Self-pumped phase conjugation in Bi12TiO20 crystal without external cavity

We present an experimental study of the self-pumped phase-conjugate-mirror (SPPCM) recording in a Bi₁₂TiO₂₀ sample due to the partial internal reflection of the pump beam from the rear face. SPPCM has been recorded under an external alternating electric field of a square-wave form with response time equal to 170 s at the pump beam intensity of 520 mW/cm². The mechanism of the SPPCM formation is similar to that of photorefractive double-phase-conjugate mirrors. The most effective SPPCM is recorded at the much larger pump-beam-incident angle than it could be supposed from the typical angular dependence of the two-wave-mixing gain factor. Strong dependence of the gain distribution on the pump-beam-propagation angle is observed. It is found that the response time strongly depends on the preliminary history of the sample. Received: 11 August 1998 / Revised version: 14 January 1999 / Published online: 28 April 1999     

Beam coupling and decoupling in degenerate two-wave mixing in a reflection geometry with photorefractive Bi2GeO20 crystals

The dependence of the effective gain on the incident angle of the beams and on the initial beam intensity ratio is studied in the experiment of degenerate two-wave mixing (TWM) in a reflection geometry with photorefractive Bi₁₂GeO₂₀ (BGO) crystals. A saturation value of the effective coupling constant ¦g¦-0.4 cm^–1 is obtained. In contrast to TWM operated in the drift mode (i.e. with a nonzero electric field applied to the crystal), for TWM operated in the diffusion mode (zero external electric field) as is our case, beam coupling is reduced by moving the crystal or the interference fringes at a constant speed. At high moving speeds, complete beam decoupling can be reached. A comparison between the theoretical and measured dependence of the effective gain on the moving speed is also made. Using this technique, complete isolation of two intersecting coherent beams inside a nonlinear medium can be achieved.     

High optical gain using counterpropagating beams in iron and terbium-doped photorefractive lithium niobate

Exceptionally high values of photorefractive gain coefficient of up to 100 cm^-1 have been observed in Fe-doped and Fe/Tb-doped crystals of photorefractive lithium niobate. It is believed that these are the highest observed coefficients of any crystalline photorefractive medium. Accurate measurements of gain coefficient have been possible for the first time by using a specially cut triangular crystal which allows the use of short interaction lengths and the complete elimination of surface reflections by having the beams incident at Brewster’s angle. Experimental results are consistent with a simple model of photorefractive beam coupling up until the onset of noise which is observed to deplete the pump for interaction lengths longer than 1 mm. Received: 28 September 1998 / Revised version: 8 January 1999 / Published online: 31 March 1999     

Photorefractive effects in long, narrow BSO crystals with applied electric field

In a photorefractive Bi₁₂SiO₂₀ crystal with high applied electric ac field of square-wave shape a fast two-wave coupling response (less than 1 s) and a slow hologram readout decay (minutes) was found for a wavelength of 633 nm. This can be explained by electron–hole transport with two trap levels. An intensity dependence of the slower complementary grating was found. Illuminating with the readout wave without applied electric field leads to a very slow grating decay (many hours). Received: 18 November 1998 / Revised version: 3 February 1999 / Published online: 7 April 1999     

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     

Light-induced scattering in SBN：Cr crystal under external electric fields and its suppression

In this paper, the enhancement of light-induced scattering in congruent SBN:Cr (Sr_{0.61}Ba_{0.39}Nb_2O_6:Cr) crystals in the presence of an externally applied electric field and its suppression are studied. If a coherent image is focalized in SBN:Cr crystal without applying external electric field, the output image will remain clear, because of the weak photorefractive effect in the crystal. When a field is applied properly along the crystal axis, markedly enhanced scattering from the signal beam and the output image dispersion can be observed due to the increase of the photorefractive two-beam coupling gain and the light-induced index change in SBN:Cr crystals. By introducing a coherent or incoherent beam with higher intensity the light-induced scattering can be suppressed through the erasure of scattering gratings. The difference between coherent and incoherent beam is that the former can also amplify the signal beam as the scattered light is removed, whereas the latter can only make the signal beam revert to its initial state. The results obtained under different experimental conditions are consistent with theoretical analysis.     

