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The photorefractivity of an indole derivative and of its polymer blends has been studied at room temperature. The indole derivative 3-[2-(4-nitrophenyl)ethenyl]-l-(2-ethylhexyl)-2-methylindole (NPEMI-E) is a typical low-molecular-weight glass-forming molecule having peculiar nonlinear optics characteristics. It is unconditionally soluble in the photoconductive poly-(N-vinyl-2,3- dimethylindole) so that all the possible blends can be studied for a weight percent (wt. %) content of NPEMI-E ranging from zero to 100. A very high and sharp maximum of the photorefractive optical gain [[GAMMA].sub.2] [approximately equal to] 2000 [cm.sup.-1] was obtained for a NPEMI-E wt. % content of about 90. On the basis of recently published theoretical calculations, we have made the hypothesis that the rapid change of [[GAMMA].sub.2] can also be ascribed to a correspondingly quick variation of the molecular electro-optic parameters of the dissolved chromophore for some well distinguished values of its concentration in the polymer matrix. Differential scanning calorimetry measurements were made and the results carefully analyzed with the aim of obtaining information on the intermolecular interactions. These last measurements also allowed rationalizing the unconditionally stable glass appearance of the obtained blends. Measurements of spectroscopic ellipsometry were also made on blends with different NPEMI-E content. OCIS codes: 160.4330, 160.5140, 160.5320, 190.3270, 190.5330, 230.4480.

Based on the restraining effect that spatial filtering has on the frequency spectrum of a beam, from the small-scale focusing theory of Bespalov and Talanov (B-T theory) we derive an expression for the pinhole diameter of the spatial filter corresponding to the fastest growing frequency. Then, compared with the theoretical pinhole diameter of the spatial filter, the restraining effect of the spatial filter on a hot image with different pinhole diameters is numerically investigated. The numerical results show that, if the pinhole diameter is larger than the theoretical one, the hot-image intensity will remain steady; once the pinhole diameter becomes smaller than the theoretical one, the hot-image intensity will begin to decrease. Moreover, as the pinhole diameter decreases, a more prominent restraining effect can be obtained. But reducing the diameter of the spatial filter would lead to greater beam energy loss. The parameters of the spatial filter must be chosen to guarantee that the scheme fulfills the demand for low beam energy loss and a satisfactory restraining effect simultaneously. OCIS codes: 090.1970, 260.5950, 070.6110, 140.3330.

We demonstrated a real-time shift-invariant VanderLugt correlator (VLC). The VLC was implemented with a 37 [micro]m thick bis-triarylamine side-chain polymer matrix photorefractive (PR) polymer composite operating at 532 nm wavelength. Correlation selectivity was enhanced by real-time edge enhancement. The advantage of the VLC is that its architecture allows overcoming the PR material response speed. A correlator of this type enables a fast shift-invariant search of a large optical image database. A high degree of shift invariance in our correlator was possible because of the material thickness. The angular bandwidth of this material was measured experimentally in a degenerate four-wave mixing setup, and it was found to be in a very good agreement with the theory. OCIS codes: 070.0070, 070.4550, 100.4550.

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 LiNb[O.sub.3] with various compositions. Bipolarons are considered to be responsible for the enhanced photovoltaic field in reduced near-stoichiometric LiNbO3, and their bulk photovoltaic constant K is estimated to be ~6.95 x [10.sup.-32] [m.sub.3]/V. Q polarons (composed of two bipolarons) are introduced to explain the photorefractive effect of congruent LiNb[O.sub.3] at both low and high light intensities.

A volume photorefractive phase mask has been fabricated with incoherent white light from an incandescent source in LiNb[O.sub.3]:Fe self-defocusing photorefractive crystal for the first time, to our knowledge. It can guide and modulate a probe white light or a laser beam and can be used to transmit an incoherent dark image as the guided modes of the waveguides induced by white-light dark spatial solitons. This also proves the existence of photorefractive nonlinearity of white light. OCIS codes: 230.7370, 210.4680, 190.5330.

The fundamentals of the photorefractive (PR) effect describing the necessary points of materials design is reviewed. Illustrations along with models developed for the PR organic materials and experimental techniques developed that feature such PR materials are explained.

An overview of research involving spatial optical solitons is presented.

Improvements in elimination of instabilities during the writing of volume gratings in a contra-directional two-beam coupling (TBC) geometry are described and discussed.

Behavior of the steady-state photorefractive harmonic gratings and the space-charge field spatial distribution are derived without restrictions. This is accomplished by solving the shallow-trap-model material equations.

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.

