Two dimensional counterpropagating spatial solitons in photorefractive crystals

We demonstrate experimentally the existence of two transverse-dimensional counterpropagating (CP) incoherent spatial solitons in a 5 x 5 x 23 mm SBN:60Ce photorefractive crystal and investigate their dynamical behavior. We carry out numerical simulations that confirm our experimental findings. Substantially different behavior from the copropagating incoherent solitons is found. A symmetry breaking transition from stable overlapping CP solitons to unstable transversely displaced CP solitons is observed. We perform linear stability analysis that predicts the threshold for the split-up transition, in qualitative agreement with numerical simulations and experimental results.     

Frequency-degenerate nonlinear light scattering in low-symmetry crystals

A considerable distinction of light-induced scattering in optically biaxial crystals comes from the fact that the principal axes of the optical indicatrix do not coincide with the crystallographic directions. The study of photorefractive scattering in Sn2P2S6 (monoclinic symmetry) revealed the particular requirements as to light polarization for excitation of parametric scattering and the polarization inhomogeneity of wide-angle scattering.     

Dynamic light beam deflection caused by space charge waves in photorefractive crystals

During holographic recording in photorefractive crystals (BSO, BGO, and BTO) by an oscillating interference pattern we observe a strong dynamic deflection of the laser beams reflected from the crystal’s surface. The theoretical treatment shows that this new effect is associated with a nonlinear interaction of space charge gratings resulting in a quasi-homogeneous oscillating space charge field which provides deformations of the crystal via the piezoelectric effect. Received: 3 August 1999 / Accepted 5 August 1999 / Published online: 8 September 1999     

Interaction of light waves on a reflecting holographic grating in cubic photorefractive crystals

The interaction of two antiparallel light waves on a reflecting grating is analyzed theoretically in arbitrarily oriented optically active cubic crystals of point group 23. The effect of nonunidirectional energy transfer on the interaction efficiency is investigated in the undepleted-pump-power approximation in (100)-, (111)-, and $(11\bar 2)$ -cut Bi₁₂TiO₂₀ crystals.     

Polarization-degenerate parametric light scattering in photorefractive crystals

Parametric scattering of two copropagating coherent light waves into cones of light with the same polarization is considered both experimentally (in photo-refractive LiNbO₃:Fe) and theoretically.     

Optical properties of gyrotropic crystals in the field of counterpropagating ultrasound waves

The oblique propagation of light through a gyrotropic crystal layer placed in the field of two counterpropagating ultrasound waves is considered. The problem is solved using Ambartsumian’s layer addition modified method. The results of studying the dependences of the amplitude and polarization characteristics on the light wavelength for different values of the problem parameters are presented. Possibilities of applications of such systems are discussed.     

Transverse instabilities in photorefractive counterpropagating two-wave mixing

Threshold analysis for transverse instabilities in photorefractive counterpropagating two-wave mixing through reflection gratings is performed. A numerical algorithm for the treatment of wave equations in this geometry is developed, displaying the emergence of running transverse waves. They appear above a threshold in the applied electric field, and their transverse wave number and oscillation frequency agree well with the values predicted by stability analysis.     

Self-trapped excitons in low-symmetry fluoride crystals

Triplet self-trapped excitons have been detected in low-symmetry crystals of the fluorides BaFCl₄, BaFBr, and LaF₃. In BaFBr crystals two types of excitons, on-center and off-center (with a symmetric or displaced configuration). The slow initiation of exciton luminescence in BaFBr is associated with the barrier transition from an on-center to an off-center configuration. In LaF₃ crystals tunnelling population of the exciton state also appears in the initiation. In LaF₃ crystals excitons appear only in the luminescence and excitation spectra while in the absorption spectra the exciton transitions are concealed beneath band-to-band transitions. A. P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 45–56, November, 1996.     

Sublattices in crystals with low-symmetry systems

Complex crystals are represented by a set of identical or different Bravais sublattices. The presence of different Bravais sublattices in crystals results in the occurrence of latent symmetry manifested through their physical and physicochemical properties. The present paper studies the consistence of sublattices with different systems, including systems with higher symmetry, in monoclinic and triclinic crystals. Conditions of consistence following from the translational symmetry of different sublattices are found, and their spatial arrangement in crystals with the given systems is described. A relationship between the vectors of sublattice displacement from each other and systems of equivalent positions of space groups is obtained. It is demonstrated at what spatial arrangement of translatory less symmetric sublattices they can be combined into a translational sublattice with higher symmetry. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 55–62, April, 2007.     

