Optimizing nonlinear beam coupling in low-symmetry crystals.

The purpose of this paper is to find the polarizations and spatial orientations of the two interacting counterpropagating coherent light waves which ensure the largest beam coupling in monoclinic photorefractive crystal. The results of calculations are presented that are verified experimentally with Sn2P2S6.

On temporal dynamics of Sn2P2S6 oscillation in semi-linear cavity.

Experimental measurements and calculations revealed an unusual type of oscillation dynamics of Sn(2)P(2)S(6) in the semi-linear cavity. It consists of a pronounced saw-tooth modulation of oscillation intensity--although it is not 100% in contrast--with the cw component being shifted in frequency with respect to the pump wave. This effect is attributed to the hybrid mode of two semi-linear oscillation geometries, one with a single pump wave and the other with two counterpropagating pump waves.

Self-diffraction from two-photon absorption gratings in Sn2P2S6.

Self-diffraction with the appearance of higher diffraction orders is discovered when writing a grating with a single sub-100 fs pulse in a nominally undoped Sn(2)P(2)S(6) sample. The short time of grating development, dependence of diffraction efficiency on the recording light intensity, correlation of wavelength dependence of efficiency with the spectrum of the two-photon absorption (TPA) constant, and a π phase shift of the diffracted beam allow for attributing the recorded grating to a dynamic amplitude grating of TPA.

Transient gain enhancement in photorefractive crystals with two types of movable charge carrier.

Considerable improvement of a transient two-beam coupling gain is reported for Sn(2)P(2)S(6), a photorefractive crystal that possesses two types of movable charge carrier. A gain enhancement occurs if the phase difference of the interacting beams is abruptly changed to pi. Enhancement is also achieved with periodic phase variations of zero and pi between two discrete states at modulation frequencies lower than the smallest of two reciprocal characteristic times of the space-charge formation.

Frequency mixing of photorefractive and ferroelectric gratings in lithium niobate crystals.

Holographically recorded photorefractive gratings in periodically poled lithium niobate crystals (PPLNs) are investigated. The principal spatial frequency K of the grating is strongly suppressed. Sideband gratings with grating vectors K +/- G appear. From the measurements the domain grating vector G and the duty cycle of the domain structure can be obtained. These findings allow for fast nondestructive quality inspection of PPLN and are of importance for any optical application combining holography and PPLN.

Holographic recording with reduced intermodulation noise in periodically poled lithium niobate.

We show that the use of periodically poled lithium niobate doped with Fe and Y ensures a considerable improvement in the quality of reconstructed images compared with the use of single-domain crystals. This improvement is due to inhibition of intermodulation noise and elimination of optical damage.

Slowing down of light in photorefractive crystals with beam intensity coupling reduced to zero.

A considerable deceleration of light pulses in crystals with nearly compensated space-charge field proves that the strong dispersion of dynamic photorefractive gratings in the vicinity of Bragg resonance is of primary importance for light slowing down.

Light pulse slowing down up to 0.025 cm/s by photorefractive two-wave coupling.

It is shown experimentally and theoretically that photorefractive wave coupling can be used for dramatic (< or approximately 0.025 cm/s) deceleration of light pulses whose width is larger than (or comparable with) the nonlinear response time. This classical nonlinear scheme exhibits similarities with the technique based on the quantum effect of electromagnetically induced transparency. The main distinctive feature of our scheme is amplification of the delayed output pulse. Advantages of the novel technique and its prospects for manipulation with light photons are discussed.

Instability of single-frequency operation in semilinear photorefractive coherent oscillators.

The transition of the single-frequency oscillation of a semilinear photorefractive coherent oscillator for sufficiently large coupling strengths into two-frequency oscillation is predicted and is observed experimentally. The critical value of the coupling strength at which the bifurcation occurs is a function of pump intensity ratio and cavity losses. The supercritical bifurcation in the oscillation spectrum is analogous to the second-order phase transition.

