Recent experiments with Bose-condensate of indirect excitons in quantum-well traps in semiconductor heterostructures
show long-scale coherence of recombination emission under appropriate CW laser pumping. We suggest that such
coherence results from the existence of high-quality polariton modes, which are formed due to strong coupling of exciton
polarization and electromagnetic field in a trap. These modes are of whispering-gallery origin and can be excited via
stimulated emission of excitons supplied by the CW laser pumping. We analyze in detail the structure and spectrum of
polariton modes taking into account polarization relaxation and radiative losses. We derive characteristic equation for
mode wave-vectors and solve it numerically for the experimental parameters. The quality factors of a small number of
modes are found to reach 10000 and higher. In the case of high density and narrow enough spectral linewidth of
excitons, which could be achieved due to Bose-Einstein condensation, some modes become unstable giving rise to
exciton lasing. We investigate their growth rates, instability thresholds, energy losses, and saturation amplitudes. Under
typical experimental conditions, we indicate lasing polariton modes which can be responsible for the observed long-scale
coherence of exciton emission.
Butt-joint diode lasers for the efficient nonlinear optical mixing in semiconductors are suggested and successfully implemented. First experimental demonstration of the sum-frequency and second-harmonic continuous-wave room-temperature generation in the InGaAs/GaAs/InGaP butt-joint diode lasers in the edge-emitting geometry is reported. Specific features of the butt-joint device, where two butt-joined diode lasers are optically coupled but injection pumped separately, are investigated and the prospects of the difference-frequency (far-infrared) generation are outlined.
A new scheme for generation of coherent radiation on the intersubband transition without population inversion between subbands is presented. The scheme is based on the resonant nonlinear mixing of the optical laser fields on the two interband transitions that are generated in the same active region and serve as a coherent drive for the infrared field. The two-wavelength lasing on the interband transitions can be achieved at substantially lower threshold current than population inversion and gain on the intersubband transition. This may ensure stable high-power room-temperature operation. Due to parametric, inversionless nature of generation, the proposed lasers are especially promising for the long- wavelength operation above 20 micron.
We investigate possible regimes of one -- or two -- color mode superradiance of femtosecond optical pulses and accompanying parametric generation of infrared pulses, which contain a few periods of field oscillations, in quantum-well heterolasers of micrometer length under cw pumping. The effects of strong magnetic field and inhomogeneous broadening of electronic levels of dimensional quantization in quantum wells are analyzed.
We analyze super-radiant lasing, i.e. superfluorescence (SF) under continuous pumping, due to collective interband recombination of electron-hole (eh) pairs in quantum wells (QWs) placed in a strong magnetic field oriented perpendicular to the well plane. In such semiconductor systems, cooperative radiation processes lead to generation of coherent femtosecond pulses and can be observed even at room temperature due to complete quantization of particle motion, high spectral density of carrier states, high spatial density of effective 'cyclotron quantum dots,' and partial suppression of intraband scattering. Our simulations based on the Maxwell-Bloch equations show that, in the case of two neighboring transitions between electron and hole Landau levels connected by scattering, two-color super-radiant lasing of the corresponding pair of resonance modes is possible under cascade cw pumping. We investigate analytically and numerically threshold conditions for this two-color lasing as well as for super-radiant lasing modified by 'discrete' inhomogeneous broadening due to fluctuations of the number of atomic layers in actual QW heterostructures. Also, we present an example of real design and discuss necessary conditions for a vertical cavity surface-emitting laser (VCSEL) to operate in the super-radiant regime.
We develop a theory of extreme spontaneous pulsations in open class D lasers corresponding to the bad-cavity limit when the medium relaxation rates are much less than the field relaxation rate TE-1. In this case it is mode superfluorescence (SF), not unidirectional one, which is responsible for the formation of an individual pulse, the latter being almost independent on pumping if it is strong enough. For the simple Fabry-Perot model of a short cylindrical sample, we investigate generation of 'hot' inhomogeneous polariton modes formed either by weak reflections from mirrors or by 'resonance' reflections from sharp boundaries of inverted medium. We pay particular attention to the cavity-free situation and describe one-mode and two-mode SF regimes, both giving rise to correlations between delay times of the counterpropagating pulses. We write down the solution of the initial value problem and analyze dynamics of forward-backward symmetrization of coherent transient emission initiated asymmetrically. We calculate analytically the reflection factors and growth rate of unstable discrete modes and continuous waves and indicate conditions which are necessary for mode SF to be much more efficient than unidirectional one.
A new type of accessible source of ultrashort optical pulses, based on the phenomenon of collective coherent recomination (superfluorescence) of electrons and holes in semiconductor heterostructures is proposed. The novel regime of an ultrafast operation of quantum-well semiconductor lasers with a low-Q cavity of length approximately 30 $min 100(mu) is analyzed, in which a quasiperiodic sequence of superradiant pulses of duration up to 30 fs and peak intensity exceeding 100 MW/cm2 is emitted under continuous pumping, with typical current density of order 104 A/cm2. It is shown that the same process of femtosecond superradiant recombination can be used for the room-temperature generation of optical coherent emission in multiple quantum-well or quantum dot GeSi/Ge structures, employing direct radiative transitions from the T-valley.
Linear coupling of helical optical modes in twisted and birefringent inhomogeneous fibers is rigorously incorporated to the newly derived polarization transfer equations, and statistical polarization parameters of partially coherent light emitted by a broadband source and transmitted through a dispersive single-mode fiber with random inhomogeneities are calculated analytically in the limiting case of slow cross talk between helical modes.
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.
We propose a new type of accessible sources of ultrashort pulses based on the phenomenon of collective coherent recombination (superradiance) of electrons and holes in semiconductor heterostructures. We find and analyze a novel regime of an ultrafast operation of quantum-well or quantum- dot semiconductor lasers in which a quasiperiodic sequence of femtosecond superradiant pulses is emitted under quasi- stationary pumping. According to our calculations for AlGaAs- GaAs heterostructures, the coherent optical pulses of duration approximately greater than 30 fs and peak intensity approximately greater than 100 MW/cm2 can be generated in a low-Q cavity of length approximately 10 - 30 micrometers. It is shown that the same process of femtosecond superradiant recombination can be used for the room-temperature generation of optical coherent emission in multiple quantum-well or quantum dot GeSi/Ge structures employing direct radiative transitions from the (Gamma) -valley.
Coherent amplification, deformation and shortening of ultrashort pulses are considered in the case of a highly doped fiber with dominant inhomogeneous broadening of a spectral line. A promising possibility of parallel multichannel processing of complicated optical signals separated in time and/or in frequency is demonstrated.
The self-consistent solution to the problem of collective spontaneous emission of an active medium in any-Q resonator, taking into account the nonlinear coupling of counter-propagating wave, is found. The optical parameters of resonator are evaluated.
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.
The general Fano-Friedrichs Hamiltonian is diagonalized via a proper extraction of all dressed decaying eigenquasistates with complex eigenquasienergies. Their non-adiabatic crossing is described using the true truncated non-Hermitian Hamiltonian with applications to DC + AC non-steady Stark effect.
A novel regime of collective spontaneous emission is an extended sample of excited molecules with quasi-equidistant energy spectrum is investigated. The limiting parameters of the ultrashort pulse and the optimal density and length of the sample are evaluated.
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