Second- and third-order nonlinearities of novel push-pull azobenzene polymers

Article abstract of DOI:10.1021/jp109936t

In this work, the second- and third-order nonlinear optical response of spin-deposited thin films of three different push-pull side chain azobenzene polymers is investigated by the second- and third-harmonic Maker fringes techniques using 30 ps laser puls

Full text of DOI:10.1021/jp109936t

ARTICLE
pubs.acs.org/JPCB
Second- and Third-Order Nonlinearities of Novel Push-Pull
Azobenzene Polymers
Hasnaa El Ouazzani,† Konstantinos Iliopoulos,† Mindaugas Pranaitis,† Oksana Krupka,‡ Vitaliy Smokal,‡
Aleksey Kolendo,‡ and Bouchta Sahraoui†,*
†Institut des Sciences et Technologies Molꢀeculaires d’Angers, MOLTECH ANJOU, CNRS UMR 6200, Universitꢀe d’Angers,
2 Bd Lavoisier, 49045 Angers cedex
‡Kyiv Taras Shevchenko National University, 60 Volodymyrska, 01033 Kyiv Ukraine
ABSTRACT: In this work, the second- and third-order non-
linear optical response of spin-deposited thin films of three
different push-pull side chain azobenzene polymers is investi-
gated by the second- and third-harmonic Maker fringes
techniques using 30 ps laser pulses at a fundamental wave-
length of 1064 nm. Measurements were carried out before and
after aligning the chromophores by corona poling of the films,
while different polarization configurations have been utilized.
Strong dependence of the response upon the structure of the
systems has been found, which is related to the different
charge transfer within the molecules. The reported findings
are compared with already published results.
Apart from the SHG, more recently, the third harmonic
1. INTRODUCTION
generation (THG) efficiency of azobenzenes has been also
Recently polymer/azobenzene nonlinear optical (NLO) sys-
tems are widely investigated, as they can be of great importance
for a variety of applications. The azobenzene moieties can be
either doped into the polymer matrixes or covalently attached to
the polymer.1-4 The latter case usually results in more stable
systems with increased density of chromophores and enhanced
nonlinear optical response.5 The strong nonlinearity emanates
from the strong charge transfer taking place within these units.6
Among the azobenzenes, the Disperse Red 1 is one of the most
widely investigated, and many studies can be found in the
literature.7-19
Intensive research has been carried out in previous years
to investigate the second-order nonlinear optical response of
azobenzene/polymer systems (e.g., refs 7-9, 20, 21). The
additional attribute of these systems of photoinduced trans-cis
and vice versa isomerization renders them promising candidates
for applications like optical data storage, surface relief gratings,
photoswitching, alignment of liquid crystals, optical elements,
and so forth.10,11,22-26 An important prerequisite to achieve high
second harmonic generation (SHG) is to make the systems non-
centrosymmetric by aligning the chromophores. This can be
done in several ways including electrode poling, corona poling,
all optical poling, and so forth.1 The corona poling is widely used,
and it is based on applying a dc electric field while heating the film
near the glass-transition temperature (Tg) to increase the mobi-
lity of the chromophores.7 Then, by still applying the electric
field, the system is cooled to room temperature. In this way, when
the electric field is switched off, the orientations of the dipole
moments remain frozen for a long time depending upon the
chemical structure and the Tg of the systems.
studied.14-19,27-32 The third-order nonlinear susceptibility (χ(3)
)
contains terms which are resonant at ω, 2ω, and 3ω, and so it is
expected to be more easily altered by changes regarding the
conformation of the molecule. In this sense, the knowledge of
the χ(3) can be utilized for the visualization of modifications con-
cerning the molecular structure.29 The advantage of the THG
technique is that it can provide information only about the elec-
tronic contribution to the χ(3) without being influenced from
generally slower contributions like molecular orientation. Azo
dyes exhibit usually high χ(3) values which are very important for
applications like optical switching and optical limiting. However,
the knowledge of the connection between several parameters like
π-delocalization, donor-acceptor groups, orientation, and the
χ
(3), which is needed for tailoring the nonlinear response to
match the needs of specific applications, is not fully acquired yet.