INVESTIGATION OF PHOTOREFRACTIVE TWO-WAVE COUPLING IN Cr-DOPED STRONTIUM BARIUM NIOBATE CRYSTAL

We present here the experimental results of photorefractive two-wave coupling in the congruent strontium barium niobate (SBN:61) crystal doped with 1000ppm Cr. Employing a He－Ne laser (632.8nm), we observed the coupling characteristics under different conditions. The crystal shows excellent photorefractive properties, with a high coupling coefficient nearly 6cm^-1 as the beam intensity ratio m is less than 100. The saturated coupling coefficient of SBN:61:Cr shows a maximum at a certain external beam crossing angle 2θ_peak, which varies with different m, showing a nearly linear dependence on m. The saturated coupling response time τ is measured to be less than 0.8 s. The response time decreased with increasing beam crossing angles no matter how large m is. We also observed the behaviour of the probe beam in reversed experimental procedures. We found that the probe beam shows a bistable state in both procedures.     

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.     

Two Beam Coupling in Semi-Insulating GaN Film Using Electroabsorption Effect

The photorefractive effect was observed in He-ion-implanted semi-insulating GaN film in the UV spectral region. The photorefractive grating is induced by the electroabsorption effect near the band-edge. Two beam coupling experiments were performed at a wavelength of 363.8 nm. The experimental results indicate that absorption grating mainly contributes to the beam coupling.     

Rayleigh wave scattering from a vertical edge of isotropic substrates

The paper numerically studies the harmonic Rayleigh wave scattering at the 90-degree corner of isotropic substrate. The finite element method is used. The main attention is paid to two cases. The first one is the apex of the substrate corner is rounded off. The second one consists in that a layer of foreign material is deposited on the face which scatters the Rayleigh wave. The dependence of the reflection and the transmission coefficients on the Poisson ratio, the angle of incidence, the fillet radius, and the layer thickness are obtained. It is found that if the Rayleigh wave is incident perpendicularly to the substrate border, then the fillet of small radius as compared to the wavelength increases the reflection coefficient and decreases the transmission coefficient by factors 1.3–1.8. At normal incidence, the Poisson ratio does not change qualitatively the dependence of the reflection and transmission coefficients on the fillet radius. But the Poisson ratio can substantially affect the angle dependence of these coefficients if the wave is incident obliquely on the corner rounded off. It is also found out that a layer can modify the conditions of scattering such that the incident wave is totally reflected without transmission and conversion into bulk waves in a wide interval of angle of incidence, although, in principle, the bulk wave generation is allowed within a part of this angle interval.     

Measurement of the optical transmission modes and losses of the poled guest–host polymer NAEC/PEK-c planar waveguides

The polyetherketone (PEK-c) guest–host polymer planar waveguides doped with (4′-nitro)-3-azo-9-ethyl-carbazole (NAEC) were prepared. The waveguide films were poled by corona-onset poling at elevated temperature (COPET), and the corona poling setup includes a grid voltage making the surface-charge distribution uniform. By using the prism-in coupling method, the dark-line spectrum given by the reflected intensity versus the angle of incidence have been obtained, and the optical transmission losses of mth modes have been measured for the poled polymer waveguides at λ=632.8 nm. The measurement result showed that the optical loss of the fundamental mode is less than 0.7 dB cm⁻¹ for the TE polarization.     

Discovery of bound exciton in ZnSe : Cu

A new bound exciton with the dissociation energy of 32 meV was observed in ZnSe : Cu by electroabsorption. The dependence of the electroabsorption peak on an electric field, the absorption and the photoluminescence spectra show that the bound exciton is associated with Cu²⁺.     

Achromatic multibeam coupling in KNbO3: Rb

We examine, both theoretically and experimentally, photorefractive coupling in KNbO₃: Rb with beams at several wavelengths in a dual grating achromatic arrangement. It is shown that, at normal incidence, the coupling gain in the multi-wavelength achromatic configuration is nearly equal to the gain of two beam coupling with a monochromatic source of the same intensity. The dependence of the achromatic gain on the angle between the bisector, of the pump and signal beams, and the normal to the crystal surface is also studied.     

