Selective modulation of infrared radiation by an Al2O3 particles — liquid crystal system

Abstract

Introduction. Liquid crystals (LC) are anisotropic optical media which have found broad application, in particular in LC displays and other photonic devices in the visible range. Little attention has been focused on studying LCs for conversion of radiation in the IR region of the spectrum, even though CO2 and other IR lasers are used in industry and medicine. Well-oriented nematic LC samples are strongly birefringent media, and are characterized by ordinary and extraordinary refractive indices no and ne. An electric field can change the orientation of LC molecules (the Frederiks effect), leading to a change in the effective refractive index along the direction of propagation of the light. In this case, the long axes of molecules of an LC with positive dielectric anisotropy are aligned parallel to the field direction, while in an LC with negative dielectric anisotropy they are aligned perpendicular to the field. This is a fundamental property of liquid crystals, and the basis for the operating principle of most photonic devices. Earlier in [1] a method was proposed for controllable filtration of IR radiation, based on a combination of the Frederiks effect and the Christiansen effect. The latter effect arises if particles are dispersed in a sufficiently transparent medium and the particle sizes are comparable with the wavelength of the incident light. In this case, the system transmits light in a narrow range of wavelengths, where the refractive indices are close for the particle material and the medium in which the particles are dispersed. Using LCs made it possible to control the position of the transmission maximum of the system when an electric field is applied. Application of this effect is also possible for a fixed value of the applied voltage (by varying its frequency). In this case, we propose using a dual-frequency LC as the medium in which the particles are dispersed, where the LC has positive anisotropy of the dielectric constant at low frequencies of the applied electric field and negative dielectric anisotropy at high frequencies. Aluminum oxide is used as the particle material; the refractive index of aluminum oxide varies considerably in the IR region of the spectrum [2]. In this paper, we present the results of a study of the Christiansen effect in an aluminum oxide particles — dual-frequency LC system.