In this direction, it is very important to characterize both the
second- and the third-order nonlinearity. In this work, we investigate
a series of push-pull side chain azobenzene polymers (Figure 1)
by the well-known SHG, THG Maker fringes setup33-35 using
the same laser excitation source. In particular, comparative study
is carried out between the novel S2, S3 systems (Figures 1, 2) and
one of the most studied quasi 1D charge transfer molecules with
enhanced first hyperpolarizability, which is the DR1 grafted to
the copolymer S (i.e. S1 system). Apart from the investigation of
the third-order and the second-order nonlinear optical response
of the systems, the impact of the electron-acceptor group and the
Received: October 16, 2010
Revised:
December 22, 2010
Published: February 16, 2011
r
2011 American Chemical Society
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the normal of the incident beam. A second polarizer positioned after
the sample offered the possibility to change the detection polariza-
tion direction between s and p thus allowing investigation of differ-
ent polarization configurations. After passing through a KG3 filter,
which cut out the fundamental beam, as well as an interference filter
(at 532 nm for SHG or at 355 for THG) to preserve only the SHG/
THG signal, the latter was detected with a photomultiplier (PMT),
which was connected with a boxcar and computer in order to be
averaged and recorded. Neutral density (ND) filters have been
always positioned before the PMT to avoid saturation.
Concerning the SHG intensity, and in the case of an isotropic
nonlinear material, neglecting reflections, the following expres-
sion stands:33-35
2p
4
2
2
1s
2p
128π5 ½t ꢀ ½tfs ꢀ ½t ꢀ
s - p
2ω
Iω2 ðLχðe2ffÞ
Þ
2
af
sa
I
ðθÞ ¼
cλ2
n22ωcos2 θ2ω
sin2 Φ þ sinh2 Ψ
ꢁ exp½-2ðδ1 þ δ2Þꢀ
ð1Þ
Φ2 þ Ψ2
where Iω is the intensity of the pump wave; λ is its wavelength;
χ(e2ff) is the effective second order nonlinear susceptibility; nω and
n2ω are the refractive indices of the pump and harmonic waves;
1s 2p
2p
sa
L is the film thickness; and t , t , and t are the Fresnel
af fs
transmission coefficients (air-film-substrate-air system) for
the fundamental and SHG beams. The phase angles Φ and Ψ are
given by the following equations:33
2πL
Φ ¼
ðnωcos θω - n2ωcos θ2ω
Þ
λ
Figure 1. Investigated azopolymers.
ꢀ
ꢁ
2πL nωkω
n2ωk2ω
Ψ ¼δ1 - δ2 ¼
-
ð2Þ
λ
cos θω cos θ2ω
where θω, θ2ω are the angles between the fundamental and doubled
frequency beams and κω, κ2ω are the extinction coefficients of
the nonlinear material at frequencies ω and 2ω. A 0.5 mm thick
Y-cut quartz slab has been used as reference material for these
measurements with χ(2) = 8.07 ꢁ 10-12 esu (1 pm/V) and
coherence length 20.5 μm.
The alignment of the chromophores, which was necessary to
break the centrosymmetrical attribute of the chromophores and
to enhance the SHG response, has been done with the corona
poling technique. During this procedure, the samples were heated to
near the glass-transition temperature, and then a high voltage was
applied through two 30 μm diameter tungsten wires, which had
been placed 1 cm over the films, inducing a strong electric field.
Then, the films were cooled down to room temperature, while
the voltage was still applied.
Figure 2. Molecule model of monomer M3.
charge transfer within the azobenzene moieties of the systems as
well as of the alignment of the chromophores with the corona
poling on the nonlinearity is presented and discussed.
For the analysis of the THG measurements and because the
third harmonic signal is in a spectral position, where the linear
absorption of the samples is important, the following equation
2. EXPERIMENTAL SECTION
has been used:35,36
2.1. Nonlinear Optical Techniques. The second/third-order
nonlinear optical response was studied by the SHG/THG Maker
fringes setup in transmission employing the fundamental exit
(1064 nm) of a 30 ps mode-locked Nd:YAG laser with 1 Hz repeti-
tion rate. The beam had Gaussian temporal and spatial profile, and
by passing through a λ/2 wave plate and a polarizer, the intensity
and the polarization were precisely adjusted. A low-power reflection
was measured with a photodiode in order to allow recording of the
incident laser intensity throughout the measurements. Then, the
laser beam was focused by a 250 mm lens on the sample, which had
been positioned near the focal plane, on a rotational stage allowing
the variation of the incident angle with a resolution of 0.5° around
sffiffiffiffiffiffi
2
R=2
I3ω
χð3Þ
¼
χðs3ÞlSc
ð3Þ
π
1-expð-Rl=2Þ I3Sω
where χ(3), χ(s3) are the third-order nonlinear susceptibilities of
the sample and the reference material (silica in this case), respec-
tively, lSc is the coherence length of fused silica, l is the film thick-
ness, R is the linear absorption coefficient, and I3ω and IS3ω are the
peak intensities of the Maker fringes pattern of the film and the
fused silica slab, respectively. The utilized reference χ(3) value
at 1064 nm for fused silica was χ(s3) = 2.0 ꢁ 10-22 m2 V-2 37
.
