Nonlinear optical properties of new photosensitive smart materials based on nematic liquid crystal with H-bonded dye–polymer complex

Optics Communications ] (]]]]) ]]]–]]]

Contents lists available at SciVerse ScienceDirect

Optics Communications journal homepage: www.elsevier.com/locate/optcom

Nonlinear optical properties of new photosensitive smart materials based on nematic liquid crystal with H-bonded dye–polymer complex A.V. Uklein a,n, A.A. Vasko a, E.V. Ouskova b, M.S. Brodyn a, V.Ya. Gayvoronsky a a b

Institute of Physics, National Academy of Sciences of Ukraine, 46 Prospect Nauki, Kiev 03680, Ukraine Department of Applied Physics, Aalto University School of Science, P.O. Box 13500, FI-00076 AALTO, Espoo, Finland

a r t i c l e i n f o

abstract

Article history: Received 18 December 2012 Received in revised form 8 January 2013 Accepted 12 January 2013

The nonlinear optical (NLO) properties of the new photosensitive heterogeneous systems based on nematic liquid crystal (LC) doped with H-bonded polymer–azo-dye complex were studied. The excitation of the heterosystem by continuous laser irradiation at 532 nm produces the refractive index variation up to 102 measured within the spatial profile analysis in the far field. The phenomenon could be attributed to the photoinduced transformation of the azo dye from trans to cis form that reduces the order parameter of the LC in the vicinity of the complex. & 2013 Elsevier B.V. All rights reserved.

Keywords: Nematic liquid crystal Azo-dye Trans–cis isomerization Nonlinear optical response Self-action effect

1. Introduction The heterogeneous systems based on liquid crystals with various types of dopants have deserved a growing interest in the last decades. The combination of the widely used LC properties with high photosensitivity of the additives allows us to design novel smart materials. The admixture of azo-dyes (AD) [1], nanoparticles of different nature (ferroelectric, ferromagnetic, metal, etc.) [2–4] and polymers [5] yields the improvement of the heterosystems quality. Thus, the optical response of LCs can be controlled by doping of the dye molecules while the other useful properties are not strongly affected. In particular, the sufficient local electric fields induced by the additives leads to enhancement of the optical nonlinearity. As it was shown in [6], the NLO susceptibility of the heterogeneous media increased by two orders versus the nominally pure matrix by the admixture of the small amount of the photoabsorbing dye molecules with a similar structure to LC [7]. Recent studies showed that combination of azo-dye molecules with polymer chains by H-bonding produces a new organic unit: polymer–dye complex. The addition of it results in even better phenomena: improvement of the optical response of the LC system [8] and the appearance of a new aligning effect [9]. The obtained heterosystem gained from all the strongest properties of the

n

Corresponding author. Tel.: þ380 509 659 386. E-mail address: [email protected] (A.V. Uklein).

components like anisotropy of LC, stability of the polymer and photosensitivity of azo-dye. The admixture of the polymer–dye complex to the LC at low concentration reorients the matrix homeotropically, being independent on the rubbing direction [9]. Due to the azo-dye resonant excitation at 457 nm laser irradiation in the power range less than 10 mW the laser beam self-action takes place. It is characterized with high NLO coefficient n2  103 cm2 =W that determines the efficiency of refractive index variation Dn (I0)¼n2I0, where I0 is the laser radiation intensity. The magnitude of the observed self-phase modulation is enough to produce aberration rings pattern formation in the far field caused by the impact of the order parameter change driven by trans–cis transformation of azodye molecules on Dn [8]. The NLO effect manifestation in the complex-doped LC is more efficient than in the azo-dye-doped LC. These promising results forced further studies with the spatial profile analysis technique that has several advantages. First, it is sensitive to slight photoinduced convergence/divergence of the laser beam, being caused by low photoinduced phase shift  50 mrad. On the contrary, the photoinduced refractive index coefficient determination from the abberation pattern needs significant NLO phase shift at least p radians or more (the threshold of a abberation ring formation) [8]. Second, with the spatial profile analysis technique the position of the sample against the waist is fixed, and it is a constant aperture of the irradiated spot in comparison with the well-known z-scan technique [10]. Consequently, adjustment of the laser beam aperture provides an efficient spatial averaging of the materials response and suppression of the optical inhomogeneities impact.

