The shallow and deep impurity levels in undoped, Rh- and Ce- doped as-grown BaTiO₃ crystals were investigated. We have found two shallow impurity levels in all the crystals by light-induced absorption spectrum studies, and have determined their thermal depths. We identified that one of the shallow levels has the same defect origins in al the crystals, while the other shallow level has the same defect origins in the undoped and Ce-doped crystals. Thermo-induced absorption spectra in these crystals were studied for the first time and proved to be a useful method for observing the absorption peaks of deep impurity levels in photorefractive crystals. The thermal depths of the deep levels in these crystals were determined by studies of grating dark-decay. The thermal levels and corresponding optical transitions in the crystals were given.

The diffraction efficiency and angular selectivity of photorefractive volume holograms, the important performances for multiplexed holograms, are strongly affected by material properties and recording conditions. In this paper the diffraction efficiency and angular selectivity of reflection gratings in photorefractive crystals are studied by means of the coupled wave theory, with respect to the recording geometry and polarization states of recording beams. The result shows that, although the angular selectivity of reflection gratings recorded with the s- polarized beams is slightly worse than that written with p-polarized ones, the diffraction efficiency of s-polarized beams is generally higher and more uniform than that of p-polarized beams over all accessible angular ranges. The theoretical results are also verified by experimental measurements. The s-polarized beams would be more suitable for recording and readout of reflection-type holograms in photorefractive crystals.

A slant grating was recorded with two laser beams incident on photorefractive crystal asymmetrically. 2D perturbative analysis for the first two higher-order harmonics of the space-charge field for steady-state photorefractive slant grating recording has been presented. Considering the contribution from the next-order harmonic at arbitrary modulation depths. Numerical results for the first two harmonics show that the next-order correction to the space- charge field is strongly dependent on the modulation depths. The dependence of the harmonics on spatial frequency, modulation depth and grating slant angles is discussed.

In this paper we demonstrate the phase conjugate characteristics of a Tb:Cu:KNSBN crystal, using the self- pumped phase conjugator, the bridge double phase conjugator and the coherently induced double phase conjugator, respectively. The experimental results show that the Tb:Cu:KNSBN crystal has high 'cat' self-pumped phase conjugate reflectivity, which is 57 percent at incident angle of 40 degrees and input intensity of 7.6 W/cm², and fast response. For the bridge double phase conjugator, two mutually incoherent e-polarized input beams enter the opposite crystal faces symmetrically with an included angle of 112 degrees. We measured the maximum phase conjugate transmissivity of 65 percent and the maximum phase conjugate reflectivity of higher than 250 percent. Moreover, we found that the temporal phase conjugate output varies with which input beam being blocked before the measurement. A peak reflectivity of as high as 300 percent was obtained when the beam ratio is 6.5. The Tb:Cu:KNSBN crystal has also been used to implement the coherently induced double phase conjugation and phase conjugate reflectivity approximately 320 has been obtained in a very large dynamic range of beam ratios.

Ring self-pumped phase conjugator with high-speed modulated incident beam is studied theoretically and experimentally. The analytical solution for the stead-state output of the phase conjugate beam in a photorefractive crystal with an arbitrary complex coupling constant is given. The theory is verified by the experimental results using a Ce:KNSBN.

In this paper, based on the point of the statistical-optics, we have developed a discussion for the Type II second harmonic generation of short pulse laser beams with phase modulations fluctuations in the new crystal such as CLBO. In it, fundamental wave is aberrated beam of which the aberration is assumed to be a Gaussian random variable. The theory on focusing the both characteristics approximate and calculated results of fundamental wave and second harmonic VS phase aberration coefficient have been presented. It is applicable in designs of harmonic generation for high power lasers.

Grating dark decay in an undoped, a Rh-doped, three Ce-doped and a (Ce, Rh)-doped BaTiO₃ crystals were systematically studied at room and elevated temperatures. Depth of the impurity levels and the dark decay time constant in these crystals were determined. The result show that the impurity levels related with photorefraction in the doped crystals have different origins from that in the undoped sample and the photorefractive center was caused by doping. In the (Ce Rh)-doped crystal, the results suggest that there are two deep levels caused by the dopings of Ce and Rh.

