K. A. Kuznetsov

Second Harmonic Generation in a Strontium Barium Niobate Crystal with a Random Domain Structure

The phenomenon of the second-harmonic (SH) generation in a strontium barium niobate (SBN) crystal in directions not obeying the conditions of phase matching for spatially homogeneous media has been studied. The appearance of the SH signal is interpreted as resulting from the quasi-phase-matching nonlinear wave interaction due to the random domain structure of the SBN crystal containing acicular ferroelectric microdomains. A theoretical model describing the SH generation in such two-dimensional, spatially inhomogeneous nonlinear optical structures is proposed. An analysis of the second-harmonic emission directivity patterns shows that the main contribution to the noncollinear SH generation in SBN is due to near-surface domains extending to a depth of up to several dozen microns.

Cascaded coherent Raman scattering by phonon polaritons

We consider the cascaded part of coherent four-wave Raman scattering in crystals without inversion symmetry as a method of spectroscopy of phonon polaritons. The theory of this effect is developed accounting for the limited dimensions of the scattering area and imaginary character of the non-linear susceptibilities. The experimental results for four-wave cascaded scattering by polaritons in crystals of LiIO 3 ,L iNbO 3 :M g and LiNbO3 : Mg : Nd are presented. The effect of narrowing of the polariton lineshape is observed in doped LiNbO3 in the region of strong polariton absorption. The probable nature of the narrowing, connected with the excitation of the electronic states during the four-wave mixing, is discussed. Copyright ” 2000 John Wiley & Sons, Ltd.

Non-Collinear Generation of Second Harmonic in a Strontium Barium Niobate Crystal with Needle-Like Microdomains

Non-collinear second harmonic generation (SHG) in a strontium barium niobate (SBN) crystal has been studied. Topology of chemically etched SBN samples is investigated by atomic force microscopy. The analysis of the angular distribution of SHG intensity shows that the main contribution to the SHG signal is induced by near-surface domains extending inward the crystal to a depth of up to several dozen micrometers. Statistical distribution of domains lateral dimensions is obtained.

BIPHOTON WAVE PACKETS GENERATED IN APERIODICALLY POLED CRYSTALS

The influence of disordering of quasi-phase-matched structures on the properties of biphoton fields generated by spontaneous parametric down-conversion in periodically poled crystals is studied in the general form, taking into consideration arbitrary one-dimensional spatial distribution of the crystal second-order susceptibility. It is shown that the spectral amplitudes of the biphoton wave function become complex and gain frequency- and angle-dependent phase shifts in the structures with unequal lengths of domains. Effects of the spectrum broadening and the phase shifting are expressed in terms of the crystal non-linear transfer function. Spectrum of the first-order correlation function is found to be strictly dependent on the transfer function module, but insensitive to the phase shifts. Availability of the phase shifts makes the second-order temporal correlation function of the broadened as-generated biphoton field to be almost the same as in case of a single-domain crystal. At the same time, possibility of strong temporal biphoton compression appears further if an additional phase-compensating element is placed after the aperiodically poled crystal.

Terahertz-radiation generation in low-temperature InGaAs epitaxial films on (100) and (411) InP substrates

The spectrum and waveforms of broadband terahertz-radiation pulses generated by low-temperature In0.53Ga0.47As epitaxial films under femtosecond laser pumping are investigated by terahertz time-resolved spectroscopy. The In0.53Ga0.47As films are fabricated by molecular-beam epitaxy at a temperature of 200°C under different arsenic pressures on (100)-oriented InP substrates and, for the first time, on (411)A InP substrates. The surface morphology of the samples is studied by atomic-force microscopy and the structural quality is established by high-resolution X-ray diffraction analysis. It is found that the amplitude of terahertz radiation from the LT-InGaAs layers on the (411)A InP substrates exceeds that from similar layers formed on the (100) InP substrates by a factor of 3–5.

Photoinduced gratings in a Sn2P2S6 ferroelectric crystal with the period depending on the optical pump power

The light scattering in the form of ring structures in Sn2P2S6 crystals at the propagation of intense laser pump radiation along the b crystallographic axis is detected. Radiation passing through a crystal is completely scattered into a cone whose angle increases with the pump power and decreases reversibly with a decrease in this power. The observed effect can be attributed to a spontaneous increase in the amplitude of photoinduced bulk diffraction gratings and to the scattering of light on them in the directions where the phase increments of the photorefractive and diffraction natures compensate each other. A similar type of scattering was observed previously in other photorefractive crystals, but the appearance of gratings with the period easily varied by varying the pump power is demonstrated for the first time.

Generation of THz radiation in epitaxial InGaAs films on InP substates of various crystallographic orientations

We study the THz wave generation by the time-domain spectroscopy method in low-temperature grown InGaAs layers on InP substrates with crystallographic orientations (100) and (411) It was found that the THz wave generation is 3-5 times more effective in the case of (411)A InP substrates as compared to the (100) substrates. In samples grown at high pressure of As4 generation of THz waves is more effective at low-frequency range less than 200 GHz.

Non-collinear generation of the second optical harmonic in strontium-barium niobate crystals with needle-like microdomains

The structure of the spatial spectra from the second harmonic (SH) of yttrium-aluminum-garnet neodymium laser radiation during quasi-phase-matched generation is studied in a spatially inhomogeneous nonlinear medium. Based on the example of the formation of the strontium-barium niobate (SBN) domain structure a theoretical model that describes the angular distribution of the intensity of the noncollinear SH generated in the polydomain crystal with stochastically distributed domains is proposed. It is shown that the characteristic spatial domain sizes can be found from the second-harmonic intensities and that the density of the domain distribution is described by the Weibull-Gnedenko distribution.

Four-wave scattering by phonon polaritons under excitation of small polarons in LiNbO3:Mg

We studied the coherent four-wave scattering of light by phonon polaritons in LiNbO3:Mg crystals before and after chemical reduction, which resulted in strong increase of small polarons. Large broadening of polariton lineshape was observed in a chemically reduced lithium niobate. To explain this effect we studied optical properties of LiNbO3:Mg in the visible and IR regions, where the undoped LiNbO3 was transparent. As a result, absorption band was found, being sensitive to the chemical reduction. We assign this band to the small polaron absorption. The values of polaron shift and the mean value of frequency of optical phonos, which interact with free electrons polaron formation, were measured. Influence of small polarons on polariton k-spectra was discussed.