Radiophysics and Quantum Electronics最新文献

We study polariton phenomena and the mechanism of self-modulation of a highly asymmetric state of the active medium and field in a superradiant laser with a low-Q Fabry–Perot cavity based on the numerical solution of the semi-classical Maxwell—Bloch equations. Features of the predicted dissipative superradiant state arising due to the formation of an inhomogeneous self-consistent population-inversion grating, which is created by the beats of counterpropagating waves of the electromagnetic field and polarization of the active medium, are found. Evidence for excitation of polariton modes by resonant Rabi oscillations of active centers located near the maxima of the population inversion and polarization of the active medium is presented.

The thermal effects appearing inevitably when laser radiation propagates through high-Q optical microresonators lead to various drifts, fluctuations and unstable regimes. In particular, thermal effects can affect strongly the generation of optical frequency combs and dissipative soliton structures. A way to offset the thermal effects is to use the self-injection effect that locks the laser radiation frequency to the eigenfrequency of the microresonator. In this work we consider the principles and dynamics of the self-injection locking effect at high pump powers in the nonlinear regimes, including those induced by thermal nonlinearity.

We use the direct numerical simulation (DNS) method to study the vortex structure of the near-surface water layer, which is saturated with air bubbles, in the presence of a stationary surface wave. A wave with a wavelength of 15 cm and a steepness of 0.2 (an amplitude of about 0.5 cm) and bubbles 400 μm in diameter (microbubbles) are considered. Complete three-dimensional fluid motion equations (Navier–Stokes equations) are solved by the DNS method simultaneously with the equations of motion of individual bubbles with allowance for their influence on the carrier flow. Under the influence of the surface wave, the flow in the near-surface layer becomes turbulent and characterized by the presence of vortex structures stretched along the wave propagation direction. To analyze the vortex structure of the flow, the instantaneous velocity gradient tensor is calculated, and its complex eigenvalues, whose imaginary part characterizes the local vorticity of the flow, are calculated, while filtering out the contribution of the purely shear component (vortex sheet). Average profiles of the eigenvalues and the fluctuations, which are obtained at the stage of the statistically stationary flow, show that the influence of the bubbles lead to intensification of small-scale vortices and turbulent pulsations in water.

We consider various aspects of interaction of vortices with a barotropic flow. When a vortex interacts with a flow, there exist three variants of the flow-core behavior: rotation, nutational oscillations, and unlimited stretching. In the first two cases, the vortex remains a localized formation, such that the ellipse semiaxes undergo oscillations near certain average values. In the third case, the shape of the vortex varies as follows: one horizontal axis increases indefinitely and the second horizontal axis tends to zero so that the vertical size of the vortex does not change and the vortex itself stretches into a filament in top view, remaining ellipsoidal. As a result, vortex formations, which are called filaments, emerge in the ocean. They emerge from the vortices which are initially almost circular in the horizontal plane and represent structures stretched in one direction and having nonzero vorticity. In this work, an analytical and graphical method for determining the regimes of behavior of three-dimensional ellipsoidal vortices is proposed for the first time for an inhomogeneous horizontal current which is linear with respect to the horizontal coordinates. Conditions for inevitable stretching of the vortices into filaments are studied. It is established that the vortex stretching is manifested in spots (domains) on 60–67% of the world ocean surface and the characteristic dimensions of these spots amount to about 200 km. The vortex stretching into filaments ensures energy pumping from mesoscale processes to submesoscale ones. According to the global oceanic reanalysis GLORYS12V1, the domain distributions in the World Ocean are plotted. It is shown that irrespective of the spatial-averaging scales, the integral area of regions in which the mesoscale vortices can stretch into filaments is dominant.

By convention, water waves are studied under the assumption of their potentiality. This approximation is not always valid in natural conditions. The vorticity is introduced by shear currents, which are ubiquitous in the ocean. It is also generated in the near-surface layer as a result of wind action. When these factors are taken into account, the models developed for potential waves require refinement and generalization. This paper is devoted to a review of advances in the field of analytical description of surface vortical waves in deep water. The presentation is based on the Lagrangian approach. The focus is on the Gerstner wave, a particular exact solution of the Euler equation. Two ways of its generalization are discussed. The first suggests consideration of weakly nonlinear steady waves with a more general vorticity distribution (Gouyon waves). The second way is to construct exact solutions for waves with inhomogeneous and non-stationary pressure distribution on a free surface (generalized Gerstner waves).

The main role in the current climate change is played by anthropogenic forcings, primarily anthropogenic emissions of greenhouse gases and aerosols. On the global scale, the response of the Earth system to these forcings is close to linear. In particular, it depends mostly on the magnitude of such forcings and only weakly on their nature and spatial localization. However, even with relatively small (in absolute value) external forcings, the response of the characteristics of the Earth system can be essentially nonlinear with the manifestation of tipping points, upon transition through which the behavior of the Earth’s climate changes qualitatively. Examples are given for linear and nonlinear mechanisms of the climate response to external forcings.