UNUSUALLY SLOW TEMPORAL EVOLUTION OF FEMTOSECOND 4-WAVE-MIXING SIGNALS IN INTRINSIC GAAS QUANTUM-WELLS - DIRECT EVIDENCE FOR THE DOMINANCE OF INTERACTION EFFECTS

Four-wave-mixing signals from excitons in high-quality GaAs quantum wells show an unusually slow temporal evolution, in stark contrast to the behavior expected for a noninteracting two-level system. We show that Coulomb interaction effects, generally neglected in the analyses of four-wave-mixing experiments, dominate the signals by 2 orders of magnitude. Numerical calculations based on extended optical Block equations for semiconductors provide good qualitative agreement with the data.