Motional Narrowing in Semiconductor Microcavities

The linewidths of exciton features in spectra from semiconductor quantum wells normally correspond directly to the width of the probