Optical amplification and its saturation in semiconductor quantum wells

We present an investigation of the optical gain and its saturation of the electronhole plasma confined in GaAs/(Ga,Al)As quantum wells. For such investigations the variable stripe length method is a simple but quite powerful tool. The spacial longitudinal variation of luminous and electron-hole pair densities in narrow stripes of various lengths has been investigated theoretically and experimentally. For this also the spectra of spontaneous luminescence (emitted in a direction perpendicular to the stripe) have been recorded at different places on the stripe. The experimental results are compared with the predictions of a theoretical model of the optical quantum well amplifier. We find that saturation is caused by carrier depopulation through stimulated electron-hole recombination, and by the loss of light travelling in the direction of the stripe due to scattering and/or reabsorption. However, we do not find evidence of intra band saturation, ie. of a non thermal electron or hole distribution

Temperature and excitation intensity dependencies of the photoluminescence spectra of GaAs/(AlGa)As disordered superlattices

The temperature and excitation intensity dependencies of the photoluminescence (PL) spectra of GaAs/Al0.3Ga0.7As superlattices (SL's) having randomly distributed well widths are studied. Our results indicate that the electronic properties of disordered SL's are similar to those of other disordered semiconductors: at low energies the electronic states are essentially localized, while at higher energies they are extended. Further, the PL spectra feature a disorder-induced fine structure, and they shift to the red with T simply following the reduction of the band gap. The PL efficiency shows a weaker decrease with increasing T than the ordered SL. The dependence of the PL spectra on the excitation intensity shows an anomalous behavior of the disorder-induced fine-structure recombination lines

Low Temperature Photocurrent Spectra of Ordered and Disordered Superlattices

We present low temperature photocurrent (PC) spectra of ordered and disordered GaAs/AlGaAs superlattices (SLs). The investigated disordered SLs were designed to provide both localized and extended states. Our results show a dramatic difference between the PC of ordered and disordered SLs. In the ordered case, the PC is observed to decrease abruptly on increasing the applied voltage V, as expected. In contrast, the total PC signal in the disordered SLs increases in steps as a function of the electric field. Our measurements indicate a transition from a conduction regime to Stark localization in the ordered case, and from the Anderson localization to a conductive regime in the disordered ones. Numerical results obtained by the transfer matrix method indicate the weak dependence and the red shift with V of the disordered SL miniband as the causes of the observed behavior