Photorefractive Bi Research Articles

Mutual transformation of the light waves by contra-directional mixing on reflection holographic grating in Bi-=SUB=-12-=/SUB=-SiO-=SUB=-20-=/SUB=- crystal of (001)-cut

The features of energy exchange during the contra-directional mixing of two linearly polarized light waves in a cubic photorefractive Bi 12 SiO 20 piezocrystal of (001)-cut are analyzed. The dependences of the relative intensity of the signal wave on the azimuth of its polarization at the entrance to the crystal during mixing with the reference wave polarized in the plane of incidence are theoretically studied for various crystal orientation angles. The dependence of the relative intensity of the signal wave on the azimuth of its polarization is experimentally studied for the cases when the vector amplitudes of light waves propagating in the crystal are parallel and perpendicular to each other. The calculation is based on the numerical solution of the coupled waves equations with and without taking into account the self-diffraction of the light waves by the reflection holographic grating recorded in the crystal. Based on a comparison of theoretical and experimental data, the influence of self-diffraction on the mutual transformation of light waves during their contra-directional mixing is analyzed. The limits of applicability of the static grating and the dynamic grating approximations for solution of the coupled waves equations are determined. Keywords: Photorefractive crystal, reflection holographic grating, coupled wave equations, self-diffraction.

Dynamic ultrasound modulated optical tomography by self-referenced photorefractive holography

Photorefractive Bi(12)SiO(20) single crystal is used for acousto-optic imaging in thick scattering media in the green part of the spectrum, in an adaptive speckle correlation configuration. Light fields at the output of the scattering sample exhibit typical speckle grains of 1 μm size within the volume of the nonlinear crystal. This heterogeneous illumination induces a complex refractive index structure without applying a reference beam on the crystal, leading to a self-referenced diffraction correlation scheme. We demonstrate that this simple and robust configuration is able to perform axially resolved ultrasound modulated optical tomography of thick scattering media with an improved optical etendue.

Novel procedure for the simultaneous determination of the Debye length and electro-optic coefficient for an optically active photorefractive Bi 12SiO 20 crystal

We present a novel procedure based on an improved physical model and a versatile numerical fitting technique, to simultaneously determinate the Debye screening length and electro-optic coefficient using a thick sample of an optically active photorefractive crystal Bi 12SiO 20 (BSO). For the computation, experimental data of gain vs. grating spacing were obtained by a two-beam coupling arrangement. Unlike former calculation models, in our work, the general expression for the diffraction efficiency in the presence of self-diffraction is considered, and the influence of the optical activity in the coupling parameters is had into account for the calculation of the gain values. The fitting of the experimental data to the predicted theoretical behavior by our model is achieved by finding the closest theoretical curve to a set of data sampled from a spline-smoothed curve of the experimental data. Both, the Debye screening length l s and the electro-optic coefficient r 41 are used as fitting parameters by searching in a rather wide range for each one of the parameters, so that, the estimation of their values is obtained in a more reliable and direct way from the same experiment. The calculations are performed in diffusion regimen and the procedure leads to l s = 0.22 μm and r 41 = 4.5 × 10 − 12 m/V. Because the optical activity can alter the maximum gain and self-diffraction effects influence the energy exchange, the procedure reveals to be physically appropriated for the simultaneous determination of these physical parameters when thick photorefractive crystals with high optical activity are considered.

Surface contouring by phase-shifting real-time holography using photorefractive sillenite crystals

This work proposes to obtain surface contouring by phase-shifting real-time holography using photorefractive sillenite crystals which provides the surface contouring from a change in phase of wavefront of the object illumination beam. The real-time holography using the photorefractive Bi 12 SiO 20 crystal and the four-frame phase-shifting technique was used to obtain the phase map, this map was filtered by an anisotropic sin/cos filter and the unwrapping process used was the cellular-automata technique. We obtained experimental results of the surfaces of the large objects with real discontinuities, and these results agree with the dimensions of the object.

Transition between superluminal and subluminal light propagation in photorefractive Bi12SiO20 crystals

We demonstrated superluminal light propagation with a negative group velocity of -5.7 m/s in a photorefractive Bi 12SiO20 crystal by using the dispersive phase coupling effect in a nondegenerate two-wave mixing process. To the best of our knowledge, this is the first experimental demonstration of superluminal light propagation at room temperature in solids by using a classical wave mixing technique. In addition, we showed the tunability of the group velocity of light between the negative (superluminal light) and the positive (subluminal light) by simply tuning the experimental conditions such as the frequency of the coupling beam, the incident intensity, and the externally applied electric fields.