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Equation 3 allows the determination of the third-order nonlinear
optical response in the case of films under the prerequisite that
the thickness of the film is much lower than the coherence length
and also that the signal coming from the substrate can be con-
sidered to be negligible compared to the signal emanating from
the film. The latter has been verified in our work by in situ remov-
ing a part of the film and measuring the signal from the substrate.
During the measurements, the films were positioned with the
thin film side facing the detector.
4-[4-(Phenylazo)-1-naphthylazo]phenol (M2). A solution of
4-[4-(phenylazo)-1-naphthylazo]phenol (1.5 g, 4.2 mmol) and
triethylamine (0.64 g, 6.3 mmol) was dissolved in THF (40 mL).
The solution was kept in an ice bath for 10 min. A solution of distilled
methacryloyl chloride (0.66 g, 6.3 mmol) in THF (10 mL) was
added slowly to the reaction mixture. After the addition of metha-
cryloyl chloride, the solution was stirred for 12 h at ambient
temperature. The solvent was removed by rotary evaporation, and
the residue was washed with a solution of sodium carbonate (0.8 g)
in water (40 mL). After removing the solvent, the resulting material
was purified by column chromatography (silica gel, ethyl acetate/
hexane 1/8). Red solid residue yield 75%, mp 90 °C.
1
2.2. Instrumentation. H NMR (500 MHz) spectra were
recorded by a Bruker Advance DRX-500 spectrometer. Chemical
shifts are in ppm from the internal standard tetramethylsilane.
UV-vis measurements were performed at room temperature
either in solutions in a quartz liquid cell or as thin films deposited
on glass substrates with a Perkin-Elmer UV/vis/NIR Lambda 19
spectrometer. A Q20 differential scanning calorimetry (DSC)
model (TA Instruments) with a continuous N2 purge was used to
determine the glass and phase-transition temperatures (Tg) of
all polymers. Two scans were run at a heating rate of 10 °C/min
up to 200 °C followed by a cooling to 20 °C giving the values
of Tg.
2.3. Synthesis and Thin Film Preparation. Standard dis-
tillation procedures were performed for triethylamine and tetra-
hydrofuran (THF) just prior to use. 2,20-Azobis(isobutyro-
nitrile) (AIBN) was recrystallized twice from absolute methanol.
Methacrylic chloride was vacuum distilled immediately before
use. Methylmethacrylate (MMA) was washed with aq NaOH to
remove inhibitors and was dried with CaCl2 under nitrogen at
reduced pressure. The chromophores 4-(4-nitrophenylazo)aniline,
N-ethyl-N-(2-hydroxyethyl)-4-(4-nitrophenylazo)aniline DR1,
and 4-[4-(phenylazo)-1-naphthylazo]phenol DO13 were pur-
chased from Aldrich and were purified by double recrystallization
from an absolute methanol solution. All other reagents and solvents
were commercially available and were used as received.
1H NMR (500 MHz, DMSO-d6): δ 9.01-8.98 (m, 2H, naph-
thalene), 8.14 (d, 2H, Ar-H), 8.05 (d, 2H, Ar-H), 7.92 (s, 2H, naph-
thalene), 7.85 (d, 2H, Ar-H), 7.78-7.81 (m, 2H, naphthalene),
7.63-7.58 (m, 3H, Ar-H), 6.35 (s, 1H, CH2), 5.9 (s, 1H, CH2),
2.07 (s, 3H, CH3). UV-vis (THF): λ = 325, 428 nm.
4-((2-Methacryloyloxyethyl)ethylamino)-40-(4-nitrophenylazo)-
azobenzene (M3). Azomonomer M3 was synthesized in the same
way as azomonomer M2. Dark purple crystals yield 60%, mp 160 °C.