Transverse instability threshold in counterpropagating light beams for a nonlinear medium with local photorefractive response

We derive the threshold conditions for the instability of counterpropagating waves in a nonlinear medium with local photorefractive response against the excitation of transverse small-angle structures. These conditions allow for all the important types of diffraction from refractive-index reflection gratings and are not limited to the case of strict frequency degeneracy of the waves. We study the dependence of the crystal-thickness threshold and the secondary wave emission angle on the crystal parameters and the pump conditions. We show that when the pump wave intensities differ considerably, excitation of standing light structures is replaced by excitation of traveling structures. Finally, we discuss the applications of the theory to experiments with the photorefractive crystals LiNbO₃ and LiTaO₃. Zh. éksp. Teor. Fiz. 111, 1611–1623 (May 1977)     

Second harmonic generation and rectification of space-charge waves in photorefractive crystals

A theory explaining the effects of second harmonic generation and rectification of space-charge waves (SCWs) in both space and time in photorefractive crystals and semi-insulating semiconductors is formulated for the first time. The theory predicts two mechanisms of SCW second harmonic excitation. An experimental technique is proposed for second harmonic detection, and the formulas required for interpreting the experimental results are derived. It is found that the effect of complete rectification of SCWs may lead to a change in the current passing through the sample by tens of percent. The results of calculations are in qualitative agreement with preliminary experimental results.     

Self-action of Bessel light beams in cubic gyrotropic photorefractive crystals

We have studied conversion and interaction of a Bessel laser beam of zero order in photorefractive crystals. We have established that in a cubic gyrotropic crystal, two Bessel light beams propagate with different wave vectors and cone angles, but with identical conicity parameters. As the power in the Bessel light beam increases, we have experimentally observed a new optical effect: self-action of a Bessel light beam. We have studied the effect of external influences and the parameters of the beam itself on the process of self-refraction in photorefractive crystals. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 4, pp. 488–493, July–August, 2006.     

Formation of spatial solitons and spatial shock waves in photorefractive crystals

An analytical model, which describes the drift and diffusion mechanisms for the formation of the nonlinear response (local and nonlocal nonlinearities) of photorefractive crystals on the microscopic level, is constructed. New types of stable self-consistent distributions of the light field intensity, i.e., spatial solitons, are found. The trajectories of their motion (self-bending) are calculated, and the possibility of observing a new nonlinear-optical effect in photorefractive crystals, viz., the formation of spatial shock waves, is demonstrated. The modulation instability appearing when plane waves propagate in photorefractive crystals is analyzed, and the characteristic spatial scales of the light field distribution formed as a result of self-interaction (fanning) are determined. The results of the analysis are confirmed by computer simulation data. Zh. éksp. Teor. Fiz. 111, 705–716 (February 1997)     

Bidirectional vectorial light amplification in cubic crystals with unshifted photorefractive gratings

We present a new method of light amplification based on vectorial two-wave mixing in cubic crystal via an unshifted photorefractive grating. The energy transfer between interacting waves is achieved in an experimental configuration that provides the amplitudes of modulation of refractive indices of opposite signs, Dn(1)=-Dn(2), for two orthogonal polarization modes of the interacting waves. The configuration demonstrates bidirectional amplification; i.e., the weak wave is always amplified, independent of its direction with respect to the strong pump beam. We present the results of numerical analysis along with the experimental results for a Bi(12)TiO(20) crystal with a dc applied electric field.     

Scattering of atoms in the field of counterpropagating light waves. Effect of initial conditions

The scattering of an atom in the field of counterpropagating light waves is studied under conditions such that the state of the atom is a superposition of the ground and excited states. For the case in which this superposition is created by the field of a traveling wave, the momentum distribution function of the atom after scattering by a standing wave is found analytically in the approximation of a short interaction time, when the atom’s motion can be neglected. Longer interactions of the atom with the field are studied numerically. We also consider the case of counterpropagating light waves consisting of Gaussian or supergaussian pulses. Zh. éksp. Teor. Fiz. 113, 563–572 (February 1998)     

Interference of copropagating and counterpropagating surface waves

In the case of propagation of copropagating and counterpropagating surface waves along the interface of two media, the existence of a substantial difference in the spatial distribution of the energy flux components associated with the interference of active and reactive components of the field was established. The interference flux of the counterpropagating waves that is connected with the reactive components is undamped along the direction of propagation of the waves.     

Magneto-optical resonances in the field of counterpropagating waves

The resonance of the intersection of sublevels in a probe laser field resonant to the cyclic transition corresponding to the D ₂ ⁸⁷Rb line has been investigated in a zero magnetic field. The strong effect of an additional laser field acting on the adjacent transition has been revealed. In a cell without the buffer gas and antirelaxation coating, the amplitude of the probe-field absorption resonance can increase by more than an order of magnitude in the presence of a counterpropagating wave. The effect is observed at the laser frequency tuned to the cross resonance, when the counterpropagating waves simultaneously act on moving atoms at the cyclic and open transitions with the common lower level. The theoretical analysis of the effect of the additional field on the electromagnetically induced absorption resonance is in agreement with the experimental data.     

Total transparency in quantum tunneling of counterpropagating waves

The problem of quantum tunneling is considered for matter waves which are impinging on the potential barrier from both sides. Possibility of the total transmission of one of incident waves at the simultaneous total reflection of the other one is studied. The analysis is carried out for two essentially different, rectangular and bell-shaped, forms of the potential barrier.