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Liquid crystal light valve with GaAs substrate operating in the transmission mode in the infrared is studied. The nonlinear phase shift of the transmitted light wave is measured as a function of applied voltage. The dynamic grating recording is achieved. A fourfold amplification of the weak signal beam is reached. The gain is increased by means of proper tilting of the cell that increases an effective pretilt of the liquid crystal molecules. The amplitude of the refractive index modulation and nonlinear coupling constant are estimated from the experimental results.

The transfer matrix method is used to analyze induced reflection gratings in photorefractive iron doped lithium niobate in a self-pumped configuration. The optical field distribution and the induced refractive index distributions inside the material are computed, and the overall transmission and reflection are determined for different orientations of the c-axis. Numerical simulations are compared with experimental results.

The complexity of photorefractive polymers arises from multiple contributions to the photo-induced index grating. Analysis of the time dynamics of the two-beam coupling signal is used to extract information about the charge species responsible for the grating formation. It has been shown in a commonly used photorefractive polymer at moderate applied electric fields, the primary charge carriers (holes) establish an initial grating which, however, are followed by a subsequent competing grating (electrons) that decreases the two-beam coupling efficiency. We show by upon using higher applied bias fields, gain enhancement can be achieved by eliminating the electron grating contribution and returning to hole gratings only.

Bragg gratings yield a single diffracted order when irradiated by a coherent beam at the appropriate Bragg angle. In many cases, nearly all of the energy of the incident beam can be coupled to the diffracted beam. Hence these gratings can form many useful optical elements, and this has been realized in 1-D, 2-D, and 3-D photonic crystals. Bragg gratings made with liquid crystals offer the added dimension of dynamic properties through the large electro-optical effect in liquid crystals. Applications for spatial light modulators are numerous, including optical switches, modulators, active optical elements (e.g., lenses), laser sources, and tunable filters. We have been exploring a number of approaches for making liquid crystal Bragg gratings, including holographic polymer-dispersed liquid crystals, cholesteric liquid crystals, and homogenous nematic liquid crystals in hybrid devices. We have studied the dynamic properties of these Bragg gratings by electrical, thermal, and optical stimulation. Modification and control of optical and dynamic properties have been obtained through combinations of liquid crystals with polymers, combinations of various dopant materials, and interactions of liquid crystals with organic and inorganic interfaces. We discuss the materials, fabrication, characterization, and physics of liquid crystal Bragg gratings and present the results of various devices we have studied in our lab. We will also discuss potential applications.

By using split-step beam propagation method, self-pumped contra-directional two-beam coupling is simulated in a photorefractive medium for arbitrary shaped beams. The simulation shows a positive influence of the photovoltaic effect on the two-beam coupling efficiency, in agreement with published experimental observations.

We demonstrate beam shaping in a hybrid acousto-optic device with adaptive electronic feedback. Cases of positive and negative feedback, and from the un-diffracted and diffracted orders are investigated. We also show the dependence of the final value of the induced grating strength in the acousto-optic cell on the feedback parameters.

Contradirectional two-beam coupling was achieved using a single laser beam incident on fibers grown from congruent iron doped lithium niobate using the laser heated pedestal growth technique. To our knowledge, this is the first time that a grating has been written in a fiber using a single laser beam and its Fresnel reflection in a 180 degree(s) contradirectional two-beam coupling geometry. Beam-coupling efficiencies were compared as a function of beam spot size for fibers and bulk crystals. At small spot sizes, the fibers outperformed the bulk crystals by reducing the diffraction effect.

In this work we present the experimental results of contradirectional two-beam coupling in a bulk crystal and single crystal fibers of iron-doped lithium niobate. Results of a reduction of the grating writing instability, a comparison of the two-beam coupling efficiency of the fibers and bulk crystal as a function of focusing geometry, as well as a comparison of theoretical and experimental results of the two-beam coupling efficiency are presented.