0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.01.011

Please cite this article as: A.V. Uklein, et al., Optics Communications (2013), http://dx.doi.org/10.1016/j.optcom.2013.01.011i

A.V. Uklein et al. / Optics Communications ] (]]]]) ]]]–]]]

In this paper we investigate the self-action phenomena of the complex-doped LC material with mentioned spatial profile analysis technique under the excitation of continuous laser irradiation with photon energy 2.33 eV (532 nm) that is at the tail of the absorption band of the dye–polymer complex, but still can induce the photoisomerization reaction.

2. Preparation and characterization of materials We investigated a typical nematic liquid crystal (NLC) 4-pentyl-40 -cyanobiphenyl (5CB; PI Chemicals Co., Ltd) doped with different concentrations of a hydrogen-bonded polymer– azo-dye complex (PADC): P4VP (CHAB)0.5. The procedure of preparation of the H-bonded complex from the polymer poly-4vynil-pyridine (P4VP, Mn: 1000 g/mol, Mw: 1200 g/mol; Polymer Source Inc.) and the azo-dye 4-cyano-40 -hydroxyazobenzene (CHAB, BEAM Co.), is simple and described in [9]. By mixing the components in the DMF (dimethylformamide) solvent with time one gets PADC. The polymer and AD create strong H-bonding and after that act as one organic unit. The complexation degree of 0.5 means that the AD molecules make a bonding only to every second side fragment of the polymer to form a complex P4VP(CHAB)0.5. To make a complexdoped LC H-bonded complex was added to LC at appropriate concentrations [9], forming the suspension 5CBþc% P4VP(CHAB)0.5 with concentration c in a range of 0.2–2 wt%. At such low concentration there is no polymer network and the PADC unit acts as a molecular additive in LC matrix not changing the basic mesogenic LC properties too much. The nematic–isotropic phase transition of LC matrix is in a range of 36.0–36.2 1C, while the one of the complex-doped LC is 35.6–36.7 1C [9]. The transition range broadening and its higher temperature mean more ordered structure of the PADC-doped LC compared to the matrix. The prepared suspension was placed between two glass substrates. The thickness of the LC cell was set by Teflon stripes to 80 mm. Unlike the nominally pure 5CB which form a rather random alignment on the uncoated glass the suspension of complex-doped LC is self-oriented [9]. It aligns homeotropically even without any orienting layers on the inner surfaces of the glass cell. To compare the properties of PADC-doped LC with the nominally pure LC or AD-doped LC we also prepared the homeotropically aligned LC and AD-doped LC with the help of polyimide SE1211 (NISSAN) aligning layers. The characterization of investigated materials is presented in Table 1. Sample (1) is a nominally pure LC matrix, and others (2–5) are the mixtures with PADC. The last column contains the transmittance T0 of these materials at 532 nm.

3. Experimental results 3.1. Optical characterization The optical density spectra of the samples (1–5) were recorded by Perkin Elmer lambda 25 spectrometer and shown in Fig. 1. Table 1 The complex concentration c, its components percentage, and spectral transmittance T0 at wavelength 532 nm of the samples: (1) nominally pure 5CB, (2–5) 5CB þc% P4VP(CHAB)0.5.

1 2 3 4 5

CHAB, wt%

P4VP, wt%

c, wt%

T0 (%)

– 0.103 0.257 0.515 1.03

– 0.0970 0.243 0.485 0.97

– 0.2 0.5 1.0 2.0

88.5 85.4 80.0 60.3 26.4

3

Optical density

2

5

2

4 3

1

2 1

0 400

500 600 Wavelength, nm

700

Fig. 1. Absorption spectra of LC matrix (1—solid line) and LC doped with different concentrations of PADC, (2)  0.2 wt%, (3)  0.5 wt%, (4)  1.0 wt%, and (5)  2.0 wt%.

