I. V. Ovchinnikov

Nonlinear optical phenomena in coherent phase of 2D exciton system

Abstract We show that spatial many-particle coherence of condensed 2D exciton system, which is represented by anomalous averages 〈 a k 1 † ⋯ a k N † 〉, can be transferred to exciton luminescence via the processes of coherent (simultaneous) recombination of many correlated excitons with production of the same number of correlated photons ( a k † is exciton creation operator). In high-quality samples 2D momentum conservation dictates the requirement ∑ i k i =0 for 2D exciton momenta. Due to momentum conservation, the photons coherently produced in such a process possess zero sum 2D momentum as well. Analyzing such a many-photon spatial coherence of the luminescence we predict new nonlinear optical effects in a coherent phase of 2D exciton system. The effects are the above-mentioned processes stimulated by external laser beams. They effectively look as if the stimulated exciton system irradiates a unidirectional beam with recoil momentum. ‘Recoil’ beams constitute a photon ‘fan’ in case of single stimulating laser beam and a photon ‘lattice’ in case of two stimulating laser beams. Quantitative estimations for exciton system in GaAs/AlGaAs double quantum wells point to possible experimental observability of the effects.

Controlling spatially indirect exciton condensate in coupled quantum wells by external fields and phonon laser

Abstract A new method for controlling and optical observing spatially and momentum space indirect exciton condensate in coupled quantum wells is proposed. The method is based on exciton dispersion engineering by external fields. Using external fields one can, at any instant, provide resonant enhancement in PL and acoustic phonon emission by tuning ‘dark’ condensate into the resonance of recombination via direct exciton level. In addition, the development of a phonon laser is a possibility.

Stimulated light backscattering from exciton Bose condensate

A new effect—light backscattering from exciton Bose-condensate—is considered. This effect is connected with the photoinduced coherent recombination of two excitons in the condensate with the production of two photons with opposite momenta. The effect of two-exciton coherent recombination leads also to the appearance of the second-order coherence in exciton luminescence connected with squeezing between photon states with opposite momenta. The estimations given for Cu2O and GaAs excitons show that the effect of stimulated light backscattering can be detected experimentally. Moreover, in the system of 2D excitons in coupled quantum wells, the effect of stimulated anomalous light transmission must also take place. Anologous effects can also take place in systems of Bose-condensed atoms in excited (but metastable) states.

Optical Properties of a Coherent Phase in Electron-Hole Systems: Stimulated Light Scattering and Multibeam Processes

A theoretical study is reported of stimulated light scattering, including wave-vector reversal and anomalous transmission, by a coherent phase in electron-hole (e-h) systems of low and high charge-carrier density. For these two cases the coherent phase is taken to be a Bose-Einstein condensate of excitons or a BCS-like state of e-h pairs, respectively. The scattering mechanism is laser-induced coherent recombination of two excitons or two coherent e-h pairs, respectively. The e-h system is assumed to exist within a GaAs/AlGaAs double quantum well or bulk GaAs. The emission rate of two photons with antiparallel momenta is estimated. Multiphoton emission due to multiexciton coherent recombination is covered. Methods for detecting the effects predicted are proposed.

Sasers: resonant transitions in narrow-gap semiconductors and in exciton system in coupled quantum wells

Abstract Two schemes for steady stimulated phonon generation (saser, i.e., phonon laser ) are proposed. The first scheme exploits a narrow-gap indirect semiconductor or analogous indirect gap semiconductor heterostructure where the tuning into resonance of one-phonon transition of electron–hole recombination can be carried out by external pressure, magnetic or electric fields. The second scheme uses one-phonon transition between direct and indirect exciton levels in coupled quantum wells. The tuning into the resonance of this transition can be accomplished by engineering of dispersion of indirect exciton by external in-plane magnetic and normal electric fields. In the second scheme the magnitude of phonon wave vector is determined by magnitude of in-plane magnetic field and, therefore, such a saser is tunable. Both schemes are analyzed and estimated numerically.

Phonon laser and indirect exciton dispersion engineering

Engineering of dispersion of indirect excitons by normal electric and in-plane magnetic fields is proposed to be used for controlling the state of many-exciton system (e.g., coherent state) and its photoluminescence and for producing an inverse population in the excitonic system. The possibility of phonon laser creation on the basis of the latter effect is discussed. Phonon number distribution appears to be a fingerprint of that in exciton system. Numerical estimations for the proposed scheme are made for GaAs/AlGaAs quantum wells.

BCS instability of a two-layer system of composite fermions

Pairing instability is considered for a two-layer electron system in a strong magnetic field with an even-fractional filling ν=1/(2m) (m is an integer) of the lowest Landau level in each of the layers. The limit of large distance d between the layers is analyzed. Microscopic analysis is carried out in the eikonal approximation in the composite-fermion formalism. It is found that the condition for pairing instability in this model is independent of d. Due to the marginal character of the composite-fermion system, pairing instability in the particle-particle (BCS) channel arises only for η<2 or η=2, but Mv0>43, where η and v0 are the parameters of the assumed electron-electron interaction, v∝v0/rη, and M is the band electron mass. In the particle-hole (isospin density wave) channel, instability is not observed.

Stimulated Multiphoton Emission from Exciton Bose Condensate

The coherent recombination of several (N) Bose-condensed excitons with simultaneous creation of N photons is considered. Due to the momentum conservation law, the total momentum of created photons is zero because of the zero momentum of excitons in Bose condensate. This requirement, in conjunction with the fact that the photon wavenumbers are fixed and equal to approximately Eg/c (Eg is the semiconductor gap and c is the speed of light), determines the mutual orientation of the wavevectors of emitted photons. This can be seen from the photon angular correlation in the experiments with several appropriately oriented detectors operating in the time-coincidence mode. It is shown that, if these processes are induced by N− 1 external laser beams (each with wavevector ki), then a unidirectional radiation with recoil wavevector k=−∑iki should emerge from the exciton system. The intensities of coherent three-and four-exciton recombination are estimated for the exciton system in Cu2O.

Cooling of excitons by coherent phonon radiation

The effect of exciton cooling by coherent phonon radiation, which is based on coherent confinement of exciton populations, is considered. Peculiarities of coherent confinement of excitons in an exciton system are analyzed, in which the umklapp processes as well as recombination and relaxation play an important role. The limitations imposed on the parameters of the system for which the effect can be realized are determined taking into account these peculiarities.