Self-action of Bessel wave packets in a system of coupled light guides and formation of light bullets

The self-action of two-dimensional and three-dimensional Bessel wave packets in a system of coupled light guides is considered using the discrete nonlinear Schrödinger equation. The features of the self-action of such wave fields are related to their initial strong spatial inhomogeneity. The numerical simulation shows that for the field amplitude exceeding a critical value, the development of an instability typical of a medium with the cubic nonlinearity is observed. Various regimes are studied: the self-channeling of a wave beam in one light guide at powers not strongly exceeding a critical value, the formation of the “kaleidoscopic” picture of a wave packet during the propagation of higher-power radiation along a stratified medium, the formation of light bullets during competition between self-focusing and modulation instabilities in the case of three-dimensional wave packets, etc. In the problem of laser pulse shortening, the situation is considered when the wave-field stratification in the transverse direction dominates. This process is accompanied by the self-compression of laser pulses in well enough separated light guides. The efficiency of conversion of the initial Bessel field distribution to two flying parallel light bullets is about 50%.     

Self-division of a pulse consisting of several light-field oscillations in a nonlinear medium with dispersion

The nonparaxial dynamics of light pulses consisting of several light-field oscillations in nonlinear media with dispersion is analyzed. It is shown that the self-action of these extremely short pulses can result in their self-division.     

Optical nonlinearity in thin films exposed to low-intensity light

Optical nonlinearity in semiconducting and insulating thin-film structures is studied by waveguide methods under self-action conditions at a light intensity less than 0.1 W/cm² and a wavelength of 630 nm. The optical properties vs. light intensity are similar for semiconducting and insulating films, multilayer thin-film structures, and semiconductor-doped glass films. It is demonstrated that the optical nonlinearity and the nonlinear optical constants depend on the interface condition.     

Slow-light switching in nonlinear Bragg-grating couplers

We study propagation and switching of slow-light pulses in nonlinear couplers with phase-shifted Bragg gratings. We demonstrate that power-controlled nonlinear self-action of light can be used to compensate for dispersion-induced broadening of pulses through the formation of gap solitons, to control pulse switching in the coupler, and to tune the propagation velocity.     

Formation of ultrashort pulses in the process of the propagation of tightly focused femtosecond light packets in a transparent condensed medium

The evolution of the space-time structure of tightly focused femtosecond light packets in a transparent condensed medium (fused silica) is analyzed by means of numerical simulation. The nonlinear self-action of light packets leads to the formation of ultrashort pulses (duration of about 10 fs) propagating in various directions. The spectral composition, time structure, and position of ultrashort pulses at the time axis depend on their propagation direction.     

A possibility of compensating the effect of self-action on the efficiency of second harmonic generation by femtosecond pulses

The feasibility of compensating the self-action of waves involved in second harmonic generation (SHG) by femtosecond pulses in the case of group matching is shown. Propagation of pulses is described in terms of the Schrödinger equation with the inclusion of the second-order dispersion and cubic nonlinearity. Analytical and numerical studies are performed indicating that, with some constraints imposed on the cubic nonlinearity, the SHG process occurs in the same way as in the absence of nonlinearity within a rather long propagation path.     

Structural features of self-action of laser radiation in the electromagnetic induced transparency mode

Structural features of the laser radiation self-focusing dynamics in the electromagnetic induced transparency (EIT) band are studied for an atomic system with a Λ-type energy level diagram. Effective nonlinearity of an EIT medium is manifested primarily as nonlinear dispersion (dependence of the group velocity on the wave amplitude). Qualitative analysis of the dynamics of self-action of laser pulses, which is confirmed by numerical simulation, shows that nonlinear evolution of a 3D wave packet follows the scenario of self-focusing, which serves as the background on which the envelope profile turnover and the formation of a shock wave occur at an advanced rate.     

Generation of polarization-squeezed light in doped resonant media

The cross-interaction effects for a two-mode light pulse in an optical matrix doped with atoms that are resonant with the light in the presence of two control optical pump pulses (M scheme) have been analyzed. The fundamental possibility of effective generation of polarization-squeezed light in this system at a simultaneous nonlinear amplification of the probe field is shown. The optimal conditions for generating polarization-squeezed light in a doped optical fiber are found for different problem parameters.     

Dynamics of the strong field of a few-cycle optical pulse in a dielectric medium

The dependence of the conditions for the dominance of different physical factors in the self-action of few-cycle optical pulses in dielectrics on the intensity, duration, and spectrum of radiation has been theoretically analyzed. It is shown that the larger the pulse width and the central wavelength, the stronger the effect of plasma nonlinearity. For example, for a quartz glass in the field of pulses with a duration of 10 fs and a central wavelength of 780 nm, this nonlinearity mechanism is dominant at intensities exceeding 3 × 10¹³ W cm^?2.     

Propagation of elliptically polarized laser pulses in isotropic gyrotropic medium with relaxation cubic nonlinearity and anomalous frequency dispersion

The self-action of elliptically polarized Gaussian laser pulses in an isotropic gyrotropic medium with an anomalous frequency dispersion and cubic Kerr nonlinearity with a finite relaxation time on the order of the pulse duration is numerically studied. It is shown that, at the output of the medium, the pulse polarization nonmonotonically varies with time. The main peak of the pulse is additionally delayed compared to the time of passing the linear medium; the value of this delay significantly depends on the polarization of the incident pulse and achieves a maximum for incident pulses whose degree of ellipticity is equal to the ratio of the material constants characterizing the local and nonlocal nonlinear optical response of the medium. It seems promising to search for possible differences in relaxation times depending on the intensity of additions to the refractive indices of the right and left circularly polarized waves by investigating the time dependence of polarization characteristics at the output of the medium.     

