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In our proposal a light intensity distribution generated by an incoherently illuminated planar amplitude grating is projected into a photorefractive crystal. This 3D distribution is mapped as an index refractive perturbation via the photorefractive effect thereby generating a volume phase grating. The self-imaging phenomenon in the Fresnel field of this volume phase grating coherently illuminated is theoretically and experimentally analyzed. A model to simulate this volume grating that considers the 3D light intensity distribution formed in the crystal combined with the photorefractive grating formation theory is proposed. A path-integral approach to calculate the self-image patterns which account for the inhomogeneous propagation through the photorefractive grating is employed. The experimental and theoretical results show that the self-images location coincides with that of the self-images generated by planar phase grating of the same period. Moreover, the self-images visibility depends on three parameters: the exit pupil diameter of the incoherent recording optical system, the external electric field applied on the crystal, and the crystal thickness. To study the visibility behavior, a phase parameter which includes the three mentioned parameters is proposed. The self-images visibility shows the typical sinusoidal dependence found in planar phase grating. A good agreement between theoretical and experimental results is observed.

We present the evolution equation of surface waves due to the two-photon photorefractive effect. It is shown theoretically that the surface waves can be supported by two-photon photorefractive media. The spatial frequency of photorefractive surface waves can be changed by an applied external electric field, which indicates a method for transforming the surface waves between high-frequency modes and low-frequency modes.

We report on a tunable color filter with surface plasmon resonance (SPR), excited by a photorefractive (PR) diffraction grating. When a white light was incident at the diffraction grating formed by a PR effect, the SPR generated at a metal-dielectric material interface was absorbed, and the reflected light showed a complementary color. When the period of the PR diffraction grating was modified by alteration of the optic configuration, the wavelength at which the SPR excitation led to a reflection minimum changed and the spectrum of the reflected light also changed. A well-known equation was used to help us understand the experimental results. All experimental results are in good agreement with the calculation predictions. This result could lead to a new type of tunable color filter.

Binary information in the form of a 256 x 256 pixel array has been stored holographically within a polymeric photorefractive composite of 130-mum thickness. Devices used consisted of 55-wt. % organic chromophore 1-(2?-ethylhexyloxy)-2, 5-dimethyl-4-(4?nitrophenylazo) benzene and up to 11.2-wt. % 2, 4, 7-trinitro-9-fluorenone (TNF) dispersed within a poly(N-vinyl carbazole) matrix. In a degenerate four-wave mixing arrangement, the refractive-index modulation and speed were 10(-3) and 200 ms, respectively. The high TNF concentration leads to short digital hologram recording times of 2 s without detriment in optical quality. Although not yet optimized, a figure of merit, M#, for holographic storage in this composite has been measured to be 0.017, and this is estimated to reach 0.18 in a revised experimental geometry.

A cw-probe Z-scan technique was employed to measure the photoinduced index change in a photorefractive SBN:60 crystal. For this experiment a three-detector data-acquisition system was used to account for temporal changes in the laser. The effects of various beam parameters such as intensity, polarization, and wavelength were studied. A theoretical simulation of the Z scan based on a band-transport model of photorefractive-index variation was also developed. This model provides reasonable agreement with the experimental results.

It is demonstrated that Er doping increases the photorefractive effect in LiTaO3. Raman and fluorescence spectra are used to establish that the upconverted green emission of Er3+ clusters acts as a gating source, significantly increasing the two-photon photorefractive effect of a monochromatic red or near-infrared pump source.

Infrared imaging materials with high speed and dynamic range are required for a variety of imaging and diagnostic techniques. We present an organic photorefractive glass that shows high diffraction efficiencies of as great as 40% with 25-mum-thick samples and fast response times of 4.3 ms under irradiation at 790 nm. Furthermore, the intensity dependence and field dependence of the grating decay are investigated.