1H NMR (500 MHz, CDCl3): δ 8.40 (d, 2H, Ar-H), 8.12-
7.92 (m, 8H, Ar-H), 6.85 (d, 2H, Ar-H), 6.12 (s, 1H, CH2),
5.61 (s, 1H, CH2), 4.38 (t, 2H, OCH2), 3.75 (t, 2H, NCH2), 3.55
(q, 2H, NCH2CH3), 1.97(s, 3H, CH3),1.28(s, 3H, CH3). UV-vis
(THF): λ = 340, 502 nm.
Polymerization. Polymers were synthesized by free-radical
polymerization. The polymerization was carried out in 10 wt %
toluene solution of M1 and methylmethacrylate (MMA) with
monomer initial mole ratios 1:3. The polymerization was con-
ducted using AIBN as a free-radical initiator (1 wt % of mono-
mer) at 80 °C 35 h in argon atmosphere. Previously, the initial
mixture was degassed with repeated freeze-pump-thaw cycles.
The polymerization was stopped by pouring the reaction mixture
into methanol. This procedure was repeated several times to
ensure removal of unreacted methacrylic monomers, and finally,
the polymer S1 was dried under vacuum at 50 °C overnight. In
the cases of copolymerization M2 with MMA and M3 with
MMA, the same synthetic procedure was used in DMF and 1,4-
dioxane solution accordingly. The copolymerization ratios in the
corresponding polymers were calculated on the basis of the
integrated peak areas of 1H NMR spectra in DMSO-d6 for S1 and
S3 and in pyridine-d5 for S2. The glass-transition temperatures
were measured by differential scanning calorimetry to be 125,
110, and 140 °C for the copolymers S1, S2, and S3, respectively.
Thin Film Processing. Thin films of S1, S2, and S3 were
obtained by spin coating of filtered solutions through a 0.4 μm
pore size nylon syringe filter on BK7 glass slides. The principle of
deposition (of the mixture with certain viscosity) is based on a
homogeneous spreading out of the solution on the rotating
substrate with an angular speed of 800 rpm. We used as solvent
1,1,2-trichlororethane for the quality of thin film formation. The
same polymer concentration of 56 g/L was used. Immediately
after the deposition, the films were cured in an oven at 50 °C and
for 180 min in order to eliminate any remaining solvent. Their
thickness was measured by a profilometer (Tencor, ALFA-Step)
and in all cases was found to be 0.4-0.5 μm.
4-(N-Ethyl-N-(2-hydroxyethyl)amino)-40-(4-nitrophenylazo)-
azobenzene. 4-(4-Nitrophenylazo)aniline (5 g, 20.6 mmol) was
dissolved in a solution of concentrated hydrochloric acid 10 mL
and dimethylformamide (DMF) 80 mL. The reaction mixture
was cooled down in an ice-water bath to 0 °C and was stirred for
15 min, and then a solution of sodium nitrite (1.45 g, 21 mmol)
in 5 mL of water was added dropwise. The reaction mixture
was stirred for 2 h at 0 °C, and then the N-ethyl-N-(2-hydroxy-
ethyl)aniline (3.5 g, 21 mmol) in 15 mL of acetic acid was added
dropwise. After stirring at 0-5 °C for 5 and 12 h at room tem-
perature, a large amount of a dark precipitate formed. The
precipitate was collected by filtration and then was washed with
saturated sodium bicarbonate solution and was dried. The crude
product was purified by column chromatography on silica gel
(toluene:acetone, 1:1.5) followed by recrystallization from THF:
hexane obtained as dark purple crystals mp 225 °C, 62%.
1H NMR (500 MHz, DMSO-d6): δ 8.46(d, 2H, Ar-H), 8.00
(d, 2H, Ar-H), 8.12 (d, 4H, Ar-H), 7.85 (d, 2H, Ar-H), 6.90
(d, 2H, Ar-H), 4.85 (t, 1H, OH), 3.55-3.65 (m, 6H, -CH2-),
1.18 (s, 3H, -CH3). UV-vis (THF): λ = 346, 520 nm.