The physical origin of the temporal fluctuation of the photorefractive effect is examined. The fluctuation is found to result, mainly, from fluctuations in the free-charge-carrier dendity, which in turn, is found to result from small fluctuations of the input total intensity and thermal excitation.

A new study investigates electro-optic phenomena in nematic liquid crystals studied by Monte-Carlo simulations and experimentally.

Research describing photorefractive multiple quantum well structures is presented. With reference to Franz-Keldysh geometry properties including saturation intensity, spatial resolution, high order diffractions and electric field dependence are investigated.

Research describing the theory of moving gratings in photorefractive polymer materials is presented. The effect of total intensity on gain coefficients is explained by the relation between space charge field and experimental parameters.

Research describing the production and dark decay of bismuth silicon oxide is presented. In particular the kinetics of index gratings as a function of write beam intensity and temperature are investigated.

Electrically biased Bi12TiO20 was studied in terms of laser pulses that are self-focusing, to determine the physics of fast photorefractive self-focusing.

The Braun model for field-dependent photogeneration efficiency was incorporated in the standard photorefractivity model and effects of space-charge field were calculated.

A new Raman and Brillouin spectroscopy study of a photorefractive single crystal of (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 with copper-ion point defects is presented.

Two beam coupling was used to determine wavelength dependence of trap density in BaTiO3:Ce crystals.

Research describing photorefractive multiple quantum well devices is presented. In particular experimental resolution is investigated by means of a lateral drift photocarrier model.

Research describing the effect of multiple trapping, low temperature growthlayers on photorefractive quantum well devices is presented. In particular the resolution and sensitivity of distributed photorefractive devices is investigated.

Research describing the formation of space-charge fields in photorefractive polymer materials is presented. In particular a derivation of the dynamic differential equation for such fields is developed with reference to the previous work of Schildkraut.

Research describing the densification mechanism of ion-implanted silica glass is presented. In particular the increase in refractive index is investigated with reference to the fabrication of optical fibre gratings.

Research describing the photorefractive response of barium titanate crystal is presented. In particular the influence of beam coupling on nonlinear grating crosstalk is investigated through coupled wave equations.

Hologram dark decay in photorefractive lithium-niobate crystals is examined.

Photorefractive polymer-dissolved liquid-crystal composites were studied for holographic Bragg grating generation.

A novel sensor is described, with a large aperture, comprising thin photorefractive crystals.

A new study investigates nonlinear absorption in Pt:ethynyl, based on an electronic transitions model obtained from earlier work.

A theoretical and experimental study of the orientational photorefractive effect in nematic liquid crystals is presented.

Grating diffraction effects on the pulse evolution in photorefractive self-pumped phase-conjugate mirrors are examined.

Two differing dynamical approaches were used to analyzed the photocurrent dark decay in photorefractive materials.

A new organic photorefractive material is presented which shows excellent photoconductive and nonlinear optical properties. The choice of a suitable plasticizer is shown to be important for dispersing charge transport.

An investigation of photorefractive damage effects in a LiNbO3 quasiphase matched waveguide is presented. This research used a novel two-beam method.

Self-enhanced diffraction was observed in new KNSBN photorefractive crystals.

Data to show that Delta n is highly dependent on the chromophore structure detail is presented, via a newly developed method to correlate birefringence measurements with Delta N 10.

Three-dimensional (3D) color holograms are recorded in a cerium-doped, strontium barium niobate (SBN:60) photorefractive crystal. These holograms are shown to reconstruct true color reproductions of the original object with an observable field of view of 37 degrees. Angle multiplexing of two or more 3D color holograms is also demonstrated with angle tuning of the reference beam corresponding to a separation angle between stored images of 0.082 degrees. Each of these results is compared with corresponding theoretical prediction.

We analyze and compare two typical recording and thermal fixing procedures of a volume hologram in a Fe:LiNbO3 crystal (low-high-low procedure and high-low procedure). We consider the kinetics of the recording, compensating, and developing processes by taking into account the ratio of the conductivities between the protons and the electrons as a function of temperature. From the analysis the optimal environmental conditions (in terms of the fixing temperature and the compensation time) for each fixing procedure can be deduced for a crystal with given material parameters.

Research was conducted to examine the photorefractive characteristics of Cu-doped (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6 crystals with different doping levels. Two-wave mixing, self-pumped phase conjugation, the growth conditions and absorption properties were the characteristics studied. Results revealed that the doping levels have much influence on the photorefractive properties.

Research was conducted to examine the low-temperature dynamic characteristics of photorefractive gratings in cobalt-doped potassium-strontium-sodium-barium-niobate crystals (Co:KNSBN). The preillumination of the sample led to a shorter write-erase cycle of the grating. Measurement of the temperature dependence of the dark decay of the photorefractive grating was performed to complete the study and to illustrate the multiple shallow levels that exist in the Co:KNSBN crystal.