We have characterized the photorefractive performance of the special 45 degree-cut Ce:BaTiO₃ crystals grown by TSSG technique to compare with the conventional 0 degrees-cut crystal. A maximum gain coefficient at 37 cm^-1 was measured using e-polarized beam incidence, however a maximum gain coefficient at 4.0 cm^-1 was measured with 0 degrees-cut BaTiO₃. Because of strong noise amplification, we find the experimental result is smaller than the theoretical value.

We derive the intensity and phase formulas of the two-, three-, and n-stage codirectional parallel-polarization degenerate two-wave mixing in photorefractive media. These formulas are valid for both the saturation and intermediate regime and can be used in investigating the problem of increasing the coherent optical gain by the multistage two- wave mixing technique.

Steady-state bright spatial solitons are predicted for photovoltaic materials with an external bias filed. These solitons, being known as the screening-photovoltaic (SP) solitons, result from both the photovoltaic effect and spatially nonuniform screening of the applied field in the biased photovoltaic-photorefractive crystal and differ from previously observed photorefractive spatial solitons in their properties and physical origin. The properties of the SP solitons are discussed in detail.

The main purpose of this paper is to distinguish the coupling coefficient from the net coupling coefficient in the two-wave mixing process. The coupling coefficient is dependent on the total intensity, whereas the net one is not. The larger the thermal emission rate or the weaker the total intensity is, the larger the difference between the two coupling coefficients will be. We present a method for measuring the net coupling coefficient and the saturation intensity in photorefractive material based on the detection of the gain of two-wave mixing at two total intensity levels. We investigate the temperature and the grating period dependence of the net coupling coefficient and the saturation intensity for Ce:Mn:LiNbO₃ and Cr:GaAs crystal, respectively.

In this paper, we took the hopping model to discuss the temporal variation of the fundamental harmonic of the space- charge field with external Sinusoidal field and square wave electric field, respectively. We presented the approximate analytical expression of the fundamental harmonic of the space-charge field versus time and the applied field amplitude. We also study the temporal variation of the real and the imaginary of the fundamental harmonic of the normalized space-charge field with different applied AC electric field, respectively. It is found that the real part of the fundamental space-charge field varies at the same frequency as the applied AC field, the imaginary part oscillates at twice the frequency. These results are in agreement with those derived from the band-transport model.

The excitonic nonlinear refractive index was calculated by using Kramers-Kronig relation and the saturating absorption of ZnSe/CdZnSe multiple quantum wells (MQWs) was studied under different pump intensities. The maximum nonlinear refractive index change is about -6.19 X 10^-3. Excitonic optical bistability in ZnSe/CdZnSe MQWs is investigated at room temperature. The result indicates that the threshold and contrast ratio for the optical bistability in ZnSe/CdZnSe MQWs are about 210Kw/cm² and 2:1, respectively. On the basis of the excitonic nonlinear theories and excitonic absorption spectra in the ZnSe/CdZnSe MQWs, we attribute the major nonlinear mechanism of the optical bistability in the ZnSe/CdZnSe MQWs to the phase space filling of excitonic states and excitonic band broadening due to exciton-exciton interactions.

The steady-state differential equations of two waves coupling in photorefractive materials for reflection geometry have been numerically solved, and the Bragg-shift are calculated for a range of the coupling intensity and the phase shift. By means of a quasi-steady state approximation, the Bragg-shift as a function of the coupling intensity has been found, and hence as a function of the recording time. There is a good agreement between experimental measurements and numerical calculation.

We propose and demonstrate simple methods to determine accurately the dispersion of electro-optic coefficients and effective charge carrier densities of photorefractive crystals. A Ce-doped BaTiO₃:Ce crystal was determined. Then by measuring the two-beam coupling coefficient at different wavelengths and crossing angles, we can determine accurately the variation with wavelength of effective charge carrier density of the crystal.

We propose and demonstrate a new configuration of mutual- coherent-pumped phase conjugation with Ce:BaTiO₃. Two coherent beams are incident into a crystal and overlap inside the crystal. The two input beams undergo stimulated photorefractive backscatterings in separated regions, in which their corresponding phase conjugations are generated. Then the four waves in the crystal undergo four-wave mixing. The intensity ratio as well as the interference modulation of the two phase conjugations is characterized as a function of the intensity ratio of the input beams. The phase conjugator exhibits a large intensity dynamic range. We theoretically analyze the modulation transfer function of the mutual-coherent-pumped phase conjugator with Ce:BaTiO₃. The theoretical analysis is in agreement with the experimental results.

