Nonlinear and electro-optical properties of liquid crystal systems doped with gold nanorods

Abstract

The present study is devoted to the investigation of the reorientation and nonlinear optical properties of 8CB liquid crystal samples doped with gold nanorods with distinct width to length ratios (aspect ratios). By using the Z-scan technique, the mechanisms behind phase distortions and nonlinear absorption by samples under homeotropic anchoring conditions are evaluated. Our results reveal that thermal effects are the leading phenomena causing distortions in the incident beam wavefront for normal light incidence. When the oblique incidence regime is concerned, nonlinearities in the refractive index play a larger role in phase shifts, due to the reorientation of the far-field director caused by the electric field of impinging light. The temperature dependence of χ (3) nonlinearities are investigated, where an anomalous behavior of the nonlinear refractive index is observed close to the nematic-smectic-A phase transition. In addition, a gradual increase in magnitude of the nonlinear absorption coefficient appears at temperatures where the smectic order is well-established. Furthermore, the effects of gold nanorod aspect ratio in the viscoelastic properties of twisted nematic cells under an AC electric field are explored, with a reduction in the Freedericksz threshold voltage being accounted as impurities are introduced in the sample. A decrease of reorientation times is reported for samples containing gold nanorods compared to the undoped liquid crystal, with a further reduction in the system containing more elongated nanoparticles. The longitudinal plasmonic extinction band was also excited in order to evaluate the contributions of the surface plasmon resonance to switch times. The Gooch-Tarry conditions and Mauguin minima are also exploited to extract the thermal dependence of the nematic birefringence of samples, where a decreased discontinuity in the nematic order parameter is observed for doped materials. A thorough discussion of the interplay between the introduction of gold nanorods and the liquid-crystalline order is performed, unveiling the possibility of novel applications for plasmonic systems.