see that the laser strength of the ITO covered
with nanotubes and than treated with SEW revealed the best laser strength. It should
be mentioned, that this treatment has been made when glass or quartz substrates have
been used. In this case at the level of energy density close to 1.5 Jcm-2 the output
signal has been changed not more than on 10%.
Following the promising test of the ITO contacts, we have considered the wide groups
of UV and IR range materials (for example, LiF, CaF2, MgF2, BaF2, ZnSe, etc.) as the
perspective candidates in order to increase the mechanical and laser strength of the
matrixes saving the transmittance spectra. The surface mechanical strength of these
materials has been increased by the factor of 5-10 depended on the quality of the
substrate. The mechanism of this improvement is now under discussion. Moreover,
the improvement in transmittance spectra in the UV and IR spectra range has been
found. Figures 1 and 2 demonstrate these results.
It should be noticed that in the IR spectra region these facts can be explained due to
little value of the imaginary part of the dielectric constant (responsible for the
absorption) of the nanotubes in the IR range.
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94
92
2
Transmition, %
90
88
86
84
82
BaF2
1 - without treatment (d=1.86 mm)
2 - with treatment (d=1.86 mm)
1
80
78
76
200
250
300
350
400
450
Lambda, nm
Fig. 1. The UV spectra of the BaF2 substrate treated with nanotubes.
2
95
Transmission, %
1
90
85
80
1 - BaF2 pure (without nanotreatment)
2 - BaF2 with nanotreatment
75
70
2
4
6
8
10
12
Lambda, micrometers
Fig. 2. The IR spectra of the BaF2 substrate treated with nanotubes.
CONCLUSION
Laser-induced processes of the fullerene- and nanotubes-doped conjugated structures
based on polyimide, 2-cyclooctylamino-5-nitropyridine, and polymer-dispersed liquid
crystal systems have been studied to apply these materials as efficient nonlinear
media. Estimated from holographic recording data, the nonlinear refraction n2 and
third order susceptibility (3) for conjugated organic strictures doped with nanoobjects
could be respectively: ~10-7 cm2kW-1 and ~10-9 esu for thin films of the fullerene- or
nanotubes-doped organic structures; ~10-6 cm2kW-1 and ~10-8 esu for the fullerene- or
nanotubes-doped polymer-dispersed liquid crystals. The high frecuency Kerr effect
provoked by efficient charge transfer complex formation has been considered as the
basic mechanism responsible for the laser-induced features. It has been discussed that
the additional polarizability of the fullerene- or nanotubes-doped structures, for
example LC, stimulates the easy control of these systems. It has been shown that
switching time can be improved by at least one-two orders of magnitude. It permits to
develop the nanoobjects-doped LC display element of new generation. Moreover, it
has been obtained that the surface mechanical and laser strength of the materials
covered with nanoobjects and treated with surface electromagnetic wave can be
improved. It predicts to apply these structures in laser technique with good advantage.
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ACKNOWLEDGEMENTS
The authors wish to thank Dr Yu.M. Voronin and Dr. A.P. Zhevlakov (Vavilov State
Optical Institute, St. Petersburg, Russia) for their help in this study. This work was
partially supported by ISTC Project IPP A-1484.
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