The diffraction efficiency of volume gratings written by two-wave mixing in a cerium-doped potassium sodium strontium barium niobate (Ce:KNSBN) photorefractive crystal is studied. It is found that the diffraction efficiency strongly depends on the polarization of writing beams and exhibits loop behavior with respect to the fringe modulation. The fringe modulations before and behind the crystal are compared. Modified coupled-wave theory is used to fit the experimental data. This research presents data that are relevant to the application of Ce:KNSBN crystals to holographic recording and optical information processing.

Research was conducted to examine high-speed photorefractive high-liquid crystal (H-LC) and low-molar-mass liquid crystal (L-LC) mixtures (HL-LCMs). The crystals and crystal mixtures exhibited the fastest response time obtained so far while maintaining a large gain coefficient of 270 cm, high resolution and the required low operating voltage. The speed of the grating generation was successfully enhanced by the reduction of the viscosity of the materials.

A study on semiconductor nanoparticles, nanocrystallites and clusters reports some of typical photorefractive figures of merit, specifically steady-state diffraction efficiency and two-wave mixing gain coefficient for the poly(N-vinylcarbazole): tricresyl phosphate: 4-nitrophenyl-L-prolinol composite system that is photosensitized with nanoparticles of the inorganic semiconductor CdS. Experimental methodology and results are presented.

Research was conducted to examine photorefractive barium-calcium titanate crystals with the congruently melting composition Ba09.77Ca0.23TiO3 (BCT) doped with iron in its state resulting from thermally annealing in a reducing atmosphere. Results of optical absorption, electron paramagnetic resonance and light-induced absorption changes indicate that the reduced samples are electron conductive and the charge transport at room temperature is governed by only one level identified as Fe2+/3+.

Research was conducted to examine the effect of dipole moment on carrier mobility with various key components of photorefractive polymers, namely, electro-optic chromophores, polymer binders and charge transport agents. Results provide evidence that carrier mobility is greatly reduced in the presence of polar molecules in accordance with the predictions of models of hopping transport in the presence of dipolar disorder.

High- (H-LC) and low-molar-mass liquid crystal (L-LC) mixtures (HL-LCMs) doped with fullerene showed high-performance photorefractivity in a thick-grating regime. The photorefractive performance strongly depended on the concentration ratio of H-LC and L-LC. A high net gain coefficient of more than 600/cm was attained with low applied dc fields and a fringe spacing of 2.8 microns. The gain coefficients and the resolution increased when the concentration of the H-LCs were increased.

The resolution of incoherent-to-coherent image conversion was studied for an incoherent-to-coherent converter that used the photorefractive fanning effect. The resolution was improved through using a high-quality imaging system and a heavy doping crystal. The resolution was not limited by the grating diffraction. A resolution as high as 90.5lp/mm was obtained. The ability of the method to convert an image with gray levels was demonstrated.

Research was conducted to examine the photorefractive and optical characteristics of Nb-doped BaTiO3. The gradual modification of the photorefractive properties of BaTiO3 was carried out through the addition of Nb which changed the charge carrier from hole to electron. The deep and shallow trap model with hole as the principal carrier explains two beam coupling gain along with electron-hole competition. The dark conductivity increased very fast and the electron became the major carrier after the compensation point, when an absorption at about 600 nanometers evolved.

Research was conducted to examine the the self-focusing process temporal behavior in photorefractive Bi12TiO20 crystal. The evolution of the diameter parallel to the beam was compared to a previously reported 1+1 dimension theory. The 1+1 theory dimensional theory permits the correct description of the self-focusing of a transversally bidimensional beam in the direction of the applied field. The experimental results allow the confirmation of the wave propagation equation that was theoretically derived.

The electric screening field in photorefractive Bi12TiO20 are analyzed with the applied electric field using a technique that employs the giant momentary readout peaks in a two-wave coupling arrangement. To measure the internal field and the strength of the screening field within the crystal, the moment of apperance of giant momentary peaks is suggested. Further, The temporal development of the internal field is established by measuring the transmission of a wave through the crystal between polarizer and crossed analyzer. The results are discussed.

A study was conducted to analyze the dynamics of photorefractive grating erasure in polymeric photorefractive materials supporting hole traps. Theoretical solution was used to determine a bi-exponential decay for the rate of optical erasing of the space-charge field. In addition, a backward, degenerate four-wave mixing configuration was utilized to examine the kinetics of the grating erasure. Experimental results indicated that shallow traps dominated the photorefractive materials.