Zhaohong Liu

Spatial modulation instability in self-defocusing photorefractive crystal LiNbO3:Fe

We report on the experimental observation of the modulational instability of quasi-plane-wave beams in the self-defocusing photorefractive crystal LiNbO3:Fe. Our analysis shows that this modulational instability is ascribable to the nonlocal response of the refractive index to the optical illumination. Theoretical results indicate that the perturbation period depends on the ratio of the optical beam intensity to that of the dark irradiance and, moreover, the perturbation period rises and reaches a saturated value as the intensity ratio r increases. Experimental and theoretical results have good agreement for the perturbation period.

Transmission of digital images consisting of white-light dark solitons

A distortion-free digital image is transmitted through parallel propagation of white-light photovoltaic dark solitons. The waveguide channels induced by white-light dark solitons can guide both the laser beam and the white-light beam well. We determine experimentally the critical separations of the dark solitons in the directions parallel and perpendicular to the crystalline c axis for the given crystal thickness.

Gap solitons in optically induced two-dimensional square photonic lattices in LiNbO3:Fe crystals

We fabricated two-dimensional square photonic lattices with a backbone structure in self-defocusing LiNbO3:Fe crystals through optical induction. We systematically observed four kinds of gap solitons and their power spectra in the nonlinear photonic lattices. Our results show that the lightwave is confined more tightly when it is launched into a low-index region at the input face of the crystal than when it is launched into a backbone ridge waveguide.

Simple optical method for determination of crystal orientation in photorefractive crystals

A simple optical method for determination of crystallographic orientations +/-x, +/-y, and +/-z in photorefractive crystals based on light amplification via backward two-wave mixing (TWM) is presented. We consider a photorefractive Fe-doped lithium niobate crystal (LiNbO(3):Fe) with a standard x-y-z cut as an example to illustrate the principle, manipulation, and experimental results of this method in detail. Also we see that the direction of the light amplification, namely, the direction of the nonreciprocal steady-state energy transfer, strongly depends on the direction of the grating vector with respect to the corresponding crystallographic axes mainly under the diffusion mechanism. Finally, a way of determining the sign of the charge carriers in photorefractive materials is proposed.

Propagation of Gaussian beam in self-defocusing photovoltaic photorefractive nonlinear crystal of LiNbO3:Fe

We investigate experimentally the propagation of Gaussian beam in self-defocusing photovoltaic photorefractive nonlinear crystal of iron-doped lithium niobate. In steady-state, the propagating beam in the low-refractive-index optical channel written by the beam itself is affected mostly by linear diffraction and anisotropic divergence of the saturated negative lenslike effect. And the latter affects the incident beam with convergent wavefront much more than that with divergent wavefront. In the case of the incident beam with the convergent wavefront, the angular width of the spatial spectrum of the output beam decreases along with the size of the output beam. We believe that the phase-type spatial filtering of the funnel-type low-refractive-index optical channel written by the propagating beam itself causes the results.