Mutual repulsion of optical beams in media with nonlocal nonlinearity

Specific features of the dynamics of reflection of optical beams in media with defocusing thermal nonlinearity have been studied. It is shown that, upon laser heating of a medium, the induced inhomogeneity profile outside the pump beam is determined by thermodiffusion and heat sink conditions at sidewalls. Signal-wave trajectory equations in a medium with an induced refractive index gradient have been derived and solved. A dependence of the nonlinear total internal reflection angle on the pump power and the initial distance between the beams has been established.     

Polarization beam splitting in media with light-induced anisotropy

The problem of light beam reflection from a refractive-index inhomogeneity induced by a high-power polarized Gaussian pump beam in a resonant medium has been simulated. A nonlinear optical method of spatial splitting of polarization components of a probe pump beam is proposed.     

Radiation force and shear motions in inhomogeneous media

An action of radiation force induced by ultrasonic beam in waterlike media such as biological tissues (where the shear modulus is small as compared to the bulk compressibility) is considered. A new, nondissipative mechanism of generation of shear displacement due to a smooth (nonreflecting) medium inhomogeneity is suggested, and the corresponding medium displacement is evaluated. It is shown that a linear primary acoustic field in nondissipative, isotropic elastic medium cannot excite a nonpotential radiation force and, hence, a shear motion, whereas even smooth inhomogeneity makes this effect possible. An example is considered showing that the generated displacement pulse can be significantly longer than the primary ultrasound pulse. It is noted that, unlike the dissipative effect, the nondissipative action on a localized inhomogeneity (such as a lesion in a tissue) changes its sign along the beam axis, thus stretching or compressing the focus area.     

在大气湍流斜程传输中拉盖高斯光束的轨道角动量的研究

大气湍流引起大气折射率随机变化,导致空间不均匀性.拉盖高斯光束在大气湍流中传输时,空间不均匀性会使光子波函数改变,引起轨道角动量的变化.本文讨论了拉盖尔高斯光束在大气斜程传输时,湍流介质改变光子轨道角动量而形成不同的光子态.计算了螺旋谐波各分量所占光束总能量的权重,分析了拉盖高斯光束的轨道角动量的变化规律.     

Lateral shift and tunneling of optical beam reflected from gradient-induced inhomogeneity

Two effects have been analytically and numerically studied: Goos-Hänchen shift, which is acquired by an optical beam reflected from a gradient inhomogeneity, and tunneling of radiation through a narrow induced inhomogeneity. A possibility of increasing the lateral beam shift in comparison with total internal reflection from a homogeneous medium is revealed. The dependence of the tunneling coefficient on the inhomogeneity parameters is determined and the critical inhomogeneity width at which the tunneling effect arises is found.     

Local heating of a liquid-like medium by a phase-conjugate ultrasound beam with self-adaptive focusing

Local heating of a liquid-like medium by means of self-adaptive focusing of a high-power phase conjugate ultrasound wave has been experimentally implemented. A sample of organic silicon polymer with a reflecting inhomogeneity was placed in water and exposed to a plane ultrasound beam so as to make the reflected acoustic field partially to enter the phase conjugation device. A parametrically amplified and conjugate wave was self-adaptively focused back to the inhomogeneity. Thermocouple measurements revealed ultrasound heating, which was localized near the inhomogeneity and depended weakly on its position within the aperture of the incident ultrasound beam. Themaximum heating value was about 6.5°C in 120 s with a frequency of ultrasound of 4.75MHz and a mean power of phase conjugate wave of about 0.1W.     

Profile of radial inhomogeneity for stationary self-trapped propagation of laser beams in a non-linear absorbing/amplifying medium with arbitrary non-linearity

Summary This paper presents an analysis of stationary self-trapped propagation of a Gaussian laser beam in radially inhomogeneous absorbing/amplifying medium with saturating non-linearity. Using the paraxial theory, the profile of radial inhomogeneity corresponding to stationary self-trapped propagation has been evaluated.     

