PHYSICAL REVIEW E 95, 052705 (2017) Highly efficient optical director reorientation of liquid-crystalline polymer induced by dye additives I. A. Budagovsky, V. N. Ochkin, S. A. Shvetsov, * and A. S. Zolot’ko † P. N. Lebedev Physical Institute, Leninsky Prospect 53, Moscow 119991, Russia A. Yu. Bobrovsky, N. I. Boiko, and V. P. Shibaev Faculty of Chemistry, Moscow State University, Leninskie Gory, Moscow 119991, Russia (Received 15 March 2017; published 31 May 2017) Light-induced director orientation of polymeric liquid-crystalline systems was investigated. The materials under study were composed of a nematic liquid-crystalline polymer (NLCP) and a small amount (0.05–0.5 wt.%) of conformationally active (azobenzene) or stable (anthraquinone) dye impurity. Light action on the homo- geneously aligned polymer films above glass transition temperature leads to the director reorientation and, consequently, to a change in the extraordinary refractive index. The effect is associated with the dye molecule excitation and related change of intermolecular forces. In the case of NLCP with conformationally active dye dopant, an extremely high orientational optical response was detected (nonlinear coefficient is n 2 ∼ 0.1 cm 2 /W). In contrast, the efficiency of orientational light action on NLCP with conformationally stable dye dopant is of the same order of magnitude as that of dye-doped low-molar-mass liquid crystals. At the normal light incidence on the NLCP doped with azo-dye, the threshold director orientation is observed which is similar to the Fréedericksz transition under the action of magnetic and electric fields. The obtained high-orientational optical response of NLCP caused by azo-dye dopant in combination with the possibility of the recording of deformed structure in the glassy state, typical for polymer compounds, reveals new opportunities in photonics applications. DOI: 10.1103/PhysRevE.95.052705 I. INTRODUCTION Smart stimuli-responsive materials, among which are nematic liquid-crystalline polymers (NLCPs), attract much attention due to the wide variety of possible practical ap- plications [1]. In this context, the light-induced orientation processes are of special interest, since light is a very convenient tool for the manipulation of structure and optical properties of condensed matter [2,3]. In these materials, high optical sensitivity is caused by the presence of light-absorbing fragments (chromophores). Due to the unique combination of photosensitivity and tendency to self-assembly the liquid- crystalline polymers are promising materials for developing optical memory systems, holographic materials, diffraction elements, etc. [4,5]. The first observation of a change in the orientation of the NLCP under the light action was reported in [6,7]. It was shown that the linearly polarized light incident on a planar film of NLCP with azobenzene light-absorbing fragments affects the polarization dependence of the absorption coefficient; the latter tends to minimum for the direction parallel to the light polarization. Although this resembles, at first glance, the director rotation of the absorbing NLCP, the phenomenon proved to be more complex. As shown in [8], the effect of linearly polarized light on an absorbing NLCP results in a decrease of the order parameter and, as in the case of amorphous azobenzene polymers, the formation of a negative optical axis (optical anisotropy n e − n o < 0) along the light field vector. Thus, the chromophore excitation leads to * Faculty of Physics, Moscow State University, Leninskie Gory, Moscow 119991, Russia. † zolotko@sci.lebedev.ru significant distortion and rearrangement of angular distribution of mesogenic groups of NLCP. Previously, studies of light-induced orientation of NLCP were performed on the highly absorbing systems containing large concentration (∼10 wt.%) of azobenzene side-chain groups [3]. In our recent communications [9,10] the orien- tation processes in optically transparent NLCPs and weakly absorbing mixtures of NLCPs and a small amount (<0.1 wt.%) of an azobenzene compound were reported. In this case, in contrast to highly absorbing NLCPs, the light-induced director reorientation without changing the order parameter was found in the NLCPs. In the transparent (undoped) NLCP, the director rotates to the light field due to the positive dielectric anisotropy at the light frequency. In this case, the refractive index of the extraordinary wave increases. In the dye-doped NLCP, the director rotation is due to the excitation of dye molecules. The typical value of light intensity required for the director reorientation is 4 orders of magnitude lower than that for the undoped NLCP. In this paper we report a detailed study of the opti- cal orientation in NLCPs with low-concentration absorbing dopants. The influence of light polarization on optical di- rector orientation is considered. Two types of dopants are used: the conformationally active azobenzene dye capable of trans-cis photoisomerizing and the conformationally stable anthraquinone dye. The comparison of light-induced response in the samples under study allows us to reveal features of orientational effects in polymeric systems. II. EXPERIMENTAL In the experiment, polyacrylate PAA (a comb-like polymer with mesogenic fragments; see Fig. 1) was used as an NLCP. Its degree of polymerization was 82; the polydispersity index 2470-0045/2017/95(5)/052705(6) 052705-1 ©2017 American Physical Society