We have demonstrated non-volatile holographic storage with two-photon, two-step recording in rare-earth doped ferroelectric crystals using cw lasers. Single photon erasure during the read out at near infrared wavelengths was found to be negligible. This progress paves the way to the realization of an economically viable, non-volatile read/write holographic recording system based on low cost semiconductor diode lasers.

We present what is, to our knowledge, the first report of a single-mode operation of broad-stripe LD using a photorefractive phase conjugator as an external cavity mirror. A single-longitudinal-mode operation of a 190 mW broad-stripe LD with 0.2 nm line-width could be obtained using an external semi-linear type phase conjugate mirror. Its wavelength was locked at 805.5 nm within 0.02 nm over 20 minutes. Further, by varying a LD's temperature, its lasing wavelength could be continuously tuned in the range between 802 nm and 809 nm.

We propose a new method for one-way parallel data transmission through a multimode optical fiber. The method consists of 'recording process,' in which each hologram that corresponds to one pixel is recorded one by one in a photorefractive crystal BSO, and 'read out process,' in which some of the holograms are illuminated simultaneously by readout beams and the corresponding pixel images are transmitted in parallel through a multimode optical fiber. In our study, a reference beam is also transmitted by the other multimode optical fiber rather than a single mode optical fiber. Although modal dispersion distorts the wave front of the reference beam, the phase conjugated beam can read out the holograms precisely by tandem arrangement of two photorefractive crystals BSO and BaTiO₃. We demonstrated experimentally a multipixel image transmission through a 5 m multimode optical fiber. The images could be transmitted more than 20 times for single recording process.

A model of phase hologram dynamics, caused by diffusion of small photochemically neutral molecules in polymeric medium is offered as an expansion of antiphase structures destruction concept. Results of experimental realization of the offered way are submitted. Variable real time dynamic regimes including hologram amplification and phase contrast inversion are demonstrated.

The phase conjugate reflectivity and the transmission in mutually pumped phase conjugators are investigated as a function of input bema power ratio in order to find out the optimum operating condition for high reflectivity and for high transmission with mutually incoherent and coherent beams. In our experimental results we show that we have the highest transmission at the ratio more than one in the modified bridge phase conjugator.

LiF crystal with F₃⁺ color center has been demonstrated to be a nonlinear material for 460 nm light and generate a phase conjugate wave by the degenerate four-wave mixing method. It is shown that the nonlinearity of the F₃⁺ centers is caused by the saturation of optical absorption.

Steady-state dark photorefractive spatial screening solitons are observed in an odd- or even-number sequence when a laser beam containing a dark stripe generated from a phase or amplitude discontinuity in the center of the beam is launched into a biased bulk strontium barium niobate (SBN) crystal. If the initial width of the dark stripe is small, only a fundamental soliton or a Y-junction soliton is generated, corresponding to the lowest order in the odd- or even-number soliton sequence. As the initial width and the bias field are increased, we observe a progressive transition from a lower-order soliton to a sequence of higher-order solitons. We show that these dark solitons induce stable waveguides which can guide an intense beam of a different wavelength into multiple channels.

A laser-diode interferometer with a self-pumped (cat) phase- conjugate mirror is presented. The phase is shifted to produce a spatially uniform phase change uncancelled by a phase conjugator between the two beams of an interferometer. An ac interferometer would be realized by means of heterodyne techniques that can be employed by modulating the phase of the conjugate wave by using the frequency-ramped laser diode (LD). In the interferometer, the uniform phase change of a single input is not reserved upon phase conjugation, while relative phases between two inputs are reversed. Effect of cross talk between two input amplitudes on phase-conjugate interferometric phase has been studied. Experimental results are shown.

In this article, our studies about the generation mechanisms of self-pumped phase conjugation (SPPC) in photorefractive crystals are reviewed. The variations of SPPC generation mechanism with operation wavelengths, doping concentrations, incident positions and incident angles, and incident configurations were investigated experimentally. It is indicated that the essential factors that determine the SPPC generation mechanisms are the backscattering gain coefficient and the scattering strength of scattering centers in the crystals.

The influences of the contradirectional two-wave mixing (CTWM) on the total-internal reflected self-pumped phase conjugator (CAT SPPC) and on the stimulate photorefractive scattering self-pumped phase conjugator (SPS SPPC) are studied. The experimental and the theoretical results both show that the CTWM is an important interaction in the SPPC process, it can obviously change the output characters of the SPPC. Especially in the SPS-SPPC, the CTWM is an indispensable interaction, it changes the structure of the SPPC in the basic.

