Voltage reduction for photorefractive two-wave mixing in InP:Fe by optical gap control

We report a method to increase the photorefractive two-wave mixing gain coefficient in semiconductors by using additional incident laser radiation. Two auxiliary beams are used to optimize the spatial distribution of an applied electrical field via modulation of the photoconductivity of the sample. By applying a voltage of 3.5 kV on a 4.3 mm thick InP:Fe crystal we obtain a two-wave mixing gain of 9 cm^–1. This is about 50% larger than that obtained without auxiliary beams.     

Holographic scattering lines observed with photorefractive BaTiO3

We investigate holographic scattering processes in BaTiO₃ pumped by two coherent copropagating waves. Four lines are observed on a screen behind the sample. The experimental results are well described by a model taking into account parametric mixing of four copropagating waves and limitations of optically induced space charge fields because of the finite trap density.     

Detection of small phase modulation using two-wave mixing in photorefractive crystals

Two-wave mixing of phase-modulated beams in photorefractive crystals under a dc electric field is studied both theoretically and experimentally. Based on the vectorial theory of light diffraction in crystals of cubic symmetry, we derived an analytical expression that describes the phase demodulation in crystals with an arbitrarily oriented electric field. The phase-demodulation technique was used for estimation of the space-charge field created in photorefractive crystals. It is shown that the space-charge-field grating created in GaP and Bi₁₂TiO₂₀ crystals is much smaller than that predicted from the one-level band-transport model of the photorefractive effect. Received: 12 December 2000 / Revised version: 8 February 2001 / Published online: 27 April 2001     

On the two-wave mixing in BaTiO3 with a self-bent pump beam

Received: 25 February 1997/Revised version: 20 June 1997     

Studies of impurity levels in Rh-doped and Ce-doped photorefractive BaTiO3

Impurity levels, light-induced and thermo-induced charge transfer processes in Rh-doped and Ce-doped photorefractive BaTiO₃ were studied. The thermal depths and optical transitions of these impurity levels in the crystals were determined by using such methods as light-induced absorption, thermo-induced absorption, and grating dark decay. The origins of these impurity levels were discussed. We demonstrated, for the first time to our knowledge, that thermo-induced absorption spectroscopy is a useful and complementary technique for investigating the impurity levels in photorefractive crystals. With this technique, the deep impurity level generated by Ce in BaTiO₃:Ce was revealed and identified. Received: 7 July 1999 / Revised version: 24 September 19999 / Published online: 2 February 2000     

Anisotropic scattering generated by photorefractive four-wave mixing in BaTiO3 crystal

A new type of anisotropic scattering with ring-and-line structure is observed for the first time from a BaTiO₃ crystal illuminated by a linearly polarized laser beam. The analysis presented herein is based on the photorefractive four-wave interaction of the incident, reflected, and scattered beams. The experimental results agree well with the theoretical analysis.     

Different temperature dependences of photorefractive parameters of Ce-doped and Rh-doped BaTiO3

eff and the electro-optic coefficient r₃₃ in Ce-doped and Rh-doped BaTiO₃ were determined by two-beam coupling measurements. It was found that the effective trap density N_eff of BaTiO₃:Ce increases whereas that of BaTiO₃:Rh decreases with increasing temperature. The electro-optic coefficient r₃₃ of both crystals increases with temperature. The photorefractive response times were also measured and found to decrease with different rates as temperature increases in the two crystals. The results were discussed by using the two-centre model for BaTiO₃:Ce and three-charge-state model for BaTiO₃:Rh. We found that the different temperature dependence of N_eff in the two crystals was due to the fact that the deep- and shallow-trap levels in BaTiO₃:Ce are caused by different impurity centres whereas those in BaTiO₃:Rh are caused by different charge states of the same impurity centre. Received: 16 July 1998/Revised version: 26 October 1998     

Manifestation of circular photogalvanic current by dynamic holography in BaTiO3

We report the first measurement of the photo-galvanic circular current antisymmetric tensor component in BaTiO₃: Co. The measurement gives a value of this coefficient, for extraordinary beam amplification, of 4×10^–9 A/W using a nonstationary measurement technique at a wavelength of 0.632 m.Professor N. Kukhtarev is on leave from the Institute of Physics in the Ukraine and is supported by the NSFDr. G. Dovgalenko is on leave from the IRPA Rotor-Control research laboratory of the Ukraine and is supported by the University of Arkansas     

Model for multiwave-pumped parametric oscillation in BaTiO3

3 crystal subjected to two counterpropagating pairs of pump beams. The model explains the characteristic features of the above effect and predicts a number of new peculiarities that we have found in additional experiments. Received: 19 March 1997/Revised version: 9 June 1997     

Light-induced scattering in photorefractive crystals

Light-induced scattering features in LiNbO₃- and BaTiO₃-crytals are compared with theories on holographic writing in photorefractive crystals. It is shown that they describe the experimental facts concerning the expected main scattering directions for a given incident polarization, the time development, the thickness and the wavelength dependence. Time records of the transmission offer a useful alternative for the determination of the photoconductivity. Furthermore, a new method for birefringence measurements is established. The high accuracy of this method is based on the automatic fulfillment of a phase matching condition by the anisotropically scattered radiation.     

