Nonlinear optics of liquid crystalline materials

Patterns, fronts and structures in a Liquid-Crystal-Light-Valve with optical feedback

We present a comprehensive analysis of the evolution of a grating envelope, in both transmission and reflection geometry, during the erasure process in various readout configurations in bulk photorefractive media. We derive a single partial differential equation that describes grating evolution. This equation shows that under certain conditions the grating envelope propagates and shares several features with either bright or dark solitons. Details of the grating envelopes directly relate to the dynamics of the observed diffraction efficiency. We derive analytical expressions for diffraction during nondestructive readout. Finally, we discuss the effects of nonzero absorption, finite dark conductivity, fringe bending, fanning, and nonlinear responses on the propagating envelope.

Evolution and propagation of grating envelopes during erasure in bulk photorefractive media

Freedericksz transition can become subcritical in the presence of a feedback mechanism that leads to the dependence of the local electric field onto the liquid crystal re-orientation angle. We have characterized experimentally the first-order Freedericksz transition in a Liquid Crystal Light Valve with optical feedback. The bistability region is determined, together with the Freedericksz transition point and the Maxwell point. We show the propagation of fronts connecting the different metastable states and we estimate the front velocity. Theoretically, we derive an amplitude equation, valid close to the Freedericksz transition point, which accounts for the subcritical character of the bifurcation.

First-order Freedericksz transition and front propagation in a liquid crystal light valve with feedback

Results from investigations are summarized into: (1) transient refractive and absorptive (two-photon) nonlinearities at 0.532 mb y theZ-scan method, and (2) reflective nonlinearity in the near-IR, of linearly nonabsorbing cyanobiphenyl liquid crystals under nanosecond laser irradiation. (1) For isotropic liquid crystals at the several-nanosecond time scale and several tens-micrometers beam-waist-diameter, transient molecular-reorientation and thermal/density refractive nonlinearities compete in changing the sign of the total transient refractive nonlinearity. For the dierent, given pulse durations, the influence of coupled thermal and density eects on nonlinear refraction depends, through buildup time, on the beam-waist diameter. Nonlinear absorption coecients depend on the incident intensity. For planar nematic layers, cumulative eects in heating (and in refractive nonlinearity) were observed even at low, 2{10 Hz pulse repetition rate. These results are useful for optical power limiting applications, and for intensity and beam-quality sensors of pulsed, high-power lasers. (2) Reflective nonlinearity of chiral-nematic (cholesteric) mirrors near selective reflection conditions for circular polarized light at =1 :064 m was studied both under free space irradiation and inside a laser resonator. Specially chosen experimental irradiation conditions make it possible to attribute the observed changing of reflectivity to athermal helix unwinding by the optical eld. The results can nd applications in laser-resonator mirrors, Q-switches and soft apertures for beam-prole formation, and also in showing the limits of use cholesteric optical elements in high-power laser beams.

Nonlinear optical response of cyanobiphenyl liquid crystals to high-power, nanosecond laser radiation

A model to describe the fanning effect based on the beam-coupling mechanism in photorefractive crystals is presented. The intensity distribution of the fanning beam in a 45°-cut BaTiO3 crystal is measured experimentally. The results show good quantitative agreement with theory.

Fanning effects in photorefractive crystals.

A quantitative discussion of the laser-heating-induced self-phase modulation in nematic liquid crystals is given. The process of the laser-heating-induced nematic-isotropic transition is analyzed. The intrinsic optical bistability due to the nematic-isotropic phase transition is observed and analyzed.

Laser-heating-induced self-phase modulation, phase transition, and bistability in nematic liquid crystals.

A model to describe fanning effects in photorefractive crystals is presented. The intensity distribution of the fanning beam for one incident beam and two incident beams in 45°‐cut BaTiO3 crystals has been studied theoretically and experimentally, with good agreement. The theoretical results suggest that, when there is a fanning effect, only at intermediate effective interaction lengths does the signal beam have a high gain in two‐wave mixing. The transient behavior of the fanning beam in a 45°‐cut BaTiO3 crystal has also been studied theoretically and experimentally.

Theoretical and experimental studies of fanning effects in photorefractive crystals

A general two‐dimensional model to describe beam fanning, which takes into consideration the finite width of the input beam and the interaction between different directions of the fanning beam, is presented. By numerical calculation of the coupled equations, the well‐known curved path of a propagating beam inside photorefractive crystals is explained, and the propagation properties of the fanning beam inside the crystals are studied and discussed.

A two‐dimensional theory and propagation of beam fanning in photorefractive crystals

It is shown that optical-electrical feedback can transform the Freedericksz transition in a nematic film from second order to first order. The criteria that indicate whether the Freedericksz transition is first or second order are obtained. The hysteresis accompanying the first-order transition versus the feedback intensity is discussed. The feedback-induced first-order Freedericksz transition has been observed experimentally. The experimental results are in good agreement with the theoretical analysis.

Hong-jun Zhang

Papers

Fanning effects in photorefractive crystals.

Laser-heating-induced self-phase modulation, phase transition, and bistability in nematic liquid crystals.

Theoretical and experimental studies of fanning effects in photorefractive crystals

A two‐dimensional theory and propagation of beam fanning in photorefractive crystals

Feedback-induced first-order Freedericksz transition in a nematic film.