R. R. Michael

Dynamics of picosecond laser‐induced density, temperature, and flow‐reorientation effects in the mesophases of liquid crystals

A detailed theoretical and experimental study of laser‐induced density and temperature changes, and flow‐reorientation effects in the nematic and smectic phases of liquid crystals is presented. Using picosecond lasers, the initial nanosecond dynamics of the photoinduced density waves, temperature buildup, and relaxations are temporally resolved. The experimentally observed relaxation phenomena and time scales are in good agreement with the theoretical expressions obtained by analytical solutions of the coupled hydrodynamical equations describing these fundamental mechanisms. Our new measurement and theory provide a quantitative account of the relative contribution from the electrostrictive and thermoelastic contributions that had not been presented in previous studies. Our study of the smectic phase has conclusively established the mechanism for the formation of erasable and permanent grating effects under short‐laser‐pulse excitation as laser‐induced electrostrictive and thermoelastic effects.

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.

Experimental studies of the dynamics and parametric dependences of switching from total internal reflection to transmission and limiting effects

We present experimental results concerning the process of switching from the total-internal-reflection to the transmission state at a glass–nematic-liquid-crystal interface. The results are in agreement with our detailed theory of the dynamics of interface switching [ J. Opt. Soc. Am B.6, 884 ( 1989)]. An optical limiting effect, caused by the transverse nonlinear phase shift that is associated with the Gaussian profile of the laser beam, is also observed.

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.

Picosecond-millisecond optical nonlinearities of liquid crystals for limiting and switching applications

We present the results of a detailed study of the optical nonlinearities of liquid crystals spanning the picosecond-millisecond time scale. These nonlinearities are utilized in some optical limiting/switching applications with infrared and also visible lasers.

The Mechanism And Dynamics Of Infrared Nonlinear TIR-Transmission Switching With A Liquid Crystal Film

We have developed a detailed theory for the two possible modes of switching at a nonlinear film bounded by two dielectric media. Because of the fact that the film is thin, and the high reflectivity of the boundary surfaces near the total internal reflection state, several factors have to be taken into account. These include reflection feedback, nonlinear Fabry Perot effects, nonlocal responses, intensity and angular dependences of time scales and switching threshold, optimal transmission, etc. Using liquid crystal as the nonlinear medium we have also performed experimental verifications of the theory.

Infrared Nonlinear Optical Power Limiting With A Nematic Liquid Crystal Film

We present a detailed theory of thermally induced transverse self-phase modulation and optical limiting with nematic liquid crystal films at CO2 (10.6μm) wavelengths. Our theory accounts for the detailed roles played by the laser parameters (such as intensity, polarization, wavelength, etc.), sample parameters (such as the thermal index gradient, thermal conductivity thickness, etc.) and the geometrical placement of the lens, sample and detector. The experimental results are in good agreement with the theoretical predictions. Our theory and observation enable us to determine optimal configurations and physical parameters for practical application of these processes.

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.

Four Wave Mixing With Gain Using Liquid Crystal Films

The extraordinarily large thermal and orientational nonlinearities of nematic liquid crystal is studied in the context of amplified reflection in four wave mixing. Greater than 100% reflection in wavefront conjugation can be obtained.