The importance of neutral donor scattering on the observed time-resolved exciton-polariton line shape in GaAs at low temperatures is demonstrated. Samples which in continuous excitation luminescence show a single polariton peak can have substantially different line shapes at short delay times after excitation with mode-locked laser pulses. A series of time-resolved spectra show at short delays after the laser pulses a doublet line shape which gradually transforms into a singlet with longer delays. We have explained this phenomenon as arising from the delay in arrival at the sample surface of polaritons in the energy region corresponding to the central ‘‘dip’’ due to neutral donor scattering. A time-dependent Boltzmann-equation model of polariton transport in the crystal incorporating only elastic neutral donor scattering has been constructed to simulate the experimental results.

• Received 10 February 1986

DOI:https://doi.org/10.1103/PhysRevB.34.1006