Vl. V. Kocharovsky

The Neutron Component in Fireballs of Gamma-Ray Bursts: Dynamics and Observable Imprints

We analyze the dynamics of a neutron-proton relativistic wind, paying particular attention to fireballs of cosmological gamma-ray bursts (GRBs). Specific effects of the neutron component depend on whether the final Lorentz factor of a plasma wind exceeds some critical value or not. In the first case, velocity decoupling of the neutron and proton flows takes place, giving rise to an electromagnetic cascade induced by pion production in inelastic collisions of nucleons. Otherwise, all nucleons in the wind behave as a single fluid. In both cases neutrons can strongly influence a GRB by changing the dynamics of a shock initiated by protons in the surrounding medium. Conditions for the decoupling of the neutron flow as well as observational consequences of the resulting pion-induced cascade are discussed, including preburst of high-energy photons and neutrinos and annihilation afterglow of a huge number of ejected electron-positron pairs. The critical value of the Lorentz factor is estimated to lie in the range expected for cosmological GRBs, so there possibly exist two different populations of bursts. A number of tests for decoupling of the neutron flow is suggested. The results obtained for the radiation-driven wind allow straightforward generalization for winds driven by other mechanisms, e.g., for the MHD winds.

Physical parameters and emission mechanism in gamma-ray bursts

Detailed information on the physical parameters in the sources of cosmological Gamma-Ray Bursts (GRBs) is obtained from few plausible assumptions consistent with observations. We consider monoenergetic injection of electrons and let them cool self-consistently, taking into account Klein-Nishina cut-o in electron- photon scattering. The general requirements posed by the assumptions on the emission mechanism in GRBs are formulated. It is found that the observed radiation in the sub-MeV energy range is generated by the synchrotron emission mechanism, though about ten per cent of the total GRB energy should be converted via the inverse Compton (IC) process into the ultra-hard spectral domain (above 100 GeV). We estimate the magnetic eld strength in the emitting region, the Lorentz factor of accelerated electrons, and the typical energy of IC photons. We show that there is a synchrotron-self-Compton constraint which limits the parameter space available for GRBs that are radiatively ecient in the sub-MeV domain. This concept is analogous to the line-of-death relation existing for pulsars and allows us to derive the lower limits on both GRB duration and the timescale of GRB variability. The upper limit on the Lorentz factor of GRB reballs is also found. We demonstrate that steady-state electron distribution consistent with the Compton losses may produce dierent spectral indices, e.g., 3/4 as opposed to the gure 1/2 widely discussed in the literature. It is suggested that the changes in the decline rate observed in the lightcurves of several GRB afterglows may be due to either a transition to ecient IC cooling or the time evolution of Klein-Nishina and/or Compton spectral breaks, which are the general features of self-consistent electron distribution.

Infrared generation in low-dimensional semiconductor heterostructures via quantum coherence

A new scheme for infrared generation without population inversion between subbands in quantum-well and quantum-dot lasers is presented and documented by detailed calculations. The scheme is based on the simultaneous generation at three frequencies: optical lasing at the two interband transitions which take place simultaneously, in the same active region, and serve as the coherent drive for the IR field. This mechanism for frequency down-conversion does not rely upon any ad hoc assumptions of long-lived coherences in the semiconductor active medium. And it should work efficiently at room temperature with injection current pumping. For optimized waveguide and cavity parameters, the intrinsic efficiency of the down-conversion process can reach the limiting quantum value corresponding to one infrared photon per one optical photon. Due to the parametric nature of IR generation, the proposed inversionless scheme is especially promising for long-wavelength (far- infrared) operation.

Gain-swept superradiance applied to the stand-off detection of trace impurities in the atmosphere.

We show that gain-swept superradiance can be used to detect low (parts per million) concentrations of various gases at distances on the order of kilometers, which is done by using pulse timing to create small regions of gain at positions that sweep toward a detector. The technique is far more sensitive than previous methods such as light detection and ranging or differential absorption light detection and ranging.

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.

Collective QED processes of electron - hole recombination and electron - positron annihilation in a strong magnetic field

The process of coherent spontaneous emission, originated from very fast collective recombination (annihilation) of electron - hole or electron - positron plasmas, is analysed. Such non-equilibrium systems of free particles and antiparticles with continuous energy spectra demonstrate cooperative features of fundamental importance for quantum electrodynamics. In particular, due to the self-consistent radiative coupling between active particles, a dense enough plasma bunch can emit an extremely short and powerful electromagnetic pulse. The phenomenon of recombination (annihilation) superradiance is expected to be very different from both usual lasing and well known Dicke superradiance in the atomic systems with discrete energy spectra. In the case of semiconductor magneto-optics, it is shown that subpicosecond, subgigawatt coherent pulses of recombination superradiance can be generated spontaneously in bulk GaAs samples placed in a strong (quantizing) magnetic field.

Room-temperature intracavity difference-frequency generation in butt-joint diode lasers

We obtain, for the first time to our knowledge, the room-temperature intracavity difference-frequency generation in the mid-infrared range (around 10 mum wavelength) in a butt-joint GaAs/InGaAs/InGaP quantum-well dual-wavelength laser diode which supports lasing at two closely spaced wavelengths in the near-infrared range close to 1 mum. We employ a special asymmetric waveguide design (which makes it possible the different-order transverse-mode lasing) and a low-doped substrate that minimize mid-infrared losses and phase mismatch for the difference-frequency nonlinear-mixing process. We explain qualitatively the physics of the intracavity mode mixing and describe in detail the design of the butt-joint diode laser. We show how to scale the effect into the far-infrared (terahertz) range, to improve the phase-matching conditions, and to enhance further the difference-frequency generation efficiency (the latter is about 1 W/(kW)2 in our first experiment).

Superradiant generation of femtosecond pulses in quantum-well heterostructures

We propose a new type of accessible source of ultra-short pulses based on the phenomenon of collective coherent recombination (superradiance) of electrons and holes in semiconductor heterostructures. We find and analyse a novel regime of ultra-fast operation of quantum-well semiconductor lasers in which a quasiperiodic sequence of femtosecond superradiant pulses is emitted under continuous pumping. According to our calculations for AlGaAs - GaAs heterostructures, the coherent optical pulses of duration 30 fs and peak intensity can be generated in a low-Q cavity of length .

Nonlinear mode mixing in dual-wavelength semiconductor lasers with tunnel junctions

The authors demonstrate and study two- and three-wavelength generations in the semiconductor diode laser with a tunnel junction separating two different quantum-well active regions integrated within a single waveguide. To avoid resonant cross absorption of the modes at different frequencies and achieve phase matching, the laser waveguide is designed to generate the first-order transverse mode at a longer wavelength and the third-order mode at a shorter wavelength. Excellent agreement with the designed and measured device parameters is observed. Intracavity nonlinear mixing leading to sum-frequency and second-harmonic generation is demonstrated.

Non-linear wave mixing in GaAs/InGaAs/InGaP butt-joint diode lasers

We suggest and fabricate butt-joint diode lasers for efficient non-linear wave mixing in semiconductors. The first experimental demonstration of sum-frequency and second harmonic continuous-wave generation in InGaAs/GaAs/InGaP butt-joint diode lasers in the edge-emitting geometry is reported.