The absorption band maximum of the 5CB matrix locates at lmax  275 nm which is far from the energy of the excitation laser quantum [11]. The admixture of the complex leads to formation of two maxima at 373 nm [9,12] and a less intensive one at 470 nm that corresponds to absorption of the PADC as a intact organic unit. The laser wavelength 532 nm is at the tail of the complex absorption band but provides the photoinduced changes in the PADC in particular and in the optical response of the whole LC cell. 3.2. Nonlinear optical characterization The photoinduced variations of the refractive index ðDn  Reðwð3Þ ÞÞ and optical absorption ðDa  Imðwð3Þ ÞÞ, where wð3Þ is the cubic NLO susceptibility, were measured with the spatial profile analysis technique described in [6]. The scheme of the experimental setup is presented in Fig. 2. The CW DPSS laser with the Gaussian spatial profile was used to irradiate the sample S. Neutral attenuator A with transmission from 1% to 50% allows us to vary the irradiation intensity up to 16 W/cm2 and control the reversibility of the NLO response. The samples were positioned at the distance 10 cm from the focusing lens L with focal length 8 cm. The typical diameter of laser beam at the sample plane was about 0.2 mm that provides the efficient spatial averaging and results in reproducibility of the experimental data at the different points at the sample transverse plane. The photodiodes 1,2,3 provide an acquisition of the input laser beam power (PD1), total transmitted power through the sample (PD2) and the power that has passed the finite diaphragm D (radius r0 ¼1 mm) in the far field (  70 cm from the sample) visualizing the light-induced convergence/divergence of the beam (PD3). The total transmittance of the studied cells is proportional to the signals ratio of the photodiodes PD2 to PD1 corrected to the apparatus function. In order to interpret the obtained variation of the total transmittance versus the incident laser power/intensity in terms of the cubic NLO response we applied the exact expression for the transmittance with account of spatial averaging across the Gaussian CW laser beam profile with peak intensity I0 [13] TðI0 Þ ¼ T 0 lnðð1þ qðI0 ÞÞ=qðI0 ÞÞ,

ð1Þ

where T0 is the linear transmission coefficient and qðI0 Þ ¼ DaðI0 ÞLeff  Imðwð3Þ ÞI0 , Leff ¼ ð1expðaLÞÞ=a is the self-action effective

Please cite this article as: A.V. Uklein, et al., Optics Communications (2013), http://dx.doi.org/10.1016/j.optcom.2013.01.011i

A.V. Uklein et al. / Optics Communications ] (]]]]) ]]]–]]]

length, a is the linear absorption coefficient, L is the thickness of the cell [13]. Eq. (1) is valid for the thin layers without beam spatial profile redistribution that was checked for our cells. The on-axis transmittance in the far field is proportional to the signals ratio PD3 to PD1 and is sensitive to the photoinduced laser beam convergence/divergence due to the self- focusing/defocusing effect in the cell [6,10]. According to the model for the cubic nonlinearity with negligible nonlinear absorption described in [10], the peak photoinduced phase shift at the exit surface of the cell is proportional to the photoinduced refractive index DjðI0 Þ ¼ kLeff DnðI0 Þ  Reðwð3Þ ÞI0 (k is the wave vector). Taking into account the Gaussian decomposition approach [10] and the spatial averaging by the transmitted beam across the diaphragm aperture the on-axis transmittance in the far field can be presented as an expansion into T a ðI0 Þ ¼ Sð1 þ a1 DjðI0 Þ þ a2 ðDjðI0 ÞÞ2 þ   Þ,