Stable self-focused beams in a photorefractive Bi_12TiO_20 crystal

We present experimental results on the self-focusing of light beams in a photorefractive, optically active Bi12TiO20 crystal. Using circular polarization of the input HeNe laser beam we succeeded to create a stable self-focused, asymmetric light-beam over several hours. Depending on external parameters, like the external electric field and the intensity ratio, a transient symmetric state is observed.

Space charge wave rectification effects in photorefractive Bi 12TiO 20 crystals

The effects of overall and spatial rectification of optically excited space charge waves (SCW) in photorefractive Bi 12TiO 20 crystals have been studied for the first time. The whole set of experimental data (frequency dependence of the amplitude of SCW, of the magnitude of the current variation caused by overall rectification and of the amplitude of the homogeneous space charge field caused by spatial rectification) are well described by the developed theory with three fitting parameters (Maxwell relaxation time, quality factor of SCW and screening factor). An extremely important role of the screening effect in the studied crystal has been revealed.

Testing of linear polarizer using photorefractive effect

A new method is developed to test linear polarizers using two-wave mixing in photorefractive Bi 12 TiO 20 (Bismuth Titanium Oxide, BTO). The extinction ratios obtained from this method was comparable with the literature values and the method is also useful for qualitative analysis of linear polarizers. Detailed theoretical and experimental analysis is given in the paper.

Faraday rotation measurement with photorefractive Bi 12TiO 20

A simple method for accurate measurement of Faraday rotation in materials with low rotation is developed using photorefractive Bi 12TiO 20. High optical activity of Bi 12TiO 20 is made use for this measurement. Detailed theoretical and experimental analysis is given.

Spatial subharmonics generation during recording of self-pumped phase-conjugate mirrors

Abstract The self-pumped phase-conjugate mirrors (SPPCM) recorded by a single beam incident in photorefractive Bi 12 TiO 20 crystals without external mirror under alternating electric field are presented. Simultaneously with the phase-conjugated wave we observe significant concentration of the scattered light on the direction of the subharmonics generation. This combined formation of the fundamental grating and its spatial subharmonics allows SPPCM formation in a much wider band of spatial frequency.

High order dark spatial solitons in photorefractive Bi 12TiO 20 crystal

In this paper we report the experimental observation of high order dark spatial solitons in a photorefractive Bi 12TiO 20 crystal. Odd or even number of solitons is generated depending of the initial field condition (phase or amplitude jump). No uniform illumination is necessary to obtain steady state solitons. Polarization dependence is observed on the output beam.

Talbot interferometer based on a birefringence grating

A 1-D coarse grating imaged onto a photorefractive Bi 12SiO 20 crystal is stored as modulation of birefringence. On this basis, the self-imaging phenomenon is implemented. When an analyzer is placed at the output of the crystal, the modulation of birefringence is converted into a modulation of transmission. In this case in the read-out process, self-images are generated and distributed along the propagation direction at those positions which coincide with the loci that the self-images occupy in case that an amplitude grating would be used instead of the combination of the birefringence grating and the analyzer. Moreover, it is demonstrated that by maintaining the read-out polarization state and the crystal configuration it is not necessary to convert the modulation of birefringence into a modulation of transmission to generate self-images. Its means that no matter the analyzer is removed, self-images are generated as well. In this case, the planes of highest visibility coincide with those planes corresponding to a conventional phase grating. It is well known that Talbot interferometers built in with ordinary gratings are not sensitive to polarization changes. On the other hand, it is showed that the birefringence grating stored in the crystal allows to implement a polarization sensitive interferometer. It means that if a birefringence material is tested, then the shifting introduced in the self-images can be used for quantitative determination of the material birefringence.

Optical properties of Bi 12TiO 20 doped with Al, P, Ag, Cu, Co and co-doped with Al+P single crystals

Large optically homogeneous photorefractive Bi 12TiO 20 (BTO) single crystals doped with Al, P, Ag, Cu, Co and Al+P-co-doping were obtained by the Top Seeded Solution Growth Method (TSSG) in a Bi 2O 3 solution. A strong bleaching effect was observed for the Al, P, Ag and Al+P-doped crystals, whereas doping with Cu and Co induced a strong photochromic effect and increased the absorption coefficients in the red spectral region. Al, P, Al+P-doped crystals increased the values of optical rotator power, while Cu and Ag-doped crystals exhibited a strong decrease in optical activity in comparison with non-doped BTO. The influences of doping elements on the optical rotation power are discussed on the basis of two structural elementary cell units – MO 4 tetrahedra and BiO n polyhedra.