Transverse effects in photorefractive two-wave mixing

In a biased photorefractive crystal, the process of two one-dimensional waves mixing, i.e., the dynamical evolution of both pump beam and signal beam, is traced by numerically solving the coupled-wave equation. Direct simulations show that the propagation and stability of the two beams are completely determined by the system parameters, such as the external bias field, the intensity and the beam waist of the pump beam. By adjusting these parameters, one can control the state of two Gaussian waves mixing. The numerical results are helpful for performing a two-wave mixing experiment.     

Nonlinearity of a response in photorefractive crystals with a square-wave applied field

The space-charge-field formation in a photorefractive crystal illuminated by a one-dimensional light pattern is studied when an alternating square-wave electric field is applied to the crystal. We derive the general nonlinear equation for the time-averaged distribution of the space-charge field and employ the simplified versions of this equation to analyze the space-charge-field formation in the case of a Gaussian beam and interference light pattern. The borders of the applicability of simplified equations and analytic expressions to describe a self-action of light beams and the large modulation effects under two-beam coupling are estimated from the numerical calculations. Received: 18 November 1998 / Revised version: 26 January 1999 / Published online: 12 April 1999     

Self-focusing in photorefractive BaTiO3 crystal under external DC electric field

Experimental observations of 1D and 2D self-focusing in a BaTiO3 crystal under externally DC applied electric field are reported. Stationary self-focusing behaviour can be obtained with and without additional uniform illumination.     

Mirrorless coherent oscillation due to vectorial four-wave mixing in LiNbO3

Mirrorless coherent oscillation in a four-wave mixing arrangement with two phase-conjugate counterpropagating pump waves is studied in a LiNbO₃:Fe crystal. Oscillation waves are observed within a wide, but limited angular window where the gain factor takes its maximum value. The spatial structure and the intensities of particular oscillating components vary continuously in time.     

Photorefractive properties and self-pumped phase conjugation in a series of BaTiO3 crystals

This paper presents the experimental results of two-wave mixing and self-pumped phase conjugation (SPPC) in barium titanate (BaTiO₃) crystals doped with Rh or Ce. It has been shown that the main parameters of SPPC depend very strongly on the concentrations of dopants, the incident angles and the crystal cut used. High gain in two-wave mixing corresponds to low phase-conjugate reflectivity in all samples. It is impossible to maintain the highest reflectivity and gain simultaneously, because the gain will definitely be limited when we try to get the highest reflectivity, and vice versa. Some qualitative explanation of the experimental results is also given. Received: 5 January 2001 / Final version: 25 May 2001 / Published online: 19 September 2001     

Photorefractive amplification and generation of light beams in BaTiO3 controlled by the intensity of the pump light

3 by photorefractive forward four-wave mixing. The intensity of a signal beam is amplified by a factor up to 3000. We show that heating of the crystal by absorption of the pump light influences the gain because of the thermo-optic effect. Thus both, amplification and generation can be controlled by the intensities of the pump beams. Received: 23 January 1998     

The effect of a high external electric field on a photorefractive ring phase conjugator

It is shown that a high external electric field often used to enhance the efficiency of photorefractive beam coupling and to decrease the photorefractive response time, may cause mode switching in photorefractive ring phase conjugator leading to a significant drop in the phase conjugate reflectivity.     

Photorefractive two-wave mixing in the presence of an incoherent beam

We present the numerical results of the photorefractive two-wave mixing in the presence of an incoherent beam by considering the influence of the ratios of the intensities and the light-excitation cross sections of the incoherent beam to the coherent beams on the space-charge field and the effective coupling coefficient. The results show that the incoherent beam can control the intensity of the coherent beams effectively. The experimental results agree with the theory well. One application is demonstrated to obtain the positive and negative coherent replicas of an incoherent image. The coherent images have a resolution of 80.6 line pairs/mm. Received: 2 March 1999 / Revised version: 26 July 1999 / Published online: 20 October 1999