Nonlinear photoluminescence in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells

We have measured the nonlinear photoluminescence (PL) spectra of ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells at low temperatures using a continuous wave technique that is based on low-power excitation and phase-locked detection. For nonresonant excitation we find a significant change of the emission energy and linewidth with increasing excitation intensity. We attribute the energy shift to the optical modulation of the internal electric fields that results in the modulation of the exciton energy via the quantum confined Stark effect, while the line broadening is due to the interaction of excitons with free carriers.