A photorefractive incoherent-to-coherent optical converter (PICOC) is demonstrated; conversion is accomplished by anisotropic self-diffraction in BaTiO(3). The setup of the PICOC is easy, and only two writing beams are required. The diffraction efficiency reaches 50%, and the resolution is 22 line pairs (lp)/mm in a typical-size crystal. Further, the resolution reaches 40 lp/mm when a BaTiO(3):Rh crystal of thickness 1.2 mm is used, and the diffraction efficiency is as high as 51%. The resolution of the PICOC can be increased effectively by reduction of the crystal thickness with no penalty for low diffraction efficiency.

We show theoretically that the group velocity of light can be slowed down by means of phase coupling in the photorefractive two-wave mixing process. It is shown that the group velocity of light propagating in a photorefractive material could be significantly reduced because of a steep variation of the phase-coupling coefficient with respect to the angular frequency difference between two coupling beams in a two-wave mixing process. The results for the case of a photorefractive Bi12SiO20 crystal are presented and discussed in detail.

Detailed analysis of the relationship between the experimental geometry and the holographic contrast in photorefractive polymers is important for applications, such as angle multiplexing in holographic data storage. In this paper the field dependent photogeneration efficiency is introduced into the complete reorientational model to provide a full account of the electric field and geometrical dependence of the index contrast. The interaction of a local grating and the photorefractive grating is also considered. A simplification for acute angles between writing beams is described. Experimental verification by use of four-wave mixing and transmission ellipsometry reveals an excellent agreement between theory and measurement.

We have characterized a two-wave mixing adaptive interferometer based on dc-biased photorefractive CdTe:Ge crystal at 1.06 and 1.55 μm. Excellent performance is shown at both wavelengths by demonstration of high sensitivity for measurement of small displacements and high cutoff frequency at low intensity. We have achieved a considerable reduction of the undesired low-frequency response using controlled heating of the crystal, which ensures depopulation of corresponding traps. The experimental data are used for measurement of the real and imaginary parts of the coupling constant, as well as the dielectric relaxation time of the crystal and the mobility-lifetime product of the free charge carriers.

Recording of efficient reflection holograms is achieved in CdTe:Ge at λ=1.064 μm and λ=1.55 μm. The gain factor measured at both wavelengths for counterpropagating two-beam coupling considerably exceeds the absorption constant and transcends all values previously reported for semiconductors with no external field. The dependences of the gain factor on intensity and grating spacing are studied. Some crystal characteristics are estimated in the frame of a single band, one mobile species approximation of space-charge formation. The homogeneity of photorefractive properties in the crystal volume is demonstrated.

Optical nonlinearities of periodically poled LiNbO(3) crystals were investigated by the single beam Z-scan technique with a continuous wave (cw) laser beam at 532 nm. The nonlinear optical absorption coefficient and refractive index change are determined to be 8.1 x 10(-6) cm/W and 2.6 x 10(-4) at 0.5 MW/cm(2) light intensity, respectively. Both sign and magnitude of the measured refractive nonlinearity are considerably different from the Z-scan results in congruent LiNbO(3). The nonlinearities in the periodically poled LiNbO(3) induced by 532 nm continuous waves are believed to be mainly due to the photorefractive effect.

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.

The influence of composition on the photorefractive effect in pure LiNbO3 crystals at low light intensity was investigated. The experimental results indicate that different defects dominate the photorefractive centers of pure LiNbO3 with various compositions. Bipolarons are considered to be responsible for the enhanced photovoltaic field in reduced near-stoichiometric LiNbO3, and their bulk photovoltaic constant kappa is estimated to be approximately 6.95 x 10(-32) m3/V. Q polarons (composed of two bipolarons) are introduced to explain the photorefractive effect of congruent LiNbO3 at both low and high light intensities.

