Photorefractive effect in sillenite crystals with shallow traps in a sign-alternating electric field

The results of a theoretical analysis of the photorefractive response in crystals with shallow traps to in a sign-alternating, square-wave electric field are presented. The numerical analysis method developed imposes no restrictions on the frequency of the external field and the period of the photorefractive grating. The parameters characterizing deep donor and shallow trap centers are estimated on the basis of investigations of two-beam interaction in a Bi₁₂SiO₂₀:Cd crystal with the application of a sign-alternating, square-wave electric field. Fiz. Tverd. Tela (St. Petersburg) 40, 2037–2043 (November 1998)     

Real-time holographic interferometry using photorefractive sillenite crystals with phase-stepping technique

This work presents a holographic interferometer that uses the photorefractive sillenite crystals in diffusive regimen whose configuration exhibits diffraction anisotropy for real-time holographic interferometry. The writing–reading process of holographic interferogram was done in real-time, connected with an interferogram-analysis method that uses the phase-stepping technique for quantitative measurement of changes on an object. The holographic interferograms from the analyzed surface were captured and they were used to calculate the phase map with four-frame technique. The unwrapping process used was the cellular-automata technique. We obtained quantitative results for some applications: measurements of micro-rotation of surfaces, punctual micro-displacements on an aluminum plate, stress on a dog's jaw, among others; adding new promising applications possibilities for basic research, dentistry and technological areas.     

Holographic imaging with multimode, large free spectral range lasers in photorefractive sillenite crystals

The holographic imaging of rigid objects with diode lasers emitting in many wavelengths in a sillenite Bi₁₂TiO₂₀ photorefractive crystal is both theoretically and experimentally investigated. It is shown that, due to the multi-wavelength emission and the typically large free spectral range of this light source, contour fringes appear on the holographic image corresponding to the surface relief, even in single-exposure recordings. The influence of the number of emitted modes on the fringe width is analysed, and the possible applications of the contour fringes in the field of optical metrology are pointed out. PACS 42.40; 42.62; 42.70     

Effect of a DC external field on the photorefractive-response dynamics in crystals with double-donor centers and shallow traps

This paper presents a theoretical analysis of the dynamics of space-charge field formation in a photorefractive crystal with double-donor centers and shallow traps. The evolution of the photorefractive-grating recording processes in the absence of an external electric field, the relaxation of the grating in the presence of a reference beam, and its subsequent development under application of an external field are considered.     

Dispersion of electrogyration in sillenite crystals

In this paper we report on the dispersion of the electrogyration of the sillenite crystals Bi₁₂SiO₂₀, Bi₁₂GeO₂₀, and Bi₁₂TiO₂₀ over the visible spectrum. We measure the electrogyration coefficient from the rotation of the polarization plane of the transmitted beam due to the externally applied electric field. Both light transmission and electric field direction are parallel to the [111] crystallographic direction in which the influence of the electrooptic effect is diminished. In all cases the electrogyratory coefficient is found to be between 0.5–3.75×10^-13 m/V and has the same dispersion pattern like optical activity and refractive index. PACS 78.20.Ek; 78.20.Jq; 42.70.Nq     

Dispersion of electro-optic coefficients in sillenite crystals

The photorefractive crystals of the sillenite family (Bi₁₂SiO₂₀ or BSO, Bi₁₂GeO₂₀ or BGO, and Bi₁₂TiO₂₀ or BTO) that belong to the cubic 23 point group are optically active, and exhibit the piezoelectric, elasto-optic, electro-optic and electrogyration effect. In this paper we measure the dispersion of the electro-optic coefficient for all the crystals of the sillenite family in the visible spectral range (500–800 nm). For this we measure by ellipsometry the polarization properties of a beam transmitted in the transverse configuration under the action of an externally applied field. The experimental data are fitted with an analytical expression of the beam polarization to find the electro-optic coefficient. The results show a normal dispersion of the electro-optic coefficient in all three sillenite crystals, similarly to other electro-optic crystals.     

Critical enhancement of nonlinear response in fast photorefractive crystals

It is shown that when approaching the spatial subharmonics generation threshold in fast photorefractive crystals (the sillenites, CdTe), a practically unlimited (singular) amplification of the nonlinear photorefractive response is possible, which results in a drastic increase in the spatial amplification of weak signals. A theory of critical spatial amplification is developed. This theory takes into account real attributes of fast photorefractive crystals such as the vectorial nature of wave coupling and nonuniform broadening of resonances owing to light absorption. The theory is applied to the analysis of the observable characteristics of critical enhancement and to optimization of the conditions of experiments aimed at the detection and investigation of this phenomenon.     