We present a new photorefractive crystal (Ce:BaTiO₃) with superior photorefractive properties. The formation of a self-pumped phase conjugate (SPPC) wave is based on backward photorefractive scattering. For a 0-degree-cut and a 45- degree-cut Ce:BaTiO₃ crystals the SPPC reflectivities of 70% and 83.5% were achieved at lambda equals 515 nm respectively. It has been experimentally demonstrated that this crystal is probably the most efficient self-pumped phase conjugator in the visible spectral regime so far.

In this paper, the theoretical and experimental studies of the orientational characteristics of electro-optic chromophore in a low glass transition temperature material are presented. The amplitude and the phase behaviors of the orientation have been discussed. The photorefractivity in a material PVK:TCP:APSS:C₆₀ is experimentally studied.

Temperature-dependent efficiency and speed of response of a Ce-doped barium titanate crystal as a self-pumped phase conjugator are investigated at 514.5 and 632.8 nm. It was observed that its efficiency and speed of response are improved at elevated temperature. Two-beam coupling measurements were also carried out at different temperatures, showing that the coupling gain coefficient as well as the corresponding speed of response of the crystal are also enhanced at elevated temperature. In addition to the photorefractive properties, the absorption coefficient of the crystal was also found to increase with temperature, implying that there are at least two deep-trap levels in the crystal.

In this paper, the dynamical evolution of dark screening solitons in PR media was investigated under steady-state conditions. Our numerical study indicates that these optical dark soliton at lower power level are stable against small perturbations whereas those at higher power level will split into pairs of dark spatial solitons. The influence of loss in refractive materials is studied in detail. The effect of the external bias field on the self-defocusing behavior of dark optical beam is also discussed.

The contradirectional two-wave mixing (CTWM) is studied and compared with the forward two-wave mixing, the results show that the CTWM offers some unique wave-coupling characters, the energy transfer direction in the process of CTWM is not only decided by the sign of the main light induced carries, but also decided by the relative value of every element in the electro-optical tensor.

We report the growth of photorefractive AlGaAs/GaAs multiple quantum well (MQW) structures by molecular beam epitaxy at low temperatures (LT). The LT MQW materials are semi- insulating as-grown. The As_Ga-related defects incorporated into the LT materials during low temperature growth provide the required deep centers for photorefractive applications. We have investigated the effect of growth conditions on optical properties of LT MQW structures. Under optimized growth condition, MQW structures have been grown semi-insulating while retaining high optical quality. The strong electro-optic effects were observed near excitonic absorption edge. The photorefractive wave mixing in AlGaAs/GaAs MQW structures was performed under Frenz-Keldysh geometry. An output diffraction efficiency higher than 0.84% and a two-wave-mixing gain more than 3000 cm^-1 have been obtained under dc electric field of 15 kv/cm.

Anomalous photorefractive behaviors that cannot be explained by previous models are observed in doped KNSBN crystals. The space-charge field increases to its maximum value and then gradually decreases as the incident light intensity increases. The superlinear dependence of the response rate on intensity, i.e. (tau) ^-1 varies direct as I^x (x greater than 1), are obtained in a Co:KNSBN crystal. The nearly linear dependence of response rate on intensity is measured in heavy reduced Co:KNSBN crystal, in which the dark decay of space-charge field is much faster and the total amount of dark decay is also larger than another one. A new model of two types of carriers with two deep and two shallow traps is proposed to explain these results.

A new configuration of mutually pumped phase conjugation was experimentally demonstrated using two nanosecond pulsed beams in BaTiO₃:Ce crystal, in which the beams travel in a geometrical path similar to the shape of a rainbow, so we refer to this as rainbow-shaped mutually pumped phase conjugation. In this configuration the two mutually incoherent pulsed beams were incident upon the +c-face to form a coupling channel without internal reflections at crystal surfaces. The highest phase conjugate reflectivity was about 24% when two incident beams have the same intensity and highest spatial resolution was 7.12 lines/mm. The dependence of the phase conjugate reflectivity on the incident angle and distance, and the grating formation time have been investigated. The photorefractive response time to the incident intensity had a sublinear dependence with a factor of 0.7776. Additionally, we estimated also the coupling strength and the interconnecting factor between two beams to be 4.512 and 0.3242, respectively.

We report our experimental studies on the photorefractive response of KNbO₃ double doped with manganese and rubidium. Two-beam coupling response time of photorefractive potassium niobate crystals is 3 to 10 times improved by double doping manganese and rubidium, when compared with that of the KNbO₃:Mn crystals, which have shown a strong photorefraction. The photorefractive two-beam coupling gain coefficient in such crystals remains almost the same as those in KNbO₃:Mn, which is 3 times higher than that of KNbO₃ doped with rubidium alone, which has a fast response time.