The spatial distribution of the power transfer achieved by contradirectional two-beam coupling using self-pumped photorefractive reflection gratings is investigated in two materials with different photorefractive gain coefficients, LiNbO3:Fe and KNbO3:Fe. Incremental portions of the volume grating are erased optically by inducing thin optical damage planes, reducing the overall two-beam coupling efficiency. By monitoring the effect of local grating disruption, the distribution of power transfer is spatially resolved throughout the crystal, and the results are found to be in agreement with our theoretical predictions.

We investigated the effect of an applied electric field on the Bragg condition of degenerate four-wave mixing in a polymeric photorefractive material with a low glass-transition temperature. For a polymeric photorefractive material the application of an external electric field as is necessary for photorefractivity leads to birefringence of the material by poling of the nonlinear optical chromophore. Because the propagation vectors of the pumping and reading beams inside the material are influenced by the refractive index of the material, the Bragg condition depends on the magnitude of the external field. Using an oriented gas model and the-coupled-mode theory, we numerically analyzed the Bragg-mismatch effect that causes a reduction in diffraction efficiency as a function of an external field. We present the boundary conditions for sample thickness and grating spacing for which the Bragg-mismatch effect must be taken into account.

We have constructed an all-optical tracking novelty filter based on the dynamic holographic properties of an efficient and fast infrared-sensitive photorefractive polymer. The photorefractive polymer was used in a two-beam coupling geometry. The polymer had a gain coefficient of 175 cm-1 at a wavelength of 780 nm and an applied field of 72 V/µm. In contrast to what has been observed in photorefractive crystals, the gain coefficient and the filter contrast are largely independent of the writing beam’s intensity ratio. We show images of a swinging pendulum observed through the novelty filter.

Incoherent-to-coherent conversion by use of the photorefractive beam-fanning effect and amplification by two-wave coupling have been demonstrated experimentally in a photorefractive Ba1-xSrxTiO3 crystal. The converted coherent image is amplified by 60×. The resolution of the amplified coherent image is ∼28 line pairs/mm. A notable advantage of the method is that incoherent-to-coherent conversion and coherence amplification of the converted coherent image are realized simultaneously in a photorefractive crystal.

We demonstrate that high-resolution incoherent-to-coherent conversion can be carried out in an incoherent-to-coherent converter based on the photorefractive fanning effect. The output coherent image has the same direction as the incident coherent beam, and thus the resolution is not limited by the grating diffraction. We obtained a resolution as high as 90.5 line pairs/mm in this experiment. The transmittance of the coherent beam relative to the intensity of the incoherent beam was measured. We also theoretically estimated the resolving power with respect to the interaction length. The numerical results agree well with the experiments.

Formulas are given for the calculation of diffraction efficiency of reflection-type gratings recorded in a photorefractive medium. The analysis incorporates the coupled-wave theory that was developed for photorefractive hologram gratings. This analysis takes into account grating slant with respect to the medium surface, light absorption during reconstruction, any incident angle of the reference beam, and any photorefractive phase shift. General solutions for signal and reference wave functions are given in a closed-form expression by use of a hypergeometric function. The optimum media parameters and recording conditions for high diffraction efficiency are obtained by the derived formulas. The diffraction properties for off-Bragg conditions are also discussed.

Using a simple ellipsometric technique, we measure the total refractive-index modulation as a function of the frequency of the applied electric field in low glass-transition-temperature photorefractive polymer composites. From these measurements we deduce the relative contributions of the poling birefringence and the Pockels and Kerr effects. By applying the oriented gas model we determine the microscopic properties of the nonlinear optical chromophore, including the anisotropic polarizability, and the first and the second hyperpolarizabilities. In the search for new high-performance materials, the technique provides a measure of the linear and the second- and third-order nonlinear optical properties simultaneously.

The photorefractive properties of poly(N-vinylcarbazole) doped with a variety of nonlinear optical chromophores and sensitizing agents are surveyed. Steady-state diffraction efficiencies of greater than 10(-3) and two-beam coupling gain exceeding the absorption loss are found in six materials combinations. The effect of the structure of the nonlinear optical chromophore on the photorefractive properties is discussed.