ð2Þ

where S is the aperture linear transmittance, ai are coefficients, that depend on the input beam parameters, the experiment geometry and the finite diaphragm aperture (see Ref. [6]). In order to compensate the impact of the photoinduced transmittance variation on refractive NLO response, we normalize the onaxis transmittance in the far field on total transmittance of the sample. The verification of such approach is described in Ref. [10] for extraction of purely refractive Z-scan from closed aperture one by division on open aperture Z-scan. Thus, the fitting of the experimental total and on-axis transmittances by expressions (1) and (2) for chosen laser intensity range allows us to obtain the magnitudes of Da and Dj as well as Dn, Imðwð3Þ Þ and Reðwð3Þ Þ. The measured photoinduced variations of the normalized total transmittance TðI0 Þ=T 0 are presented at Fig. 3(a). The normalized photoinduced on-axis transmittances calculated from the experimental data as ðT a ðI0 Þ=TðI0 ÞÞ  ðT 0 =T a ð0ÞÞ are shown in Fig. 3(b). Each point on the dependences is the result of averaging by  25 experimental data points, except of the curve 2 that was averaged by  42 data points for more clear visualization.

3

It was observed that all samples demonstrate the rise of the normalized total transmittance with increasing of the irradiation intensity that corresponds to the photoinduced bleaching (reduction of the absorption coefficient aðI0 Þ). The magnitude of the total variations are less than 0.3% for the LC matrix and the sample with 0.2 wt% of the PADC. The increase of the complex concentration in the matrix leads to the rise of the photobleaching effect efficiency. It appears to be 2%, 4% and 12% for the corresponding 0.5 wt%, 1 wt% and 2 wt% doped samples, respectively. The process is more efficient at the lower intensity irradiation. It develops until the intensity is less than 1.5 W/cm2 for the samples with 0.5 wt% and 1 wt% of PADC concentration and 3 W/cm2 for the sample with 2 wt%. At intensity 14 W/cm2 the pronounced photobleaching saturates. At the same time the on-axis transmittance variations of the LC matrix and 0.2 wt% sample is less then 2% and negligible in comparison to the others. For the samples with 0.5 wt% and 1 wt% of the PADC the monotonic rise correspondingly up to 20% and 50% of the signal was observed. Since the samples were positioned after the beam waist it means the positive variation of the refractive index (self-focusing) in the heterosystem. For the sample with the highest concentration (2 wt%) at intensities o 3 W=cm2 the self-defocusing of the beam was observed. Further enhancement of the intensity resulting the selfdefocusing effect saturation and turn to self-focusing that is similar to the observed phenomenon in the samples 0.5 wt% and 1 wt%. The estimated magnitudes of the Da, Dn, Imðwð3Þ Þ and Reðwð3Þ Þ are presented in Table 2 for two characteristic intensity ranges I (1.5–3 W/cm2) and II (8–10 W/cm2). The calculations were performed according to the technique described in [6].

Table 2 Imaginary and real parts of the cubic NLO susceptibility wð3Þ , photoinduced changes of absorption coefficient Da and the refractive index Dn for two characteristic intensity ranges I (1.5–3 W/cm2) and II (8–10 W/cm2), c—concentration of the polymer–dye complex. Range

I : 1:5C3 W=cm2

Sample c (%) Rewð3Þ

Imwð3Þ

10  4 esu

Fig. 2. Experimental setup for spatial profile analysis. A, neutral attenuator; L, lens; S, sample; D, diaphragm; Phd 1, 2, 3, photodiodes 1, 2, 3 correspondingly.