In this paper we review the currently achievable performances of holographic memories stored in photorefractive crystals. We discuss the dependence of the memory performances on the material peculiarities in three major aspects: storage capacity, data transfer rate,and image fidelity. In the recent years the research at Beijing Polytechnic University on the photorefractive holographic storage has been focused to the optimization of the storage capacity and diffraction efficiency, as well as the influence of noises on the fidelity of reconstructed images. Our research shows again that the realization of volume holographic storage technology requests materials with perfect properties.

The theory of wavelet correlation and the associative characteristic of volume holographic storage in a photorefractive crystal were introduced to construct a novel multichannel wavelet correlator. With a spherical wave as the reference beam, the system was more compact and easy to be miniaturized and utilized. Its application in fingerprint identification was studied, and experimental results were given.

A holographic interferometer architecture using a photorefractive crystals a the reference image storage device to test the refractive index distribution of the 3D temperature field of a flame is proposed. A novel method in data processing for the purpose of depressing noise and improving the quality of interferogram, spatial filtering, is introduced. Preliminary experimental studies are made to verify the effectiveness of this set-up and the method, the results are in full agreement with our theoretical expectations.

In this paper, we describe an image recognition system using a multi-channel optical correlator as the computational element and a volume holographic memory as a template database. By recording the holographic matched filters in the disk-type Ce:Fe:LiNbO₃ crystal, the multi-channel optical correlator is constructed. The system performs the multi-category pattern recognition. Experimental results are presented and discussed.

In this paper, we apply an extensive complementary encoding method, which has already been successfully used in an incoherent optical correlator based real-time target recognition processor to improve its discrimination ability, to a photorefractive crystal based Vander Lugt matched filter correlator for the same purpose. In this method, an original image and its complement image are regularly combined into an encoded image. The encoding process is applied to both the database images and the input target. The experimental results have proved that by using the extensive complementary encoding method, the discrimination ability of the photorefractive correlators can be improved effectively in a simple way.

An amorphous tripolymer containing azo-side groups has been investigated in photoinduced anisotropy and polarization holographic recording. Measurements reveal that the effect of photobleaching process should be taken into account in explaining the experimental result using the mechanism of angular hoe burning or the angular redistribution of molecules in the azobenzene side-chain polymer film and that the polymer possesses large magnitude of photoinduced birefringence. A reversible polarization hologram with high efficiency is recorded with a CW 532 nm laser in this thin film at relatively low light intensity.

We present here the experimental and numerical results to demonstrate the superior performance of optical wavelet transform correlator over conventional Fourier transform correlator. For this purpose the biomolecular material, Bacteriorhodopsin, as the recording media and the improved dual-axis joint transform correlator configuration as the preferred optical setup have been used. Compared with the dual-axis joint Fourier transform correlator, the dual-axis joint wavelet transform correlator provides better correlation performance.

Three-component is a promising direction to overcome disadvantages (e.g., unstability and critical environment requirement) of the four-component low glass transition temperature photorefractive (PR) polymer composite which consists of photosensitizer, nonlinear optical (NLO) chromophore, charge transport agent, and the plasticizer separately. Based on the well-known hole photoconductive network of poly(N-vinylcarbazole):2,4,7-trinitro-9-fluorenone, in our presented investigations, several well-performanced three-component PR composites were developed for different application purposes with the optimal designing optical chromophore exhibit both NLO response and plasticizing property. Generally, it is expected that long chain alkoxy of the chromophore may increase the plasticizing property while long alkoxy chain makes itself more easy to curl, which might weaken the plasticizing function, thus an optimum selection ofthe length ofthe alkoxy exists. From the experimental observations of 2,5-dimethyl-4-(4'—nitrophenylazo)benzene with different length of the alkoxy, it was indicated that high coupling gain (—195 cm') and index grating amplitude (—.4.3x1O) were achieved at 633nm in the composite contain l-n-butoxyl-2,5-dimethyl-4-(4'_nitrophenylazo)benzene. Besides, different substituent groups make the chromophore inequal plasticizing and different electrooptic properties. For operating at short wavelength, 3,3-diacety1-4methoxylstyrene is the more suitable selection (with a coupling gain of—38cm' at 543 urn, which is higher than these at 593 nm, 612nm and 633 urn). Doped with f3,f-diacety1-4-methoxylstyrene, a response time as short as —17 ms at illumination intensity of 1 W/cm2 at the wavelength of 633 nm was estimated, which is the most fast response reported up to now and can be used in real-time optical information processing. On the other hand, all composites mentioned above have long lifetime and high stability

