V. M. Petnikova

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.

Chirped elliptically polarized waves in an isotropic gyrotropic nonlinear medium: approximate solution to the propagation problem

An approximate solution to the nonintegrable propagation problem for chirped elliptically polarized waves in an isotropic gyrotropic medium with local and non-local parts of the cubic nonlinear optical response and second order frequency dispersion has been obtained. Conditions for excitation of waves with periodic changes in the polarization state (chirped elliptically polarized cnoidal waves) and implementation of aperiodic propagation regimes similar to polarization ‘chaos’ have been determined.

Biharmonic pumping technique for YBaCuO energy spectrum research in the vicinity of the phase transition

Abstract The temperature dependence of η(Ω) (here η is the self-diffraction process efficiency, Ω is the frequency detuning of the biharmonic pumping components) for HTSC Y-Ba-Cu-O thin films in the vicinity of the superconductive transition point Θc has been investigated. It is shown that the specific “dip” in η(Ω) for temperatures Θ (1- Θ Θ C ) 1 2 (here kB is the Boltzmann constant) for the temperature range 80 K ⩽Θ⩽ΘC is correct A = 3.12+0.22-0.26 consistent with the BCS weak-coupling limit (A ≅3.06) and ΘC = 91.0+1.3-0.9 K have been found.

Particular periodic solutions to a nonintegrable system of Schrödinger nonlinear equations and their eigenvalues

Particular solutions to the general case of a nonintegrable system of Schrödinger nonlinear equations in the form of a family of cnoidal waves are found on the assumption of linear relationship between the squared moduli of the orthogonal components of a vector field. The domains of existence of solutions and the corresponding spectra of eigenvalues are determined.

Nonlinear spectroscopy of YBaCuO and Ni thin films by a biharmonic pumping technique

Abstract High temperature superconducting (HTSC) (YBaCuO, critical temperature Tc=87 K) and metal (Ni) thin films are investigated by a picosecond spectrometer with two tunable dye lasers. The selfdiffraction process efficiency η versus the biharmonic pumping component detuning Ω has been measured at temperatures Θ=300 and 80 K. For the HTSC films at Θ=80 K a well-defined dip on the disperion curve η(Ω) for − 10 > Ω > −50 cm−1 is found. The upper limit of the region of this dip corresponds to a value of 2Δ of the superconducting energy gap. At 10 cm −1 no such effect is found. In other respects all obtained dispersion curves resemble each other. These curves consist of a central peak ( η∼10 −7 , |Ω| × 10 cm −1 ) and wide wings ( η∼10 −9 , |Ω| > 10 cm −1 ) with a complex interference structure. The YBaCuO film resonances coincide with the phonon mode frequencies (120, 335 and 580 cm−1). It is also shown that the photoexcitation kinetics must consist of some components. The characteristic timme of the most “ultrafast” component below 5 fs has been estimated and a complex subpicosecond quantum beat presence has been predicted.

Stationary and monstationary nonlinear spectroscopy of GaSe

Abstract An effective complex investigation of semiconductors by means of nonlinear spectroscopy is reported. The main nonlinearity physical mechanisms are investigated and characteristic times of some relaxation processes are measured in GaSe at 293 K.

Spatial Solitons and Shock Waves in Photorefractive Crystals with Nonlocal Nonlinearity

An analytical model has been developed to describe both drift and diffusion mechanisms of nonlinear response (local and nonlocal components of nonlinearity) of photorefractive crystals (PRCs) on a microscopic level. Some types of stable self-consistent distributions of light intensity — bright and dark photorefractive spatial solitons — have been considered. Paths of their propagation (self-bending) have been calculated and a possibility of formation of a new type of soliton-like distributions — spatial shock waves — has been shown. Analysis of plane wave modulation instability has been performed and specific spatial scales of light distribution, resulting from self-action processes (fanning), have been calculated. All the analytical results have been confirmed using computer simulation.

Picosecond nonlinear spectroscopy of quantum‐size PbTe films

We propose a new and promising technique to study quantum‐size structures. In ultrathin monocrystalline PbTe films with thicknesses L=6, 18, and 30 nm, we have examined this technique, based on two‐photon optical excitation of the renormalized electronic structure and revealed a sharp dependence of frequencies of two‐photon resonant transitions on L. We explained this result within the framework of a model taking into account the real PbTe band structure, electron–electron and electron–phonon interactions, and interband and intra band redistribution of free carriers. We have estimated the characteristic time of interband polarization decay as T2≥300 fs and have analyzed the transformation of the nonlinear response for the case L→∞.

Nonlinear spectroscopy of Y-Ba-Cu-O nonequilibrium states excited by picosecond optical pumping

Abstract Nonequilibrium states of HTSC Y-Ba-Cu-O films with a thickness of 300 nm at the temperatures Θ =80–85 K below the phase transition point Θ c ≅91 K have been investigated by a biharmonic pumping (BP) technique. It has been found that as a result of optical excitation by picosecond laser pulses with the energy density of 10 −4 -10 −3 J/cm 2 at the wavelengths of 1.064, 0.532 and 0.620 μm, thermodynamic equilibrium is attained only after a time delay of more than 1 ns. Both for the above-specified case and for the case with the equilibrium superconductive phase at Θ c , we revealed the same specific dip in the self-diffraction efficiency as a function of the BP frequency detuning Ω in the spectral range - 10 cm −1 > Ω > − 100 cm −1 . This dip conservation has been interpreted as a manifestation of a rather slow phase transition being responsible for the pairing of carriers.

Multiple-scattering Henyey-Greenstein phase function and fast path-integration

It is shown that stability of Henyey-Greenstein phase function gives a possibility to solve quickly a multiple-scattering light propagation problem with the same a priori information about interaction as in the primary problem definition.