1 2 3 4 5

0.0 0.2 0.5 1.0 2.0

0.2  0.7 4.4 5.9  54.0

II : 8C10 W=cm2

Dn

Da

Rewð3Þ Imwð3Þ

Dn

Da

10  3

m1

10  4 esu

10  3

m1

 0.04 0.5  0.6  0.2  0.03  1.5  0.5 0.6  0.34 10.3  5.3 2.3  0.59 13.7  9.7 4.1  5.22  126.0  86.5 5.1

0.04  0.04  0.07 1.4  0.19 5.4  0.09 9.6  0.39 12.0

0.6  1.1  3.1  1.5  6.5

Fig. 3. Normalized total (a) and on-axis transmittance in the far field (b) versus the laser intensity for LC matrix (1) and matrix with different concentration of polymer– dye complex 0.2 wt% (2), 0.5 wt% (3), 1.0 wt% (4), and 2.0 wt% (5).

Please cite this article as: A.V. Uklein, et al., Optics Communications (2013), http://dx.doi.org/10.1016/j.optcom.2013.01.011i

A.V. Uklein et al. / Optics Communications ] (]]]]) ]]]–]]]

The concentration dependences of the variations of the refractive index Dn and absorption coefficient Da are presented in Fig. 4. For the intensity range 1.5–3 W/cm2 the linear enhancement of the NLO parameters with the increase of the complex concentration up to 1 wt% was observed. The phenomenon could be attributed to the resonant excitation of the AD. At concentration 2 wt% the essential change of the slope and turn the sign of Dn from self-focusing to self-defocusing effect state the predominance of another mechanism that will be described below. At the intensities 8–10 W/cm2 the linear regime lasts up to PADC concentration 0.5 wt%. Further increase of the complex concentration leads to the saturation of the refractive index gain and turn in absorption coefficient dependence at 2 wt%. That phenomenon could state that the process of the resonant excitation starts to be more delocalized and sensitive to the complex environment. To compare the efficiency of the NLO response at different wavelengths 532 nm and 457 nm [8] that located within the same absorption band of the AD, we utilize dimensionless parameter Dn=ðl  aðlÞÞ. The parameter reflects the gain in photoinduced refractive index toward the resonant absorption losses at the corresponding wavelength. Its dependence on the PADC concentration is presented in Fig. 5, where circles—excitation at 457 nm, triangles—532 nm. It was shown that the peak of the ratio of the photoinduced refractive index gain to the rise of the optical absorption for both wavelengths located at the 0.5 wt%. Moreover, starting at this concentration, the irradiation of the heterosystem at 532 nm yields to higher relative efficiency of trans–cis isomerization reaction respectively to the number of the absorbed photons, despite the quantum energy is on the tail of the dye absorption band. We have compared the efficiency of the NLO response for the LC cells with PADC (1) and AD (2) (Fig. 6). The curves correspond to the averaged data points and plotted with the error bar 0.3% that is characteristic for their measurements. We have chosen the samples with 0.5 wt% of PADC and 0.25 wt% of AD. These mixtures contain the same concentration of AD since the components concentration in PADC is about 50%. The choice was driven by the linearity of the photoinduced refractive and absorptive NLO response versus the PADC concentration for both ranges of excitation. The observed photoinduced total transmittance variations of about 0.8% (1) and 1.8% (2) are about five times less than the on-axis transmittance ones, which are approximately 4.5% (1) and 10% (2). For the AD-doped LC the estimated magnitudes of the NLO parameters for both intensity ranges are wð3Þ ¼ ð1:320:1  iÞ  104 esu (I) and wð3Þ ¼ ð4:020:3  iÞ  105 esu (II). The comparison with the corresponding magnitudes for the PADC-doped LC (Table 2) shows that the efficiency of the refractive and absorptive responses of the system with PADC are wð3Þ ðLC þ PADCÞ=wð3Þ ðLC þ ADÞ  3 times higher for the range I and about six times for the range II. This fact

confirms that the local alignment of the matrix and order parameter perturbation is more efficient when the AD is connected to the polymer.