Key words: phtorefractive, polymer, three-component, grass transition temperature, coupling gain, real-time hologram

The rise times, the two-ave mixing gain coefficients and the dark decay times of KNbO₃ doped with Co, Cu, Mn and Ga were measured and compared with BaTiO₃, BaTiO₃:Ce and KNSBN:Cu under identical experimental conditions.

Distinct electroabsorption grating and photoisomerization grating as well as photorefractive grating were observed in the composite consisting of 1-n-butoxyl-2,5- dimethyl-4-(4'- nitrophenylazo)benzene: poly(N-vinylcarbazole):2,4,7- trinitro-9-fluorenone in a weight ratio of 44:55:1. Based on the quick translation technique of two-beam coupling experimental geometry, we measured the electroabsorption gratin to be 6-7 cm^-1, and photoisomeric effect also brought an absorption grating of 2-3 cm^-1. Photoisomeric effect caused index grating was separated from the others by analyzing the dynamic behavior of the gratings, and an amplitude between 1 X 10^-4-2 X 10^-4 was obtained.

We have synthesized a novel kind of multifunctional polysiloxanes containing charge-transporting agent and electro-optical chromophobe as side chains for photorefractive application. The structural characterization of this kind of polymer is presented by ¹H-NMR, IR spectra and elemental analysis.

The thermo-induced absorption spectrum was demonstrated, for the first time, to be a useful technique for the study and identification of impurity levels in photorefractive crystals. With this technique, the absorption peak generated by the Ce-doping in BaTiO₃:Ce was revealed and identified, the related impurity level and the optical transition were determined.

The research results about the transient response of the longitudinal-field multiple quantum wells photorefractive devices are reported; moreover, we also study the performances of the devices in the non-degenerate four-wave mixing geometry.

The optical limiting properties of eight-bromo-2, 3- Naphthalocyanine Zinc(II) were studied by using of double frequency ns/ps Nd:YAG laser system at 532 nm with pulse width of 21 ps. The optical limiting properties of a novel Naphthalocyanine are better than those of Fullerene toluene solution. By using the singlet excited absorption theory, optical limiting properties were analyzed, the theoretical fitting results of excited state absorption agree well with the experimental result of optical limiting. The singlet excited state absorption agree well with the experimental results of optical limiting. The singlet excited state absorption is the predominant mechanisms causing the optical limiting properties.

The burning-hole behavior of intersubband absorption in semiconductor superlattices is studied by means of the two- subband model. The Maxwell-Bloch equations of laser- superlattice interaction are derived and solved at the steady condition. The absorptive coefficient as a function of the frequency of probe laser is given. The theoretical and numerical results how that the burning hole of the absorption-frequency curve occurs when the pump laser is enough strong. The burning-hole behavior is related closely to the structure of the semiconductor superlattices.

Three functional polyphosphazenses with photo-conducting and NLO moieties as side chains were prepared by nucleophilic substitution reaction of poly(dichlorophosphazene) with photo-conducting and NLO organic molecules such as carbazole, 9-hydroxyethyl-3-(nitrophenylazo)carbazole, dispersed red and p-nitroaniline. The polymers were characterized by FTIR, UV-vis, elemental analysis, and thermal analyses.

In this paper, we analyze the characters of all damage- resistant dopants' valences and electronic shell configurations found up to now in LiNbO₃ crystals. We show that all damage-resistant dopants have only one valence state. We also show that these damage-resistant ions' electronic shell configurations are filled fully, as are the cases of the inert elements. From this point of view, we conclude that the anti-site Nb_Li⁵⁺ should be involved in the charge transport process, while the Li vacancy V_Li is not involved in the charge transport process. It also can give us some insights about how to select a damage-resistant dopant for LiNbO₃ crystal.

We propose, demonstrate and analyze the application of phase conjugate mirrors during the recording of volume holographic memories. Our results show that by using phase conjugate mirrors during recording, the uniformity of the holographic gratings is improved and the recording speed is increased. Theoretical simulations and experimental results will be presented and discussed.