A new photorefractive joint transform correlator architecture using a photorefractive crystal as a reference image storage or sequentially image updating device instead of a joint transform power spectrum recording element is proposed. Three novel and simple methods for moving target recognition and tracking with this architecture are introduced: (1) correlation with single fixed reference image; (2) correlation with a fixed synthetic reference image; (3) adaptive correlation with a sequentially updated reference image. Some specific considerations concerning the recording-erasing property of the crystals and the principle of adaptive tracking are analyzed and discussed. Preliminary experimental studies are made to verify the effectiveness of these methods, the results are in full agreement with our theoretical expectations.

We propose a method to perform two-beam coupling, in which the pump beam was incident upon the +c face and formed a curved light path toward the a face. With this special pump arrangement a part of the fanning beam could also play an important role of pumping of the signal beam and the influence of the fanning effect could be minimized. Thus a high two-beam coupling gain as high as 3.27 by 10⁵ has been obtained at the steady state and a several times higher has been observed at the beginning.

The reinjection of a small fraction of the output beam back into the laser cavity by a phase conjugate mirror (PCM) is shown to improve the beam quality of an argon laser efficiently, the output power of an argon laser is changed and the number of the longitudinal modes is reduced greatly by the reinjection of an external cavity PCM.

The intensity dependence of stationary energy transfer in photorefractive codirectional cross-polarization two-wave mixing is examined. The intensity formulae remained effect both in the saturation and the intermediate regime (before saturation) are driven, and the analytical expressions with the ratio of saturation intensity to total incident intensity as their explicit variable are obtained for the case of pump depletion. It is necessary to use these formulae in some problems, such as the temperature or intensity dependence of cross-polarization two-wave mixing. A method of measuring the coupling coefficient and saturation intensity of a cubic photorefractive crystal is presented, which is suitable for cross-polarization two-wave mixing in the intermediate regime.

In a wide region of incident angles between -60° and +70°, the self-pumped phase conjugate (SPPC) reflectivities (ri) that are greater than 60% have-been obtained in 45°-cut Ce:BaTiO3 crystal at X=5 l4nm. The SPPC buildup time was 3 sec. . A systematic study was undertaken to assess the i with respect to external angle of incidence (0) and the position (x) of the incident beam upon the crystal entrance face. We have been compared the photorefractive performance of this special 45°-cut crystal of Ce:BaTiO3 with the 0°-cut and 45°-cut BaTiO3 as well as the 0°-cut Ce:BaTiO3 crystal. Intentional doping with cerium has an important effect on the efficiency, sensitivity and spectral response of photorefractive effect. The 45°-cut Ce:BaTiO3 is sufficiently improved its photorefractive phase conjugate performance. keyword: self-pumped phase conjugation, Ce:BaTiO3 crystal

The decay processes of light-induced absorption in one undoped and two Ce-doped BaTiO₃ crystals were observed. Two decay times were found, they had not a strong dependence on intensity in our experiment. We present a model with two different shallow trap centers to explain the experimental results. The theory and experimental results agree well. The light-induced absorption coefficients and decay times corresponding to the two shallow levels are also given.

A mutually pumped phase conjugator is demonstrated with a novel photorefractive crystal of copper-doped potassium sodium strontium barium niobate, in which the beams travel in a geometric path similar to the outline of a sickle. The maximum reflectivity is up to 168%. Faster response times are measured and are also simulated as a function of incident beam power ratio. Four available input geometries are found. The phase conjugation signals exist wide range of angular and position alignment.

This paper reports a novel phenomenon in photorefractive rubidium doped potassium niobate crystals, two different two-beam coupling processes with opposite gain signs and two response time with a difference of two orders of magnitude are observed in a rubidium doped KNbO₃ crystal. Which can be explained well by the electron-hole competition.

A new formula of saturation intensity defined as the ratio of thermal emission rate to photoexcitation constant for photorefractive material is derived and a new method for the saturation intensity measurement in a photorefractive crystal based on detecting intensity gain in two-wave mixing is presented. The temperature dependence of the saturation intensity for Ce:Mn:LiNbO₃ crystal is measured. The measured values for the saturation intensity have been checked with another experiment of two-wave mixing in the crystal.

In this article, optical nonlinearity of nanometer-sized CdO organosol at wavelength of 800 nm using fs laser Z-scan method was reported and analyzed.

Photorefractive four-wave mixing (FWM) and photoconductivity properties of two new polymers are investigated. One polymer is composed of photoconducting poly(n-vinylcarbazole) (PVK), photosensitizing 2,4,7-trinitro-9-fluorenone (TNF), and optically nonlinear 4-(dicyanovinyl)-N.N-diethylaniline (DCVDEA). Its FWM diffraction efficiency and response time as a function of, respectively, electric field and writing intensity have been determined. With an 80 micrometer thick film, a FWM diffraction efficiency of 7.4 by 10^-4 has been obtained. The response time of the polymer is as short as 400 ms at a writing intensity of 0.17 W/cm². Another polymer is composed of PVK, TNF, and 4-N,N-demethyl- 4'-nitrostilbene (DANS). Its photoconductivity and photocarrier generation efficiency have been determined experimentally at different electric fields and wavelengths.