Two-dimensional diffraction on a cylinder induced by a laser beam in a nonlinear gradient medium

A wave theory was developed and a numerical simulation performed of the diffractive flow of a signal wave around a negative inhomogeneity induced by a cylindrical pump beam in nonlinear medium while keeping both waves in the potential well. It was shown that a broad pump beam reflects a narrow signal beam. A nonlinear waveguide (a dark soliton) is described as well. These phenomena can be observed in Bose-Einstein condensate (BEC) and in a gradient waveguide.     

Passage of a Gaussian Beam through a Gradient Medium

The problem of the passage of a Gaussian laser beam of light through a layer of gradient medium, whose dielectric permeability changes linearly in a direction approximately perpendicular to that of the propagation of the beam, is solved in the paraxial–optics approximation. The center of the beam in this medium propagates along a parabola, its divergence is independent of the parameter of the medium inhomogeneity, while the phase front remains approximately spherical.     

Linear Optic Response of a Noncollinear Magnetic System: Hydrodynamic Theory

A hydrodynamic theory of the linear response of a noncollinearly magnetized medium interacting with electromagnetic radiation has been developed. Linear and quadratic magnetization effects caused by the spatial inhomogeneity of the magnetic moment have been analyzed. Linear magnetization effects include an effect similar to nonreciprocal birefringence, as well as reciprocal and nonreciprocal rotation of the plane of polarization, caused by the inhomogeneity of the magnetic moment. It has been shown that an effect caused by the equilibrium spin current can appear in the considered medium. This effect is determined either by the inhomogeneity of the spin current or by the spatial dispersion of a wave. The effect associated with the spatial dispersion of the wave is linear in its wave vector and is similar to nonreciprocal birefringence. The effect associated with the inhomogeneity of the spin current describes the rotation of the plane of polarization, which, however, can occur in the system with zero average magnetization.     

Focussing of low-power laser beams in inhomogeneous plasmas with linear dielectric constant variation

In this paper we consider the propagation of a low-power laser beam in an inhomogeneous plasma with a dielectric-constant variation which linearly decreases away from the interfacez=0. It has been shown that the inhomogeneity of the plasma may cause the laser beam to focus into a line, a situation which could be of importance in x-ray laser studies and also in the ablation halo of laser fusion pellets.     

Experimental investigation of emission inhomogeneity of gyrotron cathodes basing on their current-voltage characteristics

A scheme for automatic measurement of current-voltage characteristic of gyroresonance devices has been developed. Efficient emission inhomogeneity of cathode in powerful gyrotrons has been studied basing on analysis of current-voltage characteristics. Experimental data on the parameter of efficient emission inhomogeneity for different regimes are presented, as well as the estimate for admissible efficient inhomogeneity of the cathode for powerful gyrotrons and the dependence of electron beam parameters on efficient inhomogeneity of the cathode. A method of complex measurements is proposed, which makes it possible to judge about the contribution of different physical mechanisms into emission inhomogeneity of the cathode.     

Paraxial propagation in disclinated amorphous media

We study paraxial beam propagation along the wedge axis of a disclinated amorphous medium. The defect-induced inhomogeneity results in Berry phase and curvature that are affected by the induced uniaxial anisotropy. The Berry phase manifests itself as a precession of the polarization vector. The Berry curvature is responsible for the optical spin Hall effect in the disclinated medium, where beam deflection varies sinusoidally along the paraxial direction. Its application in determining the birefringence and the magnitude of the Frank vector is explained.     

EBIC investigations of GaN layers prepared by epitaxial lateral overgrowth

GaN films prepared by lateral overgrowth are investigated by scanning electron microscopy in the electron beam induced current (EBIC) mode. A comparison of experimental and simulated dependences of induced current on beam energy has allowed us to determine not only the diffusion length, but also the donor concentration in different areas of a film. It has been found that the donor distribution is inhomogeneous and this inhomogeneity increases under fast neutron irradiation. This is indicative of the significant influence of structural defects on the rate of radiation defect accumulation. An anomalously slow signal decay outside the Schottky barrier has been found, which can be determined by charged defects formed at the merger boundary.     

