MANY-PARTICLE EFFECTS AND NONLINEAR OPTICAL-PROPERTIES OF GAAS/(AL,GA)AS MULTIPLE-QUANTUM-WELL STRUCTURES UNDER QUASI-STATIONARY EXCITATION CONDITIONS

Quasi-two-dimensional (2D) carrier systems of GaAs/(Al,Ga)As multiple-quantum-well structures are studied under quasistationary excitation conditions using the pump and probe beam and the luminescence spectroscopy. In the low- to medium-density regime the saturation of the n(z) = 1 exciton resonances dominates the nonlinear optical properties. The low-temperature saturation density is found to be N(s) almost-equal-to 4 x 10(16) cm-3, independent of the well width L(z). The carrier-induced energetic shift of the 1hh-exciton resonances as a function of L(z) shows the dimensional dependence of the screening properties of the carriers. The 2D limit is reached at well widths smaller than 50 angstrom, whereas the 3D behavior occurs already at L(z) = 190 angstrom. In the high-excitation regime, the renormalization of the fundamental band gap is investigated as a function of the electron-hole plasma density. The density and the reduced band gap are determined via systematic evaluations of both gain and luminescence spectra. The observed behavior can be described by a strict 2D theory using effective exciton parameters in order to account for the finite well widths of the structures. The same theory describes very well an n-type modulation-doped quantum well if an independent shift of each subband - according only to its specific carrier density - is assumed. The correlation enhancement of the band-to-band transitions was observed in an n-type modulation-doped sample where all excitonic features were quenched by the doping density. The study of the higher subbands reveals that both exciton bleaching and subband renormalization are due mainly to a direct occupation of the specific subband; we find that the intersubband effects via Coulomb screening are negligible.