Since the photorefractive effect was first discovered in 1966, and then photorefractive materials were used in many aspects, such as dynamic holography and optical phase conjugation, the band transport model has become the general explanation for such photorefraction. In this model, diffusion mechanism acts as a main role and in some cases drift mechanism (under applied field) as well as the photovoltaic effect (under uniform illumination) were added. However, experiments and theoretical analysis have shown that the photovoltaic effect should always be considered; in the case of ferroelectrics, e.g., LiNbO₃, this effect often exceeds the diffusion and thus dominates. In addition, the circular photovoltaic effect, if it exists, is able to create a phase shift grating. Therefore, theoretically, energy can be transferred between two beams through two-wave mixing.

We have measured the reflectivity of self-pumped phase- conjugate (SPPC) mirror as a function of wavelength in the range between 720 nm and 830 nm in 0-degree-cut KNbO₃:Ni. This is the longest wavelength for which the SPPC has been observed in KNbO₃ at room temperature. We have obtained reflectivities R equals 43% at 725 nm and R equals 1% at 831.4 nm.

We present a method of measuring the parameters of a photorefractive crystal in codirectional two-wave mixing that is suitable for two-wave mixing in the intermediate regime (before saturation). Three parameters of a crystal, the saturation exponential gain coefficient, saturation intensity and intensity absorption coefficient, can be measured simultaneously by using the method. The contribution of the intensity dependence of intensity absorption coefficient is also considered. The three parameters of a BaTiO₃ crystal are measured and the measured data are checked by another independent measurement.

Photoexcitation and relaxation dynamics of photoinduced defects in the emeraldine base (EB) form of polyaniline, poly (m-aniline) (PMAn) and the emeraldine salt (ES) form of polyaniline embedded in polymethylmathacrylate (PMMA) has been measured using femtosecond time-resolved absorption saturation technology. The relaxation of photoexcitation, observed in EB, consists of two components: an initial ultrafast relaxation (approximately 100 fs) and a following slow relaxation (approximately 30 ps). A slower photoinduced bleaching compared with that in EB and 5-ps relaxation process were observed in PMAn. The ES/PMMA film exhibits the fastest ground recovery (approximately 0.5 ps) of photoexcitation, which is about two orders of magnitude faster than that of EB and an order of magnitude faster than that of PMAn. This ultrafast polaron relaxation in ES/PMMA was observed for the first time. Optical bistability and multistability were obtained by the simulation of the excitation and relaxation processes observed in the three forms of polyaniline. The switching time is strongly affected by the relaxation dynamics of the photoexcited defects.

ZnTe/CdZnTe multiple quantum wells (MQWs) optical bistable device has been fabricated, and optical bistability in ZnTe/CdZnTe MQWs optical bistable device is investigated on transmission at room temperature. The research result indicates that the threshold and contrast ratio for the optical bistability in Zn-Te/CdZnTe MQWs optical bistable device are about 363 kW/cm² and 4:1, respectively. On the basis of the excitonic nonlinear theories, excitonic absorption spectrum in the ZnTe/CdZnTe MQWs obtained here, we attribute the major nonlinear mechanism for the optical bistability to the saturating effect of excitonic absorption.

BaTiO₃ is a typical perovskite ferroelectric with unique photorefractive, electro-optic and dielectric properties, the thin film of BaTiO₃ is suitable to be used in integrated optic and photorefractive devices. In this paper we report the growth of BaTiO₃ thin film on Si (100) substrate by atmospheric pressure metalorganic chemical vapor deposition technique. The souse materials used were Ba(thd)₂ and titanium isopropoxied. The substrate temperature was from 620 degrees Celsius to 780 degrees Celsius. The orientation of as-grown BaTiO₃ films is dependent on growth temperature. The dielectric constant ((epsilon) ) and loss tangent (tan(delta) ) of BaTiO₃ film with (001) orientation are found to be 114 and 0.02, respectively, for a 0.8 micrometer-thick film at room temperature. The remanent polarization (Pr) was 2.3 (mu) C/cm² and coercive field (Ec) was 14 KV/cm.

A theoretical explanation about the phenomenon of self- pumped phase-conjugation (SPPC) being erased is given. Controlling incident and erasure beams of SPPC, all-optical XOR and NXOR logic operations are performed experimentally.