Efeitos da adição de nanopartículas sobre as propriedades ópticas de cristais líquidos

Abstract

The present study is devoted to the investigation of the nonlinear optical properties of a smectic liquid crystal doped with gold nanorods. By using Z-scan technique, we investigate the changes in the optical birefringence of a homeotropic sample upon the laser exposure, considering the configuration of normal and oblique incidence. Our results reveal that the birefringence variations may be governed by distinct physical mechanisms, depending on the relative angle between the far-field director and the wavevector of the excitation laser beam. In particular, we observe that the position dependence of the far-field transmittance exhibits different behaviors as the incidence angle is changed, indicating that distortions in the beam wavefront may be associated with the thermal lens phenomenon or an optical-induced reorientation of the nematic director. The temperature dependence of the nonlinear refractive and absortive coeffients are investigated close to the smectic-A phase transition. A detailed analysis of the interplay between smectic order and plasmon resonance is performed, thus unveiling the capability of plasmonic liquid crystal to be used in optical devices. We also studied the nonlinear optical absorption of a smectic liquid crystal doped with gold nanorods in the presence of an external electric field. By using Z-scan technique in the configuration of fully open aperture and normal incidence, we analyze the changes in the absorption coefficient of a homeotropic sample upon laser exposure. Our results reveal that the electric field may suppress the nonlinear absorption of the samples well bellow to smectic-A-nematic transition temperature. The effects of the external electric field on the interplay between smectic order and plasmon resonance are analyzed. We have also started studies on the effects of adding carbon nanoparticles, on the reflection band of cholesteric liquid crystals, using optical spectrometry.