Using the second-order perturbation theory we have calculated the scattering assisted gain spectra in GaAs/ AlGaAs superlattice under a strong applied electric field in the Wannier-Stark-Ladder ͑WSL͒ regime. Nonequilibrium distribution function of quasi-two-dimensional carriers localized in each WSL level and indirect optical transitions between neighboring WSL levels accompanied by the emission or absorption of acoustical phonons are taken into account in our theoretical analysis and numerical calculation. We have shown that the experimentally observed down shift of the zero-gain frequency from the Bloch oscillation frequency is due to the inelastic nature of the phonon scattering and the formation of excitons when electron-hole pairs are photoexcited. Our theoretical results agree well with the experimental data which were obtained from analyzing the THz response of superlattices to the picosecond optical pulse excitation.

Phone: þ7 906 095 4402, Fax: þ7 496 524 97 01Exciton level structure and interwell relaxation are studied in Cd(Mn,Mg)Te-based asymmetric double quantum wells (ADQWs) by a steady-state optical spectroscopy in magnetic fields up to B ¼ 10 T. The as grown heterostructures with CdTe QWs and nonmagnetic interwell CdMgTe barrier were subjected to a rapid temperature annealing to introduce Mn and Mg atoms from opposite barriers inside the QWs which results in a formation of the ADQW with completely different magnetic field behavior of the intrawell excitons. The giant Zeeman effect in the QW with magnetic Mn ions gives rise to a crossing of the ground exciton levels in two QWs at B C $ 3-6 T which is accompanied by a reverse of the interwell tunneling direction. In a single-particle picture the exciton tunneling is forbidden at B < 1 T as supported by calculations. Experimentally, nevertheless, a very efficient interwell relaxation of excitons is found at resonant excitation in the whole magnetic field range, regardless of the tunneling direction, emphasizing importance of excitonic correlations in the interwell tunneling. At nonresonant excitation an unexpectedly slow relaxation of the s À -polarized excitons from the nonmagnetic QW to the s þ -polarized ground state in the semimagnetic QW is observed at B > B C , giving rise to a nonequilibrium distribution of excitons in ADQW. A strong dependence of the total circular polarization degree on the hh-lh splitting D hh-lh in the nonmagnetic QW is found and attributed to the spin dependent interwell tunneling controlled by an exciton spin relaxation. Different charge-transfer mechanisms are analyzed in details and an elastic scattering due to a strong disorder is suggested as the main tunneling mechanism with the underlying influence of the valence band-mixing.

No abstract

No abstract

No abstract