Nonlinear Optical Response of the GaAs Exciton Polariton

High-resolution differential optical transmission measurements of a thin layer of bulk GaAs show anomalous behavior of the lowest energy exciton at low excitation density and temperature. The spectrum shows induced absorption with a superlinear dependence on excitation density with no detectable change in resonance frequency or linewidth. At a temperature dependent excitation density, the anomaly undergoes a transition to a normal response due to excitation-induced dephasing. The results are fully consistent with the polariton picture generalized to include exciton-exciton interactions.