Hong-jun Zhang

Fanning effects in photorefractive crystals.

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 degrees -cut BaTiO(3) crystal is measured experimentally. The results show good quantitative agreement with theory.

HALL-PETCH RELATIONSHIP IN THE NANOCRYSTALLINE SELENIUM PREPARED BY CRYSTALLIZATION FROM THE AMORPHOUS STATE

Bulk nanocrystalline selenium (nc‐Se) specimens with various grain sizes were prepared by isothermally crystallizing as‐quenched amorphous selenium. Microhardness measurements indicated that with a decrease in grain size, from 25 to 9 nm, hardnesses of nc‐Se samples increase dramatically from 0.34 to 0.98 GPa, i.e., a normal Hall–Petch relationship is obtainable over the grain size range studied for nc‐Se samples. Structural characterization of nc‐Se using x‐ray diffraction revealed an evident lattice distortion in the nanocrystalline state, namely the lattice parameter a is increased, whereas c is decreased simultaneously with a reduction of grain size. The variation tendency of microhardness with grain size can be well correlated with the change of the unit cell volume in nc‐Se, which implies that the lattice structure of the nanocrystalline state is a factor affecting the mechanical properties.

THEORETICAL AND EXPERIMENTAL STUDIES OF FANNING EFFECTS IN PHOTOREFRACTIVE 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.

A two‐dimensional theory and propagation of beam fanning 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.

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

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

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.

Frequency locking in optical bistability with competing interactions

We study the unstable behavior of a liquid-crystal hybrid bistable device with two time delays in the feedback mechanism. We report the observation of frustrated instability, frequency locking, and hysteresis on varying the control parameters. The results of our experiments are in agreement with the theoretical analysis presented by Ikeda and Mizuno [Phys. Rev. Lett. 53, (1984)].

Mulitigrating optical phase conjugation with considerations of phase effects

We develop a numerical method to solve the coupled-wave equations for four-wave mixing phase conjugation in a photorefractive crystal with unequal complex coupling constants; by this method we can obtain both the intensities and the phases of various beams. We discuss the intensities and the phases of the interacting beams in a photorefractive crystal for the cases in which the phase shifts between the refractive-index gratings and the interference pattern are all π/2 and in which they are not all π/2.

Quasi-periodic motion in a hybrid optical bistable system with a short delay

The dynamical behavior of a hybrid optical bistable system with a short delay is calculated numerically in detail. The quasi-periodic motion and frequency locking of the system are observed experimentally.

Controlling chaos in nonlinear optical systems

Using a one-dimensional piecewise function mapping, we have developed a method for controlling chaos in an optical bistable system theoretically and numerically. This method could be applied to other nonlinear dynamical systems.