Electro-optic coefficient and wavelength dispersion in sillenite crystals

We report on the measurement of the electro-optic or Pockels coefficient of titanosillenite crystals and show that there is no sensible wavelength dispersion, at least in the explored 510–650 nm wavelength range. This result is in close agreement with a previous paper by Papazoglou et al.: Appl. Phys. B 65, 499–503 (1997) for titanosillenites but in contradiction with a recent paper by Efremidis et al.: Appl. Phys. B 95, 467–473 (2009) reporting on a large dispersion for sillenite crystals. We stress on the fact that photoinduced space-charge electrical polarization may be at the origin of a misleading apparent wavelength dependence of the electro-optic coefficient.     

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     

Secondary photorefractive centers in Sn2P2S6:Sb crystals

Secondary photorefractive centers in Sb-doped Sn2P2S6 have a lifetime comparable to the formation time of the space-charge grating. This considerably affects the dynamics of two-beam coupling and results in a new type of transient gain enhancement for preilluminated samples.     

Real-time holographic interferometry using sillenite photorefractive crystals. Study and optimization of a transportable set-up for quantified phase measurements on large objects

We present the development of a holographic interferometer that uses a photorefractive crystal of the sillenite family as holographic recording medium. The aim of this work is to achieve a transportable and breadboard instrument, with a flexible design and which is user friendly for the quantified measurements of displacements on large scattering objects. The state of the art of the use of photorefractive crystals in holographic interferometry is presented. Based on the latter, a method that is the best suited to our mentioned objectives is chosen: the real-time interferometric technique associated with the crystal configuration exhibiting diffraction anisotropy. On this basis, we have studied and compared two imaging systems, we have experimentally determined the holographic figures of merit of three sillenite crystals samples, and, finally, we have shown the use of two measurement quantification methods. The result of these investigations is a certified and transportable holographic camera prototype containing all the necessary equipment for its operation, which can be easily adapted to various applications. Among these, we show its use in the detection of defects in composite structures, in the measurement of time-evolving deformations, and in the measurement of vibration modes. Received: 4 December 1998 / Revised version: 6 March 1999 / Published online: 12 April 1999     

Dynamics of incoherent bright and dark self-trapped beams and their coherence properties in photorefractive crystals

Using the coherent density approach, we study the propagation dynamics of incoherent bright and dark beams in biased photorefractive crystals. We show that, under appropriate initial conditions, bright as well as darklike incoherent quasi-solitons can be established in this material system. Our numerical simulations demonstrate that the coherence properties of these beams can be significantly affected by the self-trapping process.     

Photo-and thermally induced optical absorption and photoconductivity of sillenite crystals

A study is reported of photo-and thermally induced changes in the optical absorption and photo-conductivity spectra of Bi₁₂SiO₂₀ crystals in the 0.5–3.5 eV spectral interval, and of the temperature dependence of optical absorption for temperatures from 80 to 600 K. An analysis is made of the electron redistribution processes between the shallow and the deep donor and acceptor levels by invoking the configurational coordinate model.     

Color imaging in photorefractive crystals

Color phase-conjugate imaging is demonstrated using a multi-colored laser beam. Speed of response, size of the image, clarity of the image, and the intensity of the image are investigated. Color images are stored and recalled without crosstalk between different colors.     

Etalon effect in wave-mixing experiments with photorefractive crystals

We present investigations of the etalon effect in sillenite photorefractive crystals with a pulsed frequency-doubled Nd:YAG laser and show its influence on two- and four-wave mixing. In four-wave mixing the etalon effect allows changes of the diffraction efficiency by more than 80%. Results are supported by theoretical considerations. Received: 15 April 1998/Revised version: 19 June 1998     

Grating oscillations in photorefractive crystals

A theoretical analysis is presented that describes wave-mixing processes in thin holographic gratings for phase-modulated beams. Experiments with photorefractive Bi(12)TiO(20) crystals in which the signals for Bragg diffraction and for the first non-Bragg diffraction order were measured support the analysis.