Holographic recording and beam coupling have been realized in ferroelectric Bi₄Ti₃O₁₂ crystals. Many interesting phenomena have been observed in this material, such as formation of multiple gratings, transient enhancement of diffraction efficiency during holographic recording with preillumination of the sample at low temperatures, and enhancement of diffraction during the readout process after a long recording time at room temperature. In this paper, photorefractive effects are studied in the temperature range from 80 K to 310 K. Light- induced absorption changes, dark decay of the gratings recorded with different writing beam intensities, and measurements of different activation energy levels show evidences which support the charge transport model of multiple trap centers in this material.

We studied the dependence of light-induced absorption on temperature in reduced Co:KNSBN crystal. The changes of light-induced absorption decrease as temperature decrease at high pump intensity, which can not be explained by previous two centers model. The dynamic behaviors of light-induced absorption reveal that there are multiple shallow levels in crystals. The thermal exciting rate and active energy of shallow levels are determined by the dependence of dynamics of light-induced absorption on pump intensity and temperature, respectively. The deep levels of reduced Co:KNSBN can be shown in transmission spectra of crystal. A model assuming both electron and hole with three shallow traps is used to explain our experimental results.

Single longitudinal mode operation of 1 W class broad-area diode lasers are achieved with phase conjugate injection locking technique. We adopted two stage injection scheme because the final output power is limited by the power of the injection beam. First, we locked a broad-area laser by the output beam from a single mode diode laser of the output power of 40 mW. We obtain 550 mW output from the broad-area laser with single longitudinal mode. Spatial pattern of the slave lasers are greatly improved by mode selection with the spatial filters at the far field between the lasers and phase conjugate mirrors. We also analyze the transverse oscillation modes in broad-area laser under locking conditions by numerical modeling. We can explain the near field and far field pattern in our experiment with our model.

We experimentally examined the crosstalk of the switching device using BaTiO₃ and interconnection mask. BaTiO₃ is used to compensate the light loss by two wave mixing.

We demonstrate one-way optical field imaging through a thick dynamic distorter using the photorefractive fanning effect without a reference beam and a four-wave mixing process. In this method, there are only one object beam and one sampling beam. The response time of the photorefractive crystal must be much longer than the fluctuation period of the dynamic distorter. Thus the crystal only responds the time-averaged intensity pattern of the rapidly varying object beam. The reconstructed images with high-fidelity are picked up.

We analyzed numerically the diffraction of the image-bearing beam from a fixed index grating in a photorefractive medium on the steady state, using a 2D model that takes into account the newly photoinduced index grating by the incident and the diffracted Fourier plane waves. In this analysis, we ignore all the absorption and assume the transverse dimensions of the medium and all the gratings to be infinite. The results point out that the intensity patterns of the diffracted image-bearing depend strongly on the following factors: (i) the feature spatial size of the incident image; (ii) the coupling coefficient and the wave number of the fixed index grating; and (iii) the thickness of the crystal and the gratings. On the other hand, they do not nearly depend on the photorefractive coupling coefficient for all the exiting photorefractive media. The effects of the spatial displacement of the diffracted images is also predicted.

We report the first demonstration, what is to our knowledge, of resonant second harmonic generation using a photorefractive phase conjugate feedback. Once the phase conjugator turns on, the SH output form self-frequency- doubled NYAB laser starts to increase and reaches up to 9-10 times of that without feedback. In addition, we find that the SH output fluctuates even if the phase conjugate feedback is constant. This phenomena is induced by the self- frequency-scanning of the SH output.

Ferroelectric lead germinate (Pb₅Ge₃O₁₁) crystals possess relatively large electrooptic coefficients and exhibit light induced conductivity. So far, only little is known about the photorefractive properties of these crystals. We present results on the growth of nominally pure and doped Pb₅Ge₃O₁₁ crystals, and of (Pb_1-xBa_x)₅Ge₃O₁₁ solid solutions. Basic parameters, such as dark and photo conductivity, are investigated in detail employing conventional and holographic techniques. Furthermore, photorefractive parameters, like the sign of the dominant charge carriers involved in the charge transport and effective charge densities, are presented and discussed, also with respect to different thermal treatments after growth, which influence the determined parameters.