Many-particle effects and nonlinear optical properties of GaAs/(Al,Ga)As multiple-quantum-well structures under quasistationary 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 ${\mathit{n}}_{\mathit{z}}$=1 exciton resonances dominates the nonlinear optical properties. The low-temperature saturation density is found to be ${\mathit{N}}_{\mathit{s}}$≊4×${10}^{16}$ ${\mathrm{cm}}^{\mathrm{-}3}$, independent of the well width ${\mathit{L}}_{\mathit{z}}$. The carrier-induced energetic shift of the 1hh-exciton resonance as a function of ${\mathit{L}}_{\mathit{z}}$ shows the dimensional dependence of the screening properties of the carriers. The 2D limit is reached at well widths smaller than 50 Å, whereas the 3D behavior occurs already at ${\mathit{L}}_{\mathit{z}}$=190 Å. 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.