Cooperative Parametric Instability of Natural Modes and Coherent Mechanism of Self-Mode-Locking in a Ring Class-B Laser

Based on recent experimental observations and numerical simulations [1-4], we develop an analytical theory of spontaneous low-threshold mode-locking in a long homogeneously-broadened fiber laser with slow relaxation of inversion. We show that the phenomenon originates from the self-consistent parametric resonance and beat-frequency-locking of paired hot modes taking place due to coherent nonlinear effects in the system of active two-level centers. An important role of the Risken-Nummedal-Graham-Haken [5-7] multimode instability is discussed.     

Spontaneous Polarization of a Gas of Two-Level Molecules and the Gibbs Quasi-Energy Distribution

We study the possibility of spontaneous formation of a polarization structure in a thermodynamically equilibrium gas of dipolarly interacting two-level molecules. Using the Maxwell-Bloch equations within the framework of the mean-field theory, we find that the antiferroelectric phase transition in a gas with a weak relaxation of the polarization is always a second-order transition. It is shown that if relaxation is neglected, then in the quasi-classical consideration of the translational motion of molecules in the polarization wave, the energy levels of a separate molecule coincide with its quasi-energies that are well known in quantum optics. Thus, to study the statistical properties of the antiferroelectric phase, we apply the generalized Gibbs distribution over quasi-energy states of the molecules. As a result, we determine the characteristic features and the possible parameters of the antiferroelectric state of a gas. In particular, it is found that, owing to the Doppler resonance of part of the molecules with the polarization wave, the properties of the gas antiferroelectrics behind the phase-transition point may radically differ from the properties of the conventional ferroelectrics in the Ginzburg-Landau theory. We also analyze the influence of polarization fluctuations for the case of a ferroelectric transition in a gas.     

Modeling of spectral features in the dynamic spectra of neutron stars

Modeling of atomic and cyclotron lines in the emergent spectra of rotating neutron stars with various distributions of temperature over the star surface is carried out. General and special relativity effects are taken into account in the radiation transfer calculations. A novel method of analysis based on the Fourier series expansion of the observed spectra over rotation frequencies is proposed. It is shown that the mutual influence of the gravitational bending of light rays and rotation of the star leads to the formation of strong features (sometimes several features at once) in the Fourier-harmonic spectrum, whereas these features remain almost invisible in both the integrated and dynamic spectra. Possible application of the obtained results to the interpretation of absorption features in the spectrum of the single neutron star 1E 1207.4 − 5209 is discussed.     

Gamma-ray bursts from evaporating primordial black holes

It is believed that the detection of gamma-ray bursts from evaporating primordial black holes is highly improbable in the near future since the expected photon flux, consisting mainly of photons with energies ? GeV, is too low. Contrary to this point of view, we show that a large fraction of the black hole power at the final stage of evaporation (the last 10³ s) can be liberated as a burst of soft γ-ray emission of duration 10^?1–10³ s and luminosity 10²⁸–10³¹ erg/s in the energy range 0.1–1 MeV. According to our calculations of the black hole evaporation rate (within the Standard Model of elementary particles), when the black hole temperature exceeds approximately 10 GeV, the charged particle outflow from a black hole forms a well-defined plasma and can be described in the hydrodynamic approximation. In this case more than half of the rest energy of a black hole can be converted into soft gamma-rays due to the presence of the magnetic field with energy density comparable to that of charged particles. We consider various mechanisms leading to such transformation and estimate their efficiency. It is shown that, at least, some of the gamma-ray bursts detected by BATSE can be associated with evaporating black holes.     

Bose–Einstein condensation in mesoscopic systems: The self-similar structure of the critical region and the nonequivalence of the canonical and grand canonical ensembles

A new hyperbolic metamaterial based on a modified semiconductor Bragg mirror structure with embedded periodically arranged quantum wells is proposed. It is shown that exciton polaritons in this material feature hyperbolic dispersion in the vicinity of the second photonic band gap. Exciton–photon interaction brings about resonant nonlinearity leading to the emergence of nontrivial topological polaritonic states. The formation of spatially localized breather-type structures (oscillons) representing kink-shaped solutions of the effective Ginzburg–Landau–Higgs equation slightly oscillating along one spatial direction is predicted.     

