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Biexponential decoherence in photon echoes: spectral evidence for nearly degenerate localized excitons

Much recent progress has been made in the understanding of the optical response of low dimensional semiconductors. However, additional complications of strong localization and their effects on dynamics and the coherent optical response remain unresolved. Using a highly disordered GaAs single quantum well (width 42 /spl Aring/), we report the nonlinear signal in a self-diffracted transient four-wave mixing (FWM) experiment extending over more than three decades in amplitude as a function of pulse delay.