Optical properties of photorefractive Bi 12TiO 20 single crystals doped with Al, P, Ag, Cu, Co and Al+P

Large optically homogeneous photorefractive Bi 12TiO 20 single crystals doped with Al, P, Ag, Cu, Co and Al+P are obtained by the top seeded solution growth method in a Bi 2O 3 solution. A strong bleaching effect is observed at Al, P, Ag and Al+P doped crystals, whereas doping with Cu and Co induced a strong photochromic effect and increased the absorption coefficients in a red spectral region. Al, P and Al+P doped crystals increased the values of optical rotation power, while Cu and Ag-doped crystals lead to strong decreasing of optical activity in comparison with non-doped Bi 12TiO 20.

One-dimensional self-focusing in photorefractive Bi 12SiO 20 crystal: theoretical modeling and experimental demonstration

We have experimentally demonstrated one-dimensional self-focusing of a laser beam in a photorefractive Bi 12SiO 20 (BSO) crystal with an externally applied DC field. Fractional change in focal length larger than 50% was measured when the biased field was varied from 2 kV/cm to 8 kV/cm. Based on the band transport model, thin crystal approximation, and gradient index (GRIN) lens approximation, we derive a mathematical expression that describes the effect quantitatively.

Improvement in quality and performance of photorefractive Bi 2TeO 5

Bismuth tellurite has a unique feature, a long-living (years) photorefractive memory that develops in the four-wave mixing process without fixing. Single crystals of Bi 2TeO 5 were grown by the Czochralski method. The Bi/Te ratio and the oxygen equilibrium were found to be the major factors modifying the crystal quality. The systematic improvement of the growth technique has led to enhanced photorefractive figure of merit for Bi 2TeO 5.

Optical activity measurements in the photorefractive Bi 12TiO 20 single crystal fibers

In the present work the specific optical activity coefficient of the photorefractive Bi 12TiO 20 single crystal fibers have been determined. The samples were illuminated by λ=0.6328 μm and λ=0.5145 μm. The proposed experimental technique is simple, fast and accurate. It is based on the automatic electronic phase shift detection in a dynamic polarimetric system. 6.62±0.06 deg/mm was obtained for λ=0.633 μm and 10.0±0.2 deg/mm for λ=0.514 μm.

Intensity controlled shaping of the beam profile using three-wave mixing in photorefractive Bi 12TiO 20

In a degenerate forward three-wave mixing configuration with Bi 12TiO 20 (BTO) the gain in the signal wave is spatially controlled by choosing the intensity ratio between the two pump waves. Depending on this ratio the gain can be both positive and negative. For high pump intensities the gain does not dependent on the signal intensity itself. The phase distribution of the signal wave is not affected. The dependence of the signal wave gain on the pump intensity ratio is used for beam shaping with a BTO crystal (10×10×10 mm 3) at a wavelength λ=633 nm. An analytical solution for this dependence in three-wave mixing was found based on the coupled wave equations for two-wave mixing, neglecting optical activity and assuming small incidence angles. The analytical results show good agreement with experiments.

Dynamic moiré patterns produced by sinusoidal gratings superposition in the Bi 12 TiO 20 crystal

In the present note a real-time holographic moire-like pattern is shown. It is produced by the superposition of two rotated sinusoidal phase gratings and the result represents a promising technique for possible metrological applications. The experiments were performed in a diffusion-only recording mechanism in a photorefractive Bi 12TiO 20 crystal illuminated by λ = 0.633 μm from a HeNe laser light source.

Phase modulation spectroscopy of space-charge wave resonances in Bi 12SiO 20

A new experimental method for the study of resonance effects and space-charge wave excitation in photorefractive Bi 12SiO 20 crystals by using a combination of frequency detuning and phase modulation technique has been developed. The accuracy of the method allows a detection of resonance peaks of diffraction efficiency within 0.5 Hz. Numerical simulations of the nonlinear differential equations describing the behaviour of the space-charge waves in photorefractive crystals have been performed and found to be in a good agreement with experiment. We have measured the photocurrent through the crystal and revealed its resonance dependence. A minimum of electric current through the sample corresponds to the main resonance detected by phase modulation technique.