4-((2-Methacryloyloxyethyl)ethylamino)-4-nitroazobenzene
(M1). Azomonomer was synthesized in the same way as reported.38
The solid was recrystallized from methanol. Dark red crystals
1
3. RESULTS AND DISCUSSION
mp 83 °C, yield 80%. H NMR (500 MHz, CDCl3): δ 8.35
(d, 2H, Ar-H), 7.92 (t, 4H, Ar-H), 6.85 (d, 2H, Ar-H), 6.1
(s, 1H, CH2), 5.6 (s, 1H, CH2), 4.38 (m, 2H, OCH2), 3.75
(m, 2H, NCH2), 3.56 (m, 2H, NCH2), 1.94 (s, 3H, CH3), 1.24
(m, 3H, CH3). UV-vis (THF): λ = 475 nm.
The nonlinear optical response of thin films of the azobenzene
polymers S1-S3 has been investigated by means of SHG/THG
Maker fringes measurements. In Figure 3, characteristic absorption
spectra of the investigated systems can be seen. As shown in the
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Table 1. Effective χ(2) Values Obtained for the S1, S2, and S3
Systems in the p-p Configuration before and after
Corona Poling with Strong Electric Field
χ
(2) (pm/V) before
corona poling
χ
(2) (pm/V) after
corona poling
sample
S1
S2
S3
0.10
n/a
26
0.56
30
0.08
Figure 3. UV-vis spectra of S1, S2, and S3 in thin solid films.
Figure 5. Comparative SHG Maker fringes patterns under the same
experimental conditions, obtained for the S3 system, under three differ-
ent polarization configurations.
Figure 4 before and after alignment of the chromophores. To
enable visual clarity of the figure, the curve obtained before the
corona poling has been multiplied by a factor of 2ꢁ104. Because
of the huge difference of the response between the two measure-
ments, different ND filters have been used to attenuate the
signals, which have been in any case taken into account during
the analysis of the experimental data. There is significant
difference of the obtained SHG magnitude between the results
before and after the corona poling as expected, which is
attributed to very efficient alignment of the molecules by the
applied electric field. Similar behavior has been found for the
other two systems S2 and S3. This result can be clearly seen in
Table 1 where the values corresponding to the p-p configura-
tion can be seen for all the systems before and after the corona
poling of the films. The increase in the nonlinearity for the S1 and
S3 system after the corona poling is about a factor of 260 and 375
for the systems S1 and S3, while in the case of the system S2, the
signal prior to corona poling has been too low to offer sufficient
signal-to-noise ratio for these measurements. In all cases, strong
nonlinearity has been found after the corona poling for all systems.
Then, comparative Maker fringes measurements were done
between the S1, S2 and S3 azobenzene polymers using different
excitation-detection polarization configurations ss, sp and pp.
For all three investigated systems, the pp configuration has
resulted in the strongest nonlinear optical response. Intermediate
efficiency has been obtained for the s-p configuration, while the
s-s corresponded to the lowest SHG efficiency. Especially in the
case of the S2 system, where the electron-donating part is missing
from the side chain, the s-s configuration resulted in very low
signal-to-noise ratio which was not adequate for measurements.
The comparison between the different configurations is nicely
Figure 4. Normalized SHG Maker fringes patterns obtained for the S1
sample under p-p polarization configuration before and after corona
poling.
figure, the π-π* band of the S1 and S3 systems is shifted to the
red compared to the S2. This is due to the electron-donor and
electron-acceptor substituents in the synthesized azobenzene
polymers S1 and S3, which increase the charge-transfer character
of the π-π* transition.39 The fact that the S3 system exhibits
maximum absorption at the longest wavelength (500 nm) among
the investigated compounds reflects the highest charge-transfer
interaction occurring between the electron-donor group (amino)
and the electron-acceptor group (nitro). This enhancement of
the charge-transfer efficiency, if compared to the S1 system,
can be attributed to the additional phenyldiazene fragment in the
side chain.32
First, SHG measurements were done before and after corona
poling of the films for all studied push-pull azobenzene poly-
mers S1-S3. In the case of the measurements after the corona
poling, the experiments were carried out directly after the poling
of the chromophores to avoid possible misalignment. However, a
very slow disorientation with time after poling has been found,
and the signal of the systems was significant and remained
unchanged even a long time after the corona poling. Theobserved
long-termstability is mainly attributed tothe relatively high Tg of the
studied systems (see Experimental Section). To illustrate the
impact of the corona poling of the chromophores to the SHG
response, the Maker fringes patterns obtained for the S1 system
using p excitation and detection polarizations can be seen in
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Table 2. Effective χ(2) Values for All Investigated Systems
under s-s, s-p, and p-p Excitation-Detection Polarization
Configurations
χ
(2) (pm/V)
sample
s-s
s-p
p-p
S1
S2
S3
3.26
n/a
10.79
0.08
9
26
0.56
30
2.16
Figure 7. Characteristic THG curve corresponding to the S3 sample
(pp polarization configuration).
