Exciton optical transitions in quantum wells with spin-orbit coupling

The energy spectrum of light-hole and heavy-hole excitons and optical absorption in a quantum well have been analyzed taking into account Rashba spin-orbit coupling. Interband and intraband exciton transitions have been considered. It has been shown that, in the presence of spin-orbit coupling, the probabilities of the interband and intraband photoelectric effects diverge in the vicinity of the threshold if the electron-hole interaction is neglected. The threshold probabilities of the interband and intraband photoelectric effects become finite when Coulomb interaction is taken into account.     

Exciton condensation in semiconductor quantum wells with a periodical modulation of potential

The appearance and properties of a structure in the density distribution of indirect excitons in coupled quantum wells in semiconductor alloys upon periodical field modulation are studied. Calculations showed that besides periodical dependence of density distribution, caused by the potential modulation, a stratification of the exciton density shafts into separate islands in the cross direction arises. Appearance of islands is the result of exciton condensed phase formation and a nonequilibrium state of the system due to the finite exciton lifetime and pumping presence. The dependence of the structure on system parameters (the pumping value, the modulated field depth and period) is investigated. Also the influence of the exciton–exciton annihilation is taken into account.     

Two-photon lasers based on intersubband transitions in semiconductor quantum wells

We propose to make a two-photon laser based on intersubband (sublevel) transitions in semiconductor nanostructures. The advantages and feasibility of such a two-photon laser are analyzed in detail using the density matrix approach. Both one-photon and two-photon gains in a three subband quantum well structure are studied on the same footing to show how the two-photon gain can be maximized, while the competing one-photon gain is minimized. The results show that a sufficient two-photon gain can be achieved to overcome one-photon competition and the loss of a conventional semiconductor cavity, making intersubband transitions one of the very few feasible approaches to two-photon lasing.     

Exciton condensation in coupled quantum wells

Bound electron-hole pairs—excitons—are Bose particles with small mass. Exciton Bose-Einstein condensation is expected to occur at a few degrees Kelvin—a temperature many orders of magnitude higher than for atoms. Experimentally, an exciton temperature well below 1 K is achieved in coupled quantum well (CQW) semiconductor nanostructures. In this contribution, we review briefly experiments that signal exciton condensation in CQWs: a strong enhancement of the indirect exciton mobility consistent with the onset of exciton superfluidity, a strong enhancement of the radiative decay rate of the indirect excitons consistent with exciton condensate superradiance, strong fluctuations of the indirect exciton emission consistent with critical fluctuations near the phase transition, and a strong enhancement of the exciton scattering rate with increasing concentration of the indirect excitons revealing bosonic stimulation of exciton scattering. Novel experiments with exciton condensation in potential traps, pattern formation in exciton system and macroscopically ordered exciton state will also be reviewed briefly.     

Exciton quantization in parabolic quantum wells

The exciton wavefunction in parabolic quantum wells is calculated using variational techniques and effective mass theory. The influences of the potential shape and of confinement on the exciton binding energies are studied. The results are in good agreement with previous calculations. The oscillator-strength of excitons in GaAs/Ga_1-xAl_xAS quantum wells has a maximum value very close to the cross-over from three to two dimensions.     

Exciton condensate emission in double quantum wells

Studies of the photoluminescence spectra of spatially indirect excitons in coupled quantum wells revealed that the emission has a directional pattern dependent on the external electric field and pumping power. The experimentally detected correlation between the spectral emission parameters of spatially indirect excitons, namely, the concentration (phase state) of such excitons, the line half-width, the degree of linear polarization, and the existence of a directional pattern, permits a suggestion that the spontaneous photoluminescence of spatially indirect excitons in the condensed state is of a coherent nature.     

Exciton relaxation in self-assembled semiconductor quantum dots

The present study focuses on the effect of excited states on the exciton–polaron spectrum for self-assembled InAs/GaAs semiconductor quantum dots. The analytical model takes into account the Coulomb interactions between the electron and the hole as well as, each carrier, the coupling with the longitudinal optical phonon field. Furthermore, the key role played by the exciton continuum in the dot spectrum is also introduced. Such an approach is well fitted to analyze recent experimental findings about single-dot spectroscopy and allows peaks assignment, line width estimation, relaxation time evaluation, etc., necessary steps toward an understanding of the internal dynamics of quantum dots.     

Exciton condensation in quantum wells: Temperature effects

Our previous theory of the formation of exciton condensed phases in a two-dimensional system is applied to the analysis of the temperature dependence of a number of parameters of the system. The pumping intensity-temperature phase diagram is constructed, some of its features characteristic of indirect excitons having a dipole moment are established, the dependences of the intensity of the radiation band corresponding to the condensed phase on the pumping intensity and temperature are calculated, and the results are compared with experimental data.     

Slow light in semiconductor quantum wells

We demonstrate slow light via population oscillation in semiconductor quantum-well structures for the first time. A group velocity as low as 9600 m/s is inferred from the experimentally measured dispersive characteristics. The transparency window exhibits a bandwidth as large as 2 GHz.     

Exciton states and relaxation dynamics in shallow quantum wells

Il Nuovo Cimento D - Optical properties of GaAs/Al x Ga1−x As shallow quantum wells have been studied in the 2–200K range, both by c.w. and time-resolved experiments. The results agree...