Search

We show experimental results for the propagation of a diffracted beam by a straight edge like initial condition in a photorefractive crystal under drift nonlinearity. We have observed that the diffracted pattern and propagation is enhanced in the form of multiple waveguides.

Existence of surface gratings associated with volume gratings in a photorefractive material is proved by analyzing reflected diffraction orders of an incident reading beam. A novel technique is used to numerically eliminate the Fabry-Perot effect from the diffracted orders.

We report on the photoelectric conversion at a non-photovoltaic photorefractive material sandwiched between transparent conductive electrodes. Its behavior is based on the light-induced Schottky effect. Its nature and performance was clearly established by wavelength-resolved photoconductivity. OCIS codes: Photoconductivity (040.5150); Photorefractive Materials (160.5320); Optoelectronics (230.0250)

High quality inverse opal structures are synthesized from the photorefractive SnO2 medium by a simple two step infiltration and calcination process. Optical characterization reveals the high index contrast and applications of them in the field of sensors and integrated optics.

Enhanced and fast photorefractive index grating is developed in compound semiconductor, when the two pico-second light pulses couple inside compound semiconductor crystal at 77 K under Voigt configuration, near the resonance condition in the presence of magnetic field.

We demonstrate image restoration and recognition via photorefractive two-beam coupling using organic photorefractive materials. Our results outperforms the simulations based on the phase-only and some optimal correlation filters.

We show a novel nonlinear optical medium, which is formed by coupling a liquid crystal layer to a photoconductive crystal. We show that this medium allows the control of the liquid crystal birefringence in a local and dynamical way, so that large optical nonlinearities are obtained on a large transverse section. The properties of the photorefractive liquid crystal light-valves are used to implement transverse pattern formation, two-wave mixing amplification and phase conjugation.

Hybrid materials, fabricated by combining liquid crystals with ferroelectric, inorganic nanoparticles, show improved photorefractive response. Three-fold increase in dielectric anisotropy and optical anisotropy was measured, as well as significant improvement in two-beam coupling gain.

We investigated the relations between the mesophase structure and the photorefractive performance in transmission and reflection grating geometries of the photorefractive composites based on highly-ordered layered and less-ordered nematic-like photoconductive polymers.

We improved the photorefractive polymer with mixing two kinds of nonlinear chromophores. Diffraction efficiency of 16 % with 0.1 mm thick sample, and shelf lifetime of more than 2 months were obtained.

Two-wave-mixing gain coefficient and time constant are measured at a wavelength of 457.8 nm. To avoid the effect of photochromism, we employed a polarization modulated interferometer. The origin of photochromism is assigned to Fe-ion-related transition.

The dependence of self-pumped photorefractive two-beam coupling in Fe:KNbO3with crystal angle has been calculated and measured at 532 nm for the a-c and b-c crystal planes using a combination of oil-immersion and relay imaging.

Photo-induced refractive index change was investigated in organic single crystals with excited-state intramolecular protontransfer molecules. The relaxation dynamics associated with the keto-enol transition could be monitored by pump-probe method.

The photorefractive effect was observed in He-ion-implanted semi-insulating GaN film in UV spectral region. The photorefractive grating is induced by the electroabsorption effect near the band-edge. Two beam coupling experiments were performed at a wavelength of 363.8 nm. The experimental results indicate that absorption grating mainly contributes to the beam coupling effect.

High performance two-photon absorbing photorefractive polymers operating at 1550 nm are described. The demonstration of thermal fixing and non-destructive read-out of the holograms in polymer composites make them attractive for practical device applications.

This paper presents investigations of the high-frequency mode of space- charge waves (SCW) in crystals of the sillenite family Bi12SiO20, Bi12GeO20 and Bi12TiO20.

Photorefractive volume holographic grating (PVHG) is predicted to have attractive applications in optical communications and optical sensors due to its flexibility for fabricating, easy erasure and repeatable use. In this paper, we present some theoretical analyses and experimental results on the diffraction properties and applications of PVHGs in LiNbO3:Fe crystal. Based on the angle selectivity of PVHG, the corresponding refractive index modulation depth and effective thickness are measured. Using these calculation values in addition to recording parameters, some special diffraction properties of PVHG can be designed. In order to avoid the polarization dependent diffraction efficiency due to the anisotropy of photorefractive crystals, double PVHGs are suggested to record in the same position of the crystal with two ordinary and extraordinary recording beams, respectively. As a filtering device, the PVHG can be well used to demodulate the wavelength reflected by a fiber Bragg grating. And the corresponding wavelength demodulation experiment is demonstrated in this paper.

We theoretically investigate the surface structure of a photorefractive grating that is formed with the use of a bulk interference pattern by photovoltaic effect in X-cut lithium niobate crystal short-circuited along z-axis. The steady-state gratings with the wave vector directed along the z-axis in the samples with dielectric and metallized (short­circuited) x-boundaries are considered. The amplitude of the surface relief grating in the sample with metallized boundary exceeds one for the case of dielectric boundary. The existence near such metallized surface the x-component of electric space-charge field causes here the predominance of perturbation for off-diagonal component ?B 13=?B 31 of the dielectric impermeability tensor at optical frequency.

