Abstract
Waveguide optical properties of a cholesteric liquid crystal (CLC) layer with a deformed lying helix (DLH) have been studied by numerical simulations using the finite difference time domain method. The DLH structure, when the helix’s axis is oriented in plane of a CLC layer, is induced by an electric field in a virtual CLC cell with periodic (planar/homeotropic) boundary conditions at one of the alignment surfaces. This in-plane helical structure is stable only in a permanently applied electric field providing the helix deformation. In this work the polarized light reflectance spectra have been studied at different electric fields and light impingement into a waveguide formed by the DLH layer. It is found that for light propagating along the helix axis the reflectance spectrum has multiple stop-bands centred at wavelengths , which is different from set of bands located at , and characteristic of CLC spectra for the Grandjean-plane textures subjected to distortion by an electric or magnetic field perpendicular to the helix axis, where j is a natural number, p is the helix pitch and is the average refractive index. Each of the higher order (j > 1) bands consists of three polarization-dependent sub-bands. In the case of an amplifying CLC DLH layer, depending on an extent of the helix deformation, the lasing modes can be excited at different edges of the sub-bands. While at the strongest deformation the lasing is preferable at the edges of the central sub-band; a lower extent of deformation makes favourable conditions for the lasing at edges of the two other sub-bands.
Highlights
Aligned cholesteric liquid crystals (CLCs) are well-known representatives of one-dimensional photonic crystals
The director field is close to that characteristic of the the Granjean-plane texture with the helix axis along the layer’s normal plane
The numerical simulations show that the optical properties of the deformed lying helix (DLH) layers are significantly richer compared to ones of the CLC Granjean-plane textures distorted by an electric field perpendicular to the cholesteric helix axis
Summary
Aligned cholesteric liquid crystals (CLCs) are well-known representatives of one-dimensional photonic crystals Their photonic properties are due to both the optical anisotropy and helical structure of the director field, which result in an appearance of a spectral band gap (stop-band), within which the light propagation along the axis of the cholesteric helix is forbidden for one of the circular polarizations [1]. The spectral structure of stop-bands and lasing modes which can be excited at different magnitudes of the applied electric field for light propagating in a waveguide regime are studied. It is worth it to mention that in [17,18], it was theoretically shown that in case of a magnetic or electric field applied perpendicular to the helix’s axis the optical spectrum is characterized by the appearance of higher order stop-bands. The field-induced bands’ composition and their spectral structure remain unknown in case of the DLH layers due to both the waveguiding regime and specific features of the director field deformation
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