V. A. Kukushkin

Difference-Frequency Pulse Generation in Quantum Well Heterolasers

It is shown that mid-to far-infrared (IR) and terahertz (THz) pulse generation via difference-frequency mixing in quantum well (QW) dual-wavelength heterolasers can be rather efficient under the modelocking regime for one or both lasing fields even at room temperature. In such a device, the long-wavelength field is produced in the process of intracavity difference-frequency mixing of two optical fields: continuous wave (CW) and pulsed (or both pulsed), due to the resonant intersubband quantum coherence in QWs, as well as due to the nonresonant second-order semiconductor bulk nonlinearity. The mode-locking regime of the optical generation allows one to significantly enhance the pulsed driving fields in comparison with those under CW operation and, therefore, substantially increase the output difference-frequency power. Within a simple model, an explicit formula for the intensity and shape of the generated IR or THz pulse is derived. It is shown that this method is capable of producing picosecond pulses at a ∼ 1-GHz repetition rate with a peak power of the order of 1 W and ≲0.2 mW at 10 and 50 µm wavelengths, respectively.

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.

Meissner effect in supercoducting cores of neutron stars

Evolution of the magnetic field of a neutron star (pulsar) under the influence of the superconductivity of a proton plasma in the stellar core is considered. The magnetic field expulsion from the core due to the Meissner effect for the second-type npe-superconductor is shown to be inefficient for the magnetic fields B0<1014G. Therefore, neglecting other expulsion mechanisms (e.g., related to the buoyancy of the Abrikosov vortices or their pinning to the Onsager-Feynman vortices in the superfluid interior of the rotating star), such magnetic fields should be frozen into the core for>1010years, i.e., during the entire lifetime of the star.

First experiments and prospects of intracavity difference -frequency generation of mid/far-infrared and terahertz radiation in diode lasers

Our experimental and theoretical investigations in the field of new semiconductor sources of coherent mid/far-IR radiation based on the intracavity nonlinear mixing of close-in-frequency optical fields in dual-wavelength diode lasers are reviewed. Perspective schemes of quantum-well heterolasers of this kind are considered, and the ways of increasing the power of difference-frequency radiation are suggested, including the use of mode-locked lasers and hybrid surface-emitting grating-outcoupled lasers.

Terahertz generation via intracavity mixing in mode-locked dual-wavelength lasers

We put forward a scheme of the efficient terahertz and mid/far-infrared pulsed generation via an intracavity difference-frequency mixing of lasing fields due to the resonant intersubband and non-resonant bulk nonlinearities in the mode-locked dual-wavelength heterolasers.

Two-color optical and mid-IR quantum-well laser

A method of mid-IR generation in optical quantum-well heterolasers is considered. In contrast to the previous studies, it is proposed to invert the inhomogeneously broadened low-frequency laser transition only in a narrow spectral range containing the entire interval of its resonance interaction with the amplified mid-IR field. This approach allows to reduce the threshold pump density to experimentally attainable values even at room temperature.

Gibbs distribution over quasi-energy levels and antiferroelectric phase in thermal gases

Generalization of Gibbs distribution over the quasienergy states of moving molecules is successfully used to describe the thermodynamic properties of antiferroelectric phase transition in an ideal (collisionless) gas of two-level molecules. According to the method, there appears a self- consistent stationary polarization wave which is well agreed with that predicted on the basis of straightforward solution of the optical Bloch kinetic equations. It is found that a novel phase of gaseous crystal of the antiferroelectric type will emerge as a result of weak first order phase transition. Basic thermodynamic properties of this antiferroelectric phase transition are outlined and some estimates for possible experiments in real gases are presented.

Effects of the Spatial Dispersion and Instabilities in the Wave Electrodynamics of Weakly Ionized Gases

Abstract The complex spectra of transverse and longitudinal modes in the conducting gas of two-level (or quasi-classical) molecules are found, taking into account the spatial dispersion due to the thermal motion of molecules. Conditions for the existence of weakly damping longitudinal waves and transverse polarization waves, as well as of electromagnetic waves with the dispersion law much different from the “vacuum” dispersion law (which is well known in spectroscopy as the Voyght one and includes Doppler broadening according to the perturbation theory) are specified. The collisionless damping modes (on the wings of the Doppler line) with the dispersion law determined by the zeroes of the complex error function are studied. The analogy with the collisionless Landau damping of the Langmuir waves in electron plasma is established. It is shown that, in the neutral gas of excited molecules, the longitudinal and transverse polarization waves possess negative energy and may lead to the dissipative and beam-type instabilities.

Antiferroelectric phase transition and nonlinear polarization structure in the thermal gas of uninverted two-level molecules

The nonlinear stationary solution of the Bloch kinetic equations with Lorentz correction to the local electric field for the thermal gas of two-level molecules is found. This solution shows that radiative coupling between high-frequency molecular dipoles can provide the second-order phase transition to the antiferroelectric state of coherent gaseous crystal.