Effect of neutral donor scattering on the time-dependent exciton-polariton photoluminescence line shape in GaAs.

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.