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 role of diffusion, photovoltaic effect and dark conductivity is investigated for a LiNbO3:Fe crystal.

Edge enhancement, a type of intensity filtering of two-dimensional optical image, was demonstrated using dye-doped photorefractive liquid crystal films. We calculate the expected images using four wave mixing geometries, which show good agreement with the image that we experimentally observed using dye-doped liquid crystal as the photorefractive materials.

Photorefractivity at 1.55 µm in a polymer composite is demonstrated using twophoton absorption. We show large diffraction efficiency and perform holographic reconstruction of distorted images utilizing thin-film devices made of this polymer.

Theoretical analysis of photorefractivity in dye-doped nematic liquid crystal film is discussed. Analytical solutions of refractive index distribution and the first order diffraction efficiency are presented. The relationship between the diffraction efficiency and the applied DC voltage shows very good agreement to the experimental result in photorefractive region.

Photorefractive polymers possessing a layered structure are shown to exhibit excellent photorefractive properties in the reflection grating geometry. A composite based on a rigid backbone of poly(p-phenyleneterephthalate) and pendent carbazole groups (PPT-CZ), the nonlinear optical chromophore piperidinodicyanostyrene (PDCST), and the sensitizer C60 shows in reflection geometry a gain coefficient of 104 cm-1 and a diffraction efficiency of 4.8 percent for an applied electric field of +60 V/mm. The investigated composites do not contain any plasticizer. Nevertheless the layered structure insures excellent stability without sacrifying the large rotational electro-optic effect connected with a very low glass transition temperature associated with the side chain motion.

The self-focusing and the screening soliton propagation for the arbitrary orientation of a (??0 )-cut cubic photorefractive crystal with respect to the external electric field have been investigated. A general system of equations for the description of light beam self-focusing in a cubic crystal is suggested. The orientation of the crystal, near to optimal one, for self-focusing of light beams is found. The phenomenon of suppression of polarization properties of the optically active crystal by nonlinear interaction of polarized components of the light beam is studied. It is shown that the optical activity of the crystal not always leads to worsening conditions for formation of the soliton.

Self-action of light beams in photorefractive waveguides is theoretically and experimentally investigated. We consider the waveguide formed by sequential diffusion of titanium and iron in Z-cut of LiNbO3 and the biased waveguide Bi12TiO20 on Bi12SiO20. It is shown that the spatial averaging of nonlinear change of refractive index is valid for different mechanisms of photorefractive effect. The wave equation in parax-ial approximation for evolution of beam amplitude shape is derived. We determine the conditions of the soliton formation for different waveguide modes taking into account two-dimensional distribution of optical field in guiding layer and the overlap integral of this field with the nonlinear changes in refractive index.

We report the first observation of self-trapping of an optical beam in an organic photorefractive monolithic glass. The orientationally enhanced photorefractive nonlinearity that gives rise to spatial solitons can be switched from self-focusing to self-defocusing simply by changing the polarization of the optical beam. Our experiment brings about the possibility of using organic materials for soliton-based applications.

It has been shown recently (see, e.g., Nature, 397, 594, 1999; 409, 490, 2001) that light pulses can be remarkably slowed down in resonant gases under very low temperatures. We show theoretically and experimentally that a similar in nature (and stronger in value) phenomenon occurs in photorefractive nonlinear media. The pulse-maximum velocity can be reduced here to the value of thickness/response time because of the two wave coupling effect. Specific properties of pulse propagation depend on the type of the photorefractive response and also duration and shape of the input pulse. Our theoretical results for the nonlocal nonlinear response are in a good agreement with experimental data obtained with BaTiO3 and Sn2P2S6 crystals.

Two-wave and four-wave mixing experiments are performed on semiconductor with DX centers, CdF2:Ga,Y. The crystal is known to exhibit strong modification of optical constants when exposed to light as a result of optically induced transformation of DX-centers. Our experimental results reveal strong ?(3) nonlinearity of this crystal and remarkable dynamic photorefractive effect at ambient temperature. With the z-scan technique we found that the 10 ?W green light beam induces the negative change of refractive index of 10-5. The diffraction efficiency reaches 50%. The grating decays in a few seconds at room temperature but the decay time decreases with temperature. The phase conjugate wave is generated with nearly 40% reflectivity, and the coherent oscillation is observed in semilinear cavity with two counter-propagating pump beams. Centro-symmetric and optically isotropic CdF2:Ga,Y is equivalent to or surpasses in some properties the well-known polar photorefractive crystals.