Degenerate parametric light scattering in periodically poled LiNbO3:Y:Fe.

The first observation of parametric light scattering patterns (rings, lines, and dots) in bulk periodically poled nonlinear media is reported. Development of the scattering patterns proves efficient photorefractive grating recording and considerable parametric gain for seed radiation in this new nonlinear material. Several novel phase-matched parametric processes, caused by the periodicity of the domain structure, are revealed.

Photorefractive response of bulk periodically poled LiNbO3:Y:Fe at high and low spatial frequencies

Polydomain periodically poled samples of iron-doped lithium niobate allow for phase grating recording with the diffraction efficiency close to that ensured by a single-domain sample with identical thickness. At the same time the optical damage induced by a finite transverse size light beam is considerably reduced in the periodically poled sample as compared with single-domain material.

Parametric amplification of a coherent light wave in photorefractive BaTiO(3) by a single pump beam.

We demonstrate a new method of light amplification based on forward four-wave mixing in photorefractive BaTiO(3). A single pump beam of combined (ordinary and extraordinary) polarization can be used for an efficient cw amplification of a signal wave with slightly shifted temporal frequency. A signal wave with an initial power of less than 1 nW is amplified ~100 times in a 3.2-mm-thick sample.

Photorefractive beam coupling in tin hypothiodiphosphate in the near infrared.

Infrared recording of phase gratings is investigated in Sn(2)P(2)S(6) crystals by radiation of a Nd(3+):YAG laser (lambda = 1.06 microm) Space-charge formation and hologram recording are dominated by diffusion charge transport. Without an applied electric field, but with preexposure of the sample to incoherent white light, the gain factor exceeds 6 cm(-1) at laser intensities above 50 W/cm(2).

Stochastic photorefractive backscattering from LiNbO(3) crystals.

Our experiment shows that stimulated backscattering from photorefractive LiNbO(3):Fe crystals occurs not only for an orientation of the polar axis that is parallel to the input beam but also for the opposite orientation, in which the spatial amplification of seed scattering through pi/2-shifted gratings is impossible. In the latter case scattering exhibits pronounced noise properties. We show analytically and numerically that the observed regularities are explained by a model including a source of low-frequency temporal noise for the space-charge field and a strong photovoltaic effect.

Spatial subharmonic generation of orthogonally polarized light waves in BaTiO(3) by phase-matched nonlinear mixing.

The development of a spatial subharmonic, i.e., of a light wave propagating at the bisector of two pump waves, with orthogonal polarizations incident upon a BaTiO(3) crystal in a plane normal to the optical axis is observed and studied. Parametric amplification of a seed wave meeting the phase-matching condition in the presence of two pump waves is shown to be the main reason for subharmonic generation in this crystal.

Mirrorless coherent oscillation due to six-beam vectorial mixing in photorefractive crystals.

A cw mirrorless oscillation is observed in a photorefractive LiNbO(3):Cu crystal when the crystal is exposed to two nearly counterpropagating pump waves from an Ar(+) laser. This oscillation is shown to result from a parametric vectorial six-beam interaction in a medium with a photovoltaic nonlinearity.

Degenerate stimulated parametric scattering in LiTaO(3).

Degenerate nonlinear light scattering in LiTaO(3) caused by parametric forward four-wave mixing of waves with orthogonal polarization is reported. Scattered light is located on a cone whose apex angle is defined by the phase-matching condition. The photorefractive-type nonlinearity involved in this process is caused by the excitation of spatially oscillating photovoltaic currents directed normal to the optical axis and the subsequent formation of a volume space-charge field modulating the refractive index.

Copper-vapor laser with self-starting LiNbO(3) nonlinear mirror.

A copper-vapor laser with a self-starting nonlinear mirror in a photorefractive LiNbO(3):Fe crystal is described. The nonlinear mirror is a reflection-type volume phase grating recorded in the crystal by a superradiance beam of the active copper-vapor tube.