Nonlinear nonequilibrium processes in a silicon switch of high-power microwave radiation

The nonlinear dynamic interaction of high-power microwave radiation with a high-purity compensated silicon sample, driven or not by a laser pulse initiating nonequilibrium photoconductivity, has been investigated numerically. It is shown that this effect can be used in semiconductor switches of the phase of electromagnetic waves in the presence of strong thermal self-action of microwaves. Original Russian Text ? G.G. Denisov, Vl.V. Kocharovsky, M.L. Kulygin, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 1, pp. 98–102.     

Features of Superradiance in a Cyclotron Quantum-Dot Heterolaser Under Continuous Pumping

We study the generation of femtosecond superradiance (SR) pulses under continuous pumping, caused by the collective recombination of electron-hole (eh) pairs in quantum wells placed into a strong magnetic field that is perpendicular to the quantum-well plane. Such a superradiant semiconductor laser can operate even at room temperature owing to the complete quantization of the moving particles, the maximum possible spectral density of carrier states, the high volume density of effective cyclotron quantum dots, and the partial suppression of intraband scattering. In a multilayer laser heterostructure having an optical confinement factor of the order of 0.2 and located in a magnetic field of 10-50 T, generation of a quasi-periodic or chaotic sequence of coherent pulses with a peak power about 1 W, pulse duration about 100 fs, and a period-to-duration ratio of order 10 is expected. It is shown that dichromatic superradiant generation of a pair of modes resonant to two neighboring transitions between the corresponding electron and hole Landau levels is possible over a broad range of pumping powers. We performed numerical and analytical studies of monochromatic and dichromatic superradiance thresholds including studies with allowance for their modification caused by inhomogeneous broadening due to thickness fluctuations of the quantum wells and barriers in actual heterostructures.     

Weibel Mechanism of Magnetic-Field Generation in the Process of Expansion of a Collisionless-Plasma Bunch with Hot Electrons

Radiophysics and Quantum Electronics - We perform qualitative physical analysis and particle-in-cell two-dimensional numerical simulation of the Weibel mechanism of magnetic-field generation due to...     

Saturation of relativistic Weibel instability and the formation of stationary current sheets in collisionless plasma

We have studied the features of formation and the possible stationary structures of a self-consistent magnetic field in a relativistic collisionless plasma, which are characteristic of a simple geometry of the Weibel instability that is well known in the nonrelativistic case. The universal condition is established, the growth rate is determined, and the criteria of saturation of the Weibel instability are analyzed for a broad class of anisotropic particle distribution functions (for definiteness, in application to an electron-positron plasma). A nonlinear equation of the Grad-Shafranov type describing the potential current structures is derived and its solutions are analytically studied. Special attention is paid to spatially harmonic, nonlinear current configurations with parameters determined by the properties of the initial homogeneous plasma subject to the Weibel instability. It is demonstrated that the magnetic field energy density in the obtained solutions (both harmonic and nonharmonic) can be comparable with the kinetic energy density of plasma particles.     

Cooperative recombination of a quantized high-density electron-hole plasma in semiconductor quantum wells

We investigate photoluminescence from a high-density electron-hole plasma in semiconductor quantum wells created via intense femtosecond excitation in a strong perpendicular magnetic field, a fully quantized and tunable system. At a critical magnetic field strength and excitation fluence, we observe a clear transition in the band-edge photoluminescence from omnidirectional output to a randomly directed but highly collimated beam. In addition, changes in the linewidth, carrier density, and magnetic field scaling of the photoluminescence spectral features correlate precisely with the onset of random directionality, indicative of cooperative recombination from a high-density population of free carriers in a semiconductor environment.