P. Y. Yan

Optically-induced molecular reorientation in nematic liquid crystals and nonlinear optical processes in the nanoseconds regime

We present a detailed theoretical analysis and experimental study of purely optically-induced nematic axis reorientation and the associated nonlinear optical processes such as self-phase modulations and optical switching. It is shown that under a sufficiently intense laser field (∼ 100 MW/cm2), nematic liquid crystals will respond in the nanosecond regime. Single nanosecond laser pulse self-phase modulation and intensity switching effects are observed. The magnitude and dynamics of the response are in agreement with theoretical expectations. Optical power limiting and switching effects associated with self-phase modulations are also demonstrated.

Low-power (10.6-μm) laser-beam amplification by thermal-grating-mediated degenerate four-wave mixing in a nematic liquid-crystal film

We present a detailed theoretical analysis of degenerate four-wave mixing by a laser-induced thermal grating in a nematic liquid-crystal film. In particular, we show that the coupling of the (strong) pump beam to the first-order diffracted beam can give rise to substantial amplification of a (weak) probe beam. Experimental verification of this effect with a CO2 laser beam is also made. A probe-beam gain of greater than 20 can easily be observed in a 120-μm film with a pump intensity of the order of a few watts per square centimeter.

Simultaneous occurrence of phase conjugation and pulse shortening in stimulated scattering in liquid crystal mesophases

We have observed for the first time simultaneous occurrence of phase conjugation and pulse shortening in stimulated back scattering of nanosecond laser pulses from thin film of smectic and nematic liquid crystals, and from thick samples of isotropic cholesterics. Aberration correction capability and high compression ratio are obtained.

Thermal grating-mediated wave mixing and beam amplification in nematic liquid crystal thin films

A model is presented of thermal grating-mediated wave mixing and amplification. The model includes a strong pump beam, a weak probe beam, and a first-order diffracted beam. The coupled Maxwells wave equations and the thermal diffusion equation are solved using a self-consistent formalism. The influence of various input beam parameters (the pump to probe beam intensity ratio, beam intensities, crossing angle, wavelength) and sample parameters (the thermal nonlinear coefficient, thermal conductivity, sample thickness) on the wave mixing effects is considered. Some recently observed infrared beam amplification effects have been qualitatively described by the theory of the optimum configuration for signal (probe) beam amplification with nematic liquid crystals. The results are important for optical phase conjugation and self-oscillation processes involving infrared lasers, and demonstrate the particular usefulness of liquid crystals for these applications. >

Laser Spot Size Dependence, Nonlocality And Saturation Effects In Transverse Optical Bistability

We present here a theoretical discussion of the effect of nonlocality and saturation in transverse optical bistability. It is found that the switching conditions, switching power and characteristics are considerably different from the situation where these effects are not accounted for. We also include an analytical study of the transverse self-phase modulation effect and relate the key parameters explicitly to the transverse bistability switching condition and bistability.

Infrared Optical Wave Mixing And Beam Amplification With Liquid Crystal Nonlinearity

We present a detailed theory and experimental study of infrared optical wave mixing based on thermal nonlinearity in nematic liquid crystal films. Because of the longer wavelength of infrared laser, lower scattering loss, transparency and other unique physical characteristics, very efficient degenerate optical wave mixing effects can be realized in nematic liquid crystal films. Applications to fairly fast (submillisecond) phase conjugations, beam and image amplification and self-pumped phase conjugation are also discussed.