4. Discussion According to Ref. [8] the origin of self-action nonlinear optical response in self-oriented complex-doped LC can be the lightinduced change of the order parameter S of the complex-doped LC that followed by a change in the refractive indices and in the birefringence. There are several possible reasons of the order parameter change:

 photoinduced conformational changes of the AD molecules initiating the order parameter variations [14] due to its perturbation in the vicinity of the transformed AD;  photoinduced changes in LC anchoring that affect director pretilt and bulk profile, leading to the order parameter change;  the photoinduced AD orientational ordering variation which affects the polymer backbone, that in-turn effects LC orientational ordering, resulting in DS. The described mechanism can explain the observed results on self-focusing. Despite several possibilities, the initial driving force

2.0 1.8 1.6 Δn/λα (λ))

4

1.4 1.2 1.0 0.8 0.6 0.4 0.2

0.4

0.6 c, wt. %

0.8

1.0

Fig. 5. The parameter Dn=ðl  aðlÞÞ versus the polymer–dye complex concentration at l ¼ 457 nm ðmÞ and 532 nm ðÞ excitation wavelength. The parameter reflects the gain in photoinduced refractive index toward the resonant absorption losses.

Fig. 4. Photoinduced variations of the refractive index Dn (solid line, right axis) and absorption coefficient Da (dashed line, left axis) versus the concentration c (wt%). The intensity range I (1.5–3 W/cm2) is shown in (a) and intensity range II (8–10 W/cm2) is shown in (b). The points are connected for guidance purposes only.

Please cite this article as: A.V. Uklein, et al., Optics Communications (2013), http://dx.doi.org/10.1016/j.optcom.2013.01.011i

A.V. Uklein et al. / Optics Communications ] (]]]]) ]]]–]]]

5

Fig. 6. Normalized total (a) and on-axis transmittance in the far field (b) versus the laser intensity for LC matrix with 0.25 wt% of CHAB (1) and 0.5 wt% of P4VP(CHAB)0.5 (2). Both mixtures contain the same concentration of the azo-dye.

of all mentioned photoinduced changes is AD trans–cis isomerization [15] modified by the presence of the polymer that strongly affects the LCs order parameter in the vicinity of the complex and leads to the sharp variations of Dn and Da. Light irradiation causes AD transformation from rod-like to U-shape molecules. This transformation disorders LC and decreases LC order parameter. Since the initial orientation of LC in our samples is homeotropic and irradiation occurred at normal incidence, thus any decrease in S is leading to increase in birefringence that results in the observed self-focusing with NLO refractive coefficient n2 in a range of 10  4–10  2 cm2/W. The most doped 2 wt% LC cell demonstrates the self-defocusing effect with the NLO refractive coefficient n2  8:4  102 cm2 =W in the intensity range I. Unlike the self-focusing effect of the rest of the samples, the phenomenon could appear due to excitation of the trans state of the azo-dye without consequent conformation. As it was mentioned in [16], the trans isomers could induce the negative nonlinearity in nematic LC. Thus, at high concentration the complex units could appear to be close enough to impede and decrease the probability of the trans–cis transformation. Moreover, the initially existing cis isomer in the equilibrium state before irradiation could be forced to relax to trans state at irradiation, increasing by this way the LCs order parameter. As a result, the presence of the high concentration of the excited trans isomers leads to the photoinduced reduction of the refractive index of the heterogeneous system, which thus shows selfdefocusing. This suggestion needs more experimental proofs. We suppose that application of two-color excitation [17] can produce more comprehensive control of the sign and magnitude of the NLO response. Starting from PADC concentration 0.5 wt% the relative photoinduced refractive index variations toward the absorption are more efficient at excitation with the quantum energy 2.33 eV (532 nm) than with 2.71 eV (457 nm). Thus, the optical quality parameter for the indicated heterosystems is even higher at longer wavelength which is on the tail of the absorption band of the trans–cis isomerization reaction in comparison to 457 nm (see Figs. 1 and 5). Finally, taking into account the significant photobleaching phenomenon described in Section 3.2, the optical quality of the self-oriented complex-doped LC is actually even better at 532 nm irradiation making the system very promising for application at similar excitation conditions.