Thermal signatures of a localized inhomogeneity in a 2-D participating medium subjected to an ultra-fast step-pulse laser wave

The transport of a train of short-pulse radiation through a 2-D rectangular participating medium consisting of local inhomogeneities is investigated in this article. A collimated beam of step temporal profile is considered. The pulse wave consists of 1 and 4 pulses. The absorbing and scattering participating medium contains a square-shaped local inhomogeneity. The inhomogeneity differs from the rest of the medium by its scattering albedo. The pulse width and the period of the laser wave are of the order of a nano-second. Transmittance and reflectance signals are analyzed for the effects of the extinction coefficient and the scattering albedo. Heat flux distributions inside the medium are also studied. The finite-volume method is used to solve the problem.     

Exact solution of damped nonlinear Schrödinger equation for a parabolic density profile

An exact single envelope soliton solution of the damped nonlinear Schrödinger equation in a medium with a parabolic density profile has been obtained. It is found that the condition of reflection of the soliton from the density hump is determined by its initial velocity and the scale lengths of the inhomogeneity.     

Forward light reflection from a moving inhomogeneity

The reflection of monochromatic and quasi-monochromatic pulsed light incident on a moving inhomogeneity in the optical characteristics of a medium having plasma-type dispersion has been analyzed. The velocity V of the inhomogeneity, induced in the medium by an intense laser pulse, has been changed by varying its carrier frequency. It has been shown that the usual back-reflection mode, when the reflected radiation pulse moves in the direction opposite the direction of incident radiation, is implemented only if the velocity V is less than the critical value V _min, which depends on the carrier frequency of the incident radiation pulse. It has been found that reflected radiation moves in the same direction as the incident radiation in a certain range of the velocity V _min < V < V _max (forward reflection). In this case, the reflected radiation pulse begins to lag behind a fast-moving inhomogeneity. When V _max < V < c, where c is the speed of light in vacuum, the group velocity of the incident radiation pulse is less than the speed of inhomogeneity, and there is no reflection. Analytical treatment is supported by numerical simulation.     

Controlling spiral wave with target wave in oscillatory systems

Spiral waves have been controlled by generating target waves with a localized inhomogeneity in the oscillatory medium. The competition between the spiral waves and target waves is discussed. The effect of the localized inhomogeneity size has also been studied.     

Temporal growth of a parametric excitation by a self-focused laser beam

The effect of self-focusing of the pump laser beam on the temporal growth of a parametric excitation has been investigated in the paraxial region. The two equations for the signal and idler modes have been decoupled by assuming the near self-trapping condition and a linearly varying phase mismatch. By employing the WKBJ approximation, it is found that the growth rate is a strong function of the radial intensity inhomogeneity of the pump laser beam. The condition for validity of the first-order approximate theory employed here has been derived.     

Steady-state self-focusing of Gaussian electromagnetic beams in an inhomogeneous nonlinear medium: Effect of absorption and initial curvature of the beam

The authors have investigated the effect of linear absorption and initial curvature of an electromagnetic Gaussian beam on focusing/defocusing in an inhomogeneous nonlinear medium. Numerical computations have been made for linear inhomogeneity and saturating nonlinearity, characteristic of dielectrics and collisional plasmas. The maximum and minimum of the beam width keep decreasing with increase in distance of propagation (or absorption), till the beam becomes very weak and diverges steeply; penetration in an overdense medium also decreases with increasing absorption. Converging beams initially converge and then go in: (i) the oscillatory divergence (ii) self-focusing or (iii) steady divergence mode, depending upon the initial values of beam width and axial irradiance. The maximum penetration in an overdense medium has the highest values for −0.7<(df/dξ)_ξ=0<0.4 and falls sharply, outside these limits.