ISSN 1063-7745, Crystallography Reports, 2013, Vol. 58, No. 1, pp. 144–149. © Pleiades Publishing, Inc., 2013. Original Russian Text © M.I. Barnik, A.R. Geivandov, S.P. Palto, V.S. Palto, B.A. Umanskii, N.M. Shtykov, 2013, published in Kristallografiya, 2013, Vol. 58, No. 1, pp. 128–134. 144 INTRODUCTION One important property of chiral liquid crystals (CLCs) is their ability to form, under certain condi- tions, a single-domain layer with a helical distribution of the director (local optical axis) [1–3]. The geometry in which the heliõ axis is oriented along the normal to the layer is of special interest. In this case, the single- domain CLC layer is one-dimensionally inhomoge- neous. In any plane parallel to the layer plane (xy), the molecules are oriented in the same way, undergoing helical twist only along the layer normal (z axis) (Fig. 1a). The helicity of the supramolecular structure, characterized by a helix pitch P, together with the optical anisotropy Δn = n || – n ⊥ , related to the differ- ence in the principal refractive indices n || (along the director) and n ⊥ (perpendicular to the director), give rise to numerous optical effects, which can be used in optical devices and laser technology [4, 5]. For exam- ple, a large variety of electro-optic effects in both fer- roelectric liquid crystals (LCs) and nematic CLCs is related to the helix deformation in an electric field (Fig. 1b) directed perpendicular to the helix axis [6– 8]. According to [8], the weak higher harmonics that are produced in the spatial distribution of the director under an electric field result in significant changes in the light polarization state at the CLC layer output. Thus, in the case of nematic CLCs, the spatial distri- bution of the director in the electric field is character- ized by not only the fundamental spatial frequency q 0 = 2π/P, but also odd harmonics with frequencies q m = 2π(2m + 1)/P, where m are natural numbers. In the case of a helix pitch comparable to visible wave- lengths, we can speak of induced additional “nano- structured” deformation of the CLC director field. The relaxation times of the induced deformation are very fast due to the extreme smallness of its period. The times appear to be shorter by several orders of magnitude than the times of traditional electro-optic effects in nematic LCs (NLCs). However, as the study [8] showed, for a helix pitch of about ~1 μm, the light at the output of a nematic CLC layer has a very high spectral dispersion of polarization states and can- not be blocked by a polarizer (analyzer) in a wide spec- tral range. Thus, the fast electro-optic effect is appli- cable only in a very narrow spectral range. In [9], the LIQUID CRYSTALS Compensation for the Spectral Dispersion of Light Polarization States in Electro-Optic Modulators Based on Chiral Liquid Crystals M. I. Barnik, A. R. Geivandov, S. P. Palto, V. S. Palto, B. A. Umanskii, and N. M. Shtykov Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia e-mail: palto@hotmail.ru Received March 23, 2012 Abstract—The effect of compensation for the spectral dispersion of light polarization states at the output of a single-domain layer of a chiral liquid crystal (CLC) is experimentally studied. It is shown that such disper- sion can be decreased significantly with the aid of phase plates of two types that have different signs of the spectral dispersion of birefringence. The dispersion compensation allows one to significantly increase the operating spectral range of fast light modulators based on chiral nematic liquid crystals (NLCs). DOI: 10.1134/S1063774513010045 (a) E = 0 (b) E x y Fig. 1. Schematic representation of the helical distribution of LC director (the director indicating the preferred local orientation of molecule axis is shown as a cylinder) with (a) switched-off and (b) switched-on electric fields (indi- cated by arrows). One helix turn is shown.