5. Conclusions We have investigated the nonlinear optical properties of the self-orienting heterogeneous systems based on the nematic LC with different concentrations of H-bonded polymer–azo-dye complex. The effects of photobleaching and self-focusing with characteristic values of Da  10 m1 and Dn  102 were observed. The sharp enhancement of the NLO parameters started from 0.5 wt% of the complex was established. Such behavior could be attributed to the trans–cis isomerization reaction that reduces the order parameter of the LC in the vicinity of the complex. It was shown that the efficiency of the pronounced phenomenon is higher when the azo-dye is connected to the polymer. The observed results make these materials very promising for application in photonic and optoelectronic devices. References [1] E. Ouskova, N. Aryasova, V. Boichuk, D. Fedorenko, K. Slyusarenko, Yu. Reznikov, Molecular Crystals and Liquid Crystals 527 (2010) 43/[199]. [2] E. Ouskova, O. Buchnev, V. Reshetnyak, Yu. Reznikov, H. Kresse, Liquid Crystals 30 (10) (2003) 1235. [3] E. Ouskova, O. Buluy, C. Blanc, H. Dietsch, A. Mertelj, Molecular Crystals and Liquid Crystals 525 (2010) 104. [4] D. Lysenko, E. Ouskova, S. Ksondzyk, V. Reshetnyak, L. Cseh, G.H. Mehl, Yu. Reznikov, European Physics Journal E: Soft Matter and Biological Physics 35 (5) (2012) 33 1–7. [5] I.A. Budagovsky, A.S. Zolot’ko, V.N. Ochkin, M.P. Smayev, A.Y. Bobrovsky, V.P. Shibaev, M.I. Barnik, Journal of Experimental and Theoretical Physics 106 (1) (2008) 172. [6] V. Gayvoronsky, S. Yakunin, V. Nazarenko, V. Starkov, M. Brodyn, Molecular Crystals and Liquid Crystals 426 (2005) 231. [7] V.Ya. Gayvoronsky, S.V. Yakunin, V.M. Pergamenshchik, V.G. Nazarenko, Laser Physics Letters 3 (11) (2006) 531. [8] E. Ouskova, M. Kaivola, Optical Materials Express 2 (8) (2010) 1056. [9] E. Ouskova, J. Vapaavuori, M. Kaivola, Optical Materials Express 1 (8) (2011) 1463. [10] M. Sheik-Bahae, A.A. Said, T. Wei, D.J. Hagan, E.W. van Stryland, Journal of Quantum Electronics 26 (4) (1990) 760. [11] S.-T. Wu, Journal of Applied Physics 84 (8) (1998) 4462. [12] J. Vapaavuori, V. Valtavirta, T. Alasaarela, J.-I. Mamiya, A. Priimagi, A. Shishido, M. Kaivola, Journal of Materials Chemistry 21 (39) (2011) 15437. [13] W.L. Smith, CRC Handbook of Laser Science and Technology, vol. 3, CRC Press, Inc., Boca Raton, FL, pp. 229–258. [14] I.P. Pinkevich, Yu.A. Reznikov, V.Y. Reshetnyak, O. Yaroshchuk, International Journal of Nonlinear Optical Physics 1 (3) (1992) 447. [15] D. Statman, I. Janossy, Journal of Physical Chemistry 118 (7) (2003) 3222. [16] I.A. Budagovsky, V.N. Ochkin, M.P. Smayev, A.S. Zolot’ko, A.Y. Bobrovsky, N.I. Boiko, A.I. Lysachkov, V.P. Shibaev, M.I. Barnik, Liquid Crystals 36 (1) (2009) 101. [17] H.C. Lin, C.W. Chu, M.S. Li, A.Y.-G. Fuh, Optics Express 19 (14) (2011) 13118.

Please cite this article as: A.V. Uklein, et al., Optics Communications (2013), http://dx.doi.org/10.1016/j.optcom.2013.01.011i