Wave processes in microinhomogeneous elastic media with hysteretic nonlinearity and relaxation

The nonlinear propagation of an initially harmonic acoustic wave in a microinhomogeneous medium containing defects with quadratic hysteretic nonlinearity and relaxation is studied by the perturbation method. The frequency dependences of the effective nonlinearity parameters are determined for the self-action of the quasi-harmonic acoustic wave and the higher harmonic generation processes.     

Self-focusing of laser radiation in cluster plasma

The self-focusing of laser radiation in plasma with ionized gaseous clusters is studied both analytically and numerically. An electrodynamic model is proposed for cluster plasma in a field of ultrashort laser pulse. The radiation self-action dynamics are studied using the equation for wave-field envelope with allowance for the electronic nonlinearity of the expanded plasma bunches and the group-velocity dispersion in a nanodispersive medium. It is shown that, for a laser power exceeding the self-focusing critical power, the wave-field self-compression occurs in a medium with dispersion of any type (normal, anomalous, or combined). Due to the strong dependence of the characteristic nonlinear field on the size of ionized cluster, the corresponding processes develop faster than in a homogeneous medium and give rise to the ultrashort pulses.     

Self-action dynamics of ultrashort electromagnetic pulses

The self-action dynamics of three-dimensional wave packets whose width is on the order of the carrier frequency is studied under fairly general assumptions concerning the dispersion properties of the medium. The condition for the wave field collapse is determined. Self-action regimes in a dispersion-free medium and in media with predominance of anomalous or normal group velocity dispersions are numerically investigated. It is shown that, for extremely short pulses, nonlinearity leads not only to the self-compression of the wave field but also to a “turn-over” of the longitudinal profile. In a dispersionless medium, the formation of a shock front within the pulse leads to the nonlinear dissipation of linearly polarized radiation and to self-focusing stabilization. For circularly polarized radiation, the wave collapse is accompanied by the formation of an envelope shock wave.     

Nonlinear Dynamics of Wave Beams and Packets in an Ionizable Medium

We present a review of the studies on nonlinear dynamics of the plasma–field system formed in the processes of breakdown of a gas by high-intensity laser or microwave radiation. The ionization instability dominating these processes significantly modifies the known effects of self-action of waves in a medium and gives rise to a number of new effects which are absent for other nonlinearity mechanisms. We describe the most important among these effects, such as the ionization–field instability of a plane wave, the self-channeling of radiation in the form of surface or leaky waves, and the self-conversion of the spectrum of the ionizing radiation. The results of numerical simulations of the dynamics of nonequilibrium freely-localized discharges created by focused microwave and laser pulses are presented.     

Spinor model of a perfect fluid

Different characteristics of matter influencing the evolution of the uUniverse haves been simulated by means of a nonlinear spinor field. We have considered two cases where the spinor field nonlinearity occurs either as a result of self-action or due to the interaction with a scalar field.     

Self-action of ultrasonic pulses in a marble rod

The nonlinear effects due to the self-action of finite-amplitude ultrasonic pulses (limitation of both amplitude and carrier phase delay) propagating in a marble rod are studied experimentally and theoretically. On the basis of the measured amplitude dependences, a nonlinear equation of state is proposed for the propagation medium. With the use of this equation, the experimental results are analytically described and the nonlinear parameters of marble are determined. It is shown that the dissipative and reactive high-frequency nonlinearities of marble are caused by different mechanisms.     

Study of light-induced reorientation of the nematic liquid crystal director by birefringence dynamics

It is proposed to apply the birefringence method to measure the threshold of the light-induced Freedericksz transition and the nonlinearity enhancement factor of nematic liquid crystals by director relaxation dynamics in the light field. A birefringence change is recorded by interference of ordinary and extraordinary components of the light beamaffecting the liquid crystal director orientation. The Freedericksz transition thresholds and nonlinearity enhancement factors are measured for the samples doped with comb-shaped polymers with various degrees of polymerization. The determined thresholds are in agreement with the results obtained by the aberrational self-action method.     

Self-focusing of wave packets and envelope shock formation in nonlinear dispersive media

The self-action of three-dimensional wave packets is analyzed analytically and numerically under the conditions of competing diffraction, cubic nonlinearity, and nonlinear dispersion (dependence of group velocity on wave amplitude). A qualitative analysis of pulse evolution is performed by the moment method to find a sufficient condition for self-focusing. Self-action effects in an electromagnetically induced transparency medium (without cubic nonlinearity) are analyzed numerically. It is shown that the self-focusing of a wave packet is accompanied by self-steepening of the longitudinal profile and envelope shock formation. The possibility of envelope shock formation is also demonstrated for self-focusing wave packets propagating in a normally dispersive medium.     

Dislocation nonlinearity and nonlinear wave processes in polycrystals with dislocations

Based on the modification of the linear part of the Granato–Lücke dislocation theory of absorption, the equation of state of polycrystalline solids with dissipative and reactive nonlinearity has been derived. The nonlinear effects of the interaction and self-action of longitudinal elastic waves in such media have been theoretically studied.