At low light intensity contrasts, the profile of the refractive index pattern under an ac field exhibits a gradient dependence on the light intensity. At high contrasts, however, the situation is dramatically different and new features arise. The light-induced space-charge density distribution exhibits a strong spatial localization or delta-type singularity, coupled with sharp discontinuities of the space-charge field and the index profiles. The effect shows a clear threshold behavior depending on the ratio of the width of the light beam and the mobility-lifetime-ac field product. The new features are even more striking when a narrow light beam propagates through such a self-generated discontinuous refractive index pattern. These include a combination of diffraction together with self-focusing and defocusing effects, and even with the self-trapping along the index discontinuity.

We show that specific features of holographic recording such as a non-monotonic behavior of diffraction efficiency and the development of a latent image observing in some photorefractive crystals can be explained on the base of the band-transport model including deep donor centers (that allow double ionization) and shallow traps. It is shown that the cause of the some observing effects of the enhancement and the development of the photorefractive gratings is a spatial shift of charge gratings relative to one another without changing their amplitudes.

Absorption saturation features and the correspondent nonlinear change of refractive index in a Cr4+:YAG crystal are studied applying the Z-scan technique in steady-state at wavelength 1.06 µm with taking into account of the crystal nonlinear absorption anisotropy.

Basing on the vectorial theory of light diffraction in photorefractive crystals of cubic symmetry, we derived analytical expressions that describe the outgoing intensity of coherent beams coupled with the dynamic grating and subjected to a fast phase modulation. It is shown that the linear transformation of fast phase excursions into intensity modulation is achieved in crystals with non-local photorefractive response after a proper polarization filtering of the transmitted beams when an alternating external field is applied to the crystal. Theoretical results are supported by the experiments with Bi12TiO20 crystals of various optical configurations.

We have grown Bi12GeO20 single crystals from melts of different GeO2 content. The absorption of the crystals increases with decreasing GeO2 content in the melt. Furthermore, the samples show a significant variation of the photoconductivity and the effective trap density. Our results can be described by a one-center model with the intrinsic antisite defect BiGe (Bi3+/4+ on Ge site associated with a hole) as dominant photorefractive center.

Summary form only given. Self-trapping of 1D and 2D optical beams in photorefractive InP has been experimentally observed back in 1996. Here we present the first theory describing the resonant self-focusing effects in photorefractive semiconductors. The theory is based on a models that relies on two donors levels, one deep trap level and conduction of both electrons and holes.

We apply a periodically phase modulated two-wave mixing technique to obtain simultaneously the absorption and refractive index contributions to the diffraction efficiency. The technique allows us to measure the amplitude and phase of the refractive and absorption index changes, and therefore their relative contributions to the diffraction efficiency with high sensitivity.

The existence of multiple active levels in a photorefractive Bii2TiO20 crystal is here investigated at 514.5nm wavelength. We carry out two-wave mixing experiments using symmetrically incident beams of equal intensities. A large amplitude fast phase modulation in one of the beams reduces the fringes visibility and improves the detection of the generated frequency modulated signals in both (R and S) output directions. Diffraction efficiencies of the phase (photorefractive) and the absorption (photochromic) gratings are quantitatively computed as functions of the grating period. Results show that the absorption grating has two distinct components: one associated to the photorefractive trap density modulation and another related to local light-induced effects between different levels. The photorefractive grating was also investigated at 633nm and 594nm (besides 514.5nm) and a significant quenching of the photorefractive effect was observed at these wavelengths.

Edge-enhanced correlation provides sharper correlation peaks than conventional correlators, leading to easier target identification. We present results of real-time all optical edge-enhanced joint transform correlation in photorefractive polymers in a unique forward two-beam coupling geometry.

In energy exchanging photorefractive polymers, we analyze the dependence of the induced change in refractive index on the initial two-wave intensity ratio and the gain coefficient. Using this, we demonstrate programmable edge enhancement.

Single beam polarization interferometry was introduced to measure the Pockels coefficients in a BSO single crystal. The linear superposition principle of the induced birefringence and the optical activity was employed in the analysis of the Pockels effect measurement.

Charge redistribution between two different photorefractive centers in CdTe:Ge allows for enhancement of two-beam coupling at 1.064 µm and 1.32 µm by the additional illumination of the sample. The gain factor reached at 1.064 µm without external electric field is only 20% smaller than the theoretical limit for CdTe. The "net" gain is obtained at 1.064 µm as well as at 1.32 µm and 1.3 times amplification of the signal wave is achieved at 1.06 µm with no applied field.

The temporal properties of the light-induced erasure decay and dark decay of asgrown and reduced BaTiO3:Rh are investigated as a function of temperature.

We fabricate semi-insulating lnGaAs/GaAs multiple quantum wells and measure the two-wave mixing gain and the four-wave mixing diffraction efficiency at the wavelengths around the excitonic resonance of 0.93 µm. The important properties including spatial resolution, saturation intensity and response time are experimentally investigated. The photorefractive response time of 100 µs is obtained at the wavelength of 924.5 nm with the intensity of 7 mW /cm2.