Table 3. Third-Order Nonlinear Susceptibility (χ(3)) under
All Polarization Configurations
χ
(3) (10-22 m2/V2)
polarization configurations
sample
p-p
s-s
s-p
p-s
S1
S2
S3
2432
147
2420
148
1584
118
1574
112
Figure 6. Normalized SHG Maker fringes curves enabling the compar-
ison between the systems S1, S2, and S3 under identical experimental
conditions.
618
612
416
432
configurations. It was found that the ss and pp polarization
configurations led to higher signals, while the difference between
them was within the experimental error. On the contrary, when
different excitation and detection polarizations (ps or sp config-
urations) were used, the signal was reduced to significantly lower
values. As an example, the obtained THG signal as a function of
the incident angle can be seen in Figure 7 corresponding to the
S3 sample and p polarization for both excitation and detection.
The χ(3) values for the polarization configurations pp, ss, sp, and
ps of the samples S1, S2, and S3, obtained before the orientation
of the chromophores, according to the procedure described in
the Experimental Section, are presented in Table 3. In all cases,
the values were found to be very high, indicative of very efficient
third harmonic generation of the azobenzenes, which were up to
3 orders of magnitude higher than the values obtained for
the reference material. The value reported by Yoon et al.42 for
poly(1,4-phenylenevinylene) containing Disperse Red is about
7 times lower than that reported here, which can be attributed to
the different structures of the systems as well as to the fact that
the measurements reported there were carried out at 1907 nm,
in transparent region for the system. Moreover, the χ(3) values of
the S2 and S3 systems found here are several times lower if
compared with the S1 system. More specifically, in the case of the
pp configuration, the efficiency of the S2 and S3 systems is
decreased by about 16 and 4 times, respectively, with respect to
the S1 system. In agreement with the SHG results, the non-
linearity of the S1 and S3 is much higher than that of S2 because
of the enhanced charge transfer of these systems. A comparative
second series of experiments have been done for all possible
polarization configurations after corona poling of the samples,
but the results have been found in all cases to be identical, which
illustrated in Figure 5, where the obtained curves are seen for the
system S3, under identical experimental conditions for all three
different configurations, after corona poling of the films. The curves
corresponding to the sp and ss configurations are multiplied by a
factor of 5 and 25, respectively. The big difference in the signal levels is
obvious, while the maximum SHG signals correspond to an angle of
about 55° and the minimum to normal incidence of the laser beam.
From Maker fringes patterns obtained for all the azobenzene
polymers and for the ss, pp, and sp configurations, the effective
values of χ(2) have been determined using quartz as reference
sample following the details provided in the Experimental
Section and are shown in Table 2. Characteristic curves obtained
for these systems are presented in Figure 6 for the pp polarization
configuration. The χ(2) values of the S1 and S3 systems are similar
between these two systems but are much higher than that of the
system S2. In particular for the pp configuration, the difference is
about a factor of 50. This large enhancement can be attributed to
the strong acceptor moieties in the para position and electron-
donor group (amino) and results in the highest charge transfer in
the polymer system, which has consequently a strong impact on
the NLO response. The obtained results are in good agreement
with already reported results in the literature. In ref 40, pDR1M
azobenzene homopolymer films were studied, and comparative
investigation was carried out between the pp and ss configura-
tions, which revealed higher signal with the former configuration
in agreement with the present findings (see Figure 5). The value
found here for the reference S1 system is in agreement with
already published values in the literature.9,40,41
Then, THG measurements were carried out in fresh, noncorona-
poled films of the same compounds using different polarization
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Table 4. Third-Order Nonlinear Susceptibility (χ(3)) for the
Systems S1-S3 before and after Corona Poling
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χ
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after corona poling
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S2
S3
2432
147
2442
144
618
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: bouchta.sahraoui@univ-angers.fr.
’ ACKNOWLEDGMENT
The authors would like to thank the COST action MP0702
(Towards Functional Sub-Wavelength Photonic Structures) for
financial support. They would like to thank also C. Cassagne,
A. Mahot, and D. Guichaoua for their technical help.
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