New methods of highly efficient controlled generation of radiation by liquid crystal nanostructures in a wide spectral range

Abstract

Abstract. We report the recent results of research focused on a new kind of soft matter -the liquid-crystal nanocomposites with controllable mechanical and nonlinear optical properties. These are promising media for implementation of ultra-compact photonic devices and efficient sources of coherent radiation in a wide spectral range. We overview the technology of preparation of nematic-liquid-crystal media saturated with disclination defects. The defects were formed in different ways: by embedding nanoparticles and molecular objects, by exposure to alpha-particle flux. The defect locations were controlled by applying an electric field. We also present and discuss the recently discovered features of nematic-liquid-crystal media: a thermal orientation effect leading to the fifth-order optical nonlinearity, enormous second-order susceptibility revealed by measurements, and structural changes upon exposure to laser radiation. We report on efficient generation of harmonics, sum and difference optical frequencies in nematic-liquid-crystal media. In addition, transformation of laser radiation spectra to spectral supercontinua, and filamentation of laser beams were also observed in nematic-liquid-crystal media. We conclude that most nonlinear optical effects result from changes of the orientational order in the examined nematic liquid crystals. These changes lead to the symmetry breaking and disclination appearances.

Generation of harmonics and supercontinuum in nematic liquid crystals

“…The third optical harmonic in a nonlinear medium without inversion centre may arise both as a result of nonlinear mixing of three pump waves with the same frequency w (due to the cubic susceptibility) and in the cascade (stepwise) way during second-harmonic generation and nonlinear mixing of two waves with frequencies 2w and w [15]. This combined process with a susceptibility c casc ( ) 3 can be described in terms of effective cubic nonlinearity as a relation…”

“…The direction of optical axis in NLCs is related to the director, which is defined as the vector of preferred orientation of the long axes of NLC molecules and depends on many external and internal factors. The orientation of NLC molecules is determined by applied electrostatic and light fields (the Freedericksz effect [2]), heat flows (thermal orientation effects [16]), the shape of the volume occupied by the NLC and the boundary conditions on the surface [6], and the internal disclinations [3]. All these factors may lead to complex NLC anisotropy, inhomogeneous in space and time.…”

“…All these factors may lead to complex NLC anisotropy, inhomogeneous in space and time. Due to the nonlinear orientational susceptibility, spatial optical solitons (the so-called nematicons [17]) and filaments [3,17] can be formed in NLCs under laser irradiation; as a result, the general pattern of nonlinear optical conversions in NLCs is complicated even more.…”

“…Based on the positive results of the aforementioned preliminary experiments [3,11], it was concluded that an efficient laser frequency converter can be designed based on microscopic NLC volume, integrated with an optical fibre. The experimental optical scheme developed by us is shown in Fig.…”

“…Nonlinear optical properties of liquid crystals (LCs) [1,2] and their nanocomposites [3] are fairly complex and diverse. Study of these properties is of great interest for researchers specialising in photonics, nonlinear optics, physics of nanoparticles, laser physics, and information and telecommunication technologies.…”