Photorefractive effect of a novel conjugate polymer containing transition metal complex

The photorefractive effect (PR) of a novels σ-π alternating polymer having 2,2'-bipyridyl in the polymer backbone and their ruthenium complexes has been investigated. The ruthenium complex was used as the charge generator, the σ-π alternating polymer backbone as the charge transporting channel and second-order nonlinear (NLO) optical chromophore. The photorefractive properties were demonstrated by two-beam coupling (2BC), degenerated four-wave mixing (DFWM) and field-induced orientation birefringence at wavelength of 532 nm. This polymer shows a enhanced photorefractive effect due to the efficient photoinduced metal-to-ligand charge transfer (MLCT) inside the ruthenium complex. A net optical gain of about 22 cm^-1 and the diffraction efficiency about 10% were obtained at the external electric field of 30 V/μm. Received 21 December 1999 and Received in final form 7 July 2001     

Spatial solitons in photorefractive semiconductors with the Franz-Keldysh effect

The features of the nondiffractive propagation of light beams in photorefractive semiconductors under conditions for manifestation of the Franz-Keldysh effect are investigated. It is shown that the combined influence of electrorefraction and electroabsorption in such materials leads to the formation of inhomogeneous spatial solitons, for which the directions of energy propagation and the phase velocity are different. The dependence of the soliton shape on the parameters of the photorefractive semiconductor and the external electric field strength is established. Zh. Tekh. Fiz. 68, 80–84 (October 1998)     

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     

Reconstruction of parasitic holograms to characterize photorefractive materials

Photoinduced light scattering is a serious drawback that limits the applicability of thick holographic recording media but provides valuable information on the recording medium. As long as there is no correlation between the scattering centers in the crystal, photoinduced light scattering may be explained to result from the interference pattern of the incident beam and the field scattered from a single point-like scattering center. The hologram of this ellipsoidally scattered wave field will have practically the same structure in the reciprocal space modified by a response function which reflects the anisotropic properties of the recording medium. We studied photoinduced light scattering in LiNbO₃:Fe, a model system for photorefractive materials. The transmitted intensity in the stationary state of the scattering process is investigated as a function of the reconstruction angle at different wavelengths and polarizations of the reconstructing beam. The experimental results are analyzed by a simple phenomenological model based on the Ewald construction and can be used to choose suitable conditions at which holographic scattering can be minimized as well as to extract some physical parameters of the crystal. Received: 27 September 2000 / Revised version: 1 December 2000 / Published online: 21 February 2001     

Self-bending of photorefractive solitons

Self-bending of photorefractive solitons is caused by diffusion in photorefractive crystals and becomes an important effect when the beam size is in the range of the charge carriers diffusion length. In this paper we present an experimental and numerical examination of the beam bending dependence on relevant parameters such as the applied electric field and the beam intensity. We demonstrate that the bending dependence on the electric field in the low saturation regime has the form of a square function at low values of the field and becomes linear for higher values. For stronger saturation the curve gets the form of a square root function. The bending dependence on the beam intensity has a maximum at defined intensity. The experimental data are compared with numerical simulations, giving a good qualitative agreement.     

Internal reflection mode scanning near-field optical microscopy with the tetrahedral tip on metallic samples

An internal reflection mode is introduced for scanning near-field optical microscopy (SNOM) with the tetrahedral tip. A beam of light is coupled into the tip and the light specularly reflected out of the tip is detected as a photosignal for SNOM. An auxiliary STM mode is used to control the distance during the scanning process and to record the topography of the sample simultaneously with the SNOM image. Images were obtained of different metallic samples which show a contrast in the order of 10% of the total reflected photosignal. In images of metallic samples an inverted contrast is consistently obtained compared to images previously obtained of comparable samples in a transmission mode. The contrast shows a pronounced dependence on the polarization of the incident beam with respect to the orientation of the edges of the tip. In the case of gold surfaces, the photosignal as a function of distance between the tip and the surface shows a pronounced peak in the near-field range of 0–20 nm which is tentatively attributed to the excitation of surface plasmons on the gold surface. The pronounced near-field effects and the strong contrast in the near-field images and the resolution well below 50 nm are an indication of a highly efficient coupling of the incident beam to a local excitation of the tip apex which is essential for the function of the tip as a probe for SNOM. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 21 October 1999     

Anisotropic conical scattering in photorefractive BaTiO3: diffraction efficiency measurements

The simultaneous presence of beam fanning and anisotropic conical scattering hinders the exact measurements on anisotropic conical diffraction. In this paper, a method is described for the measurement of diffraction efficiency of the gratings responsible for anisotropic conical scattering in a photorefractive BaTiO₃ crystal. Dependence of diffraction efficiency on the angle of incidence of the incident beam and the build-up processes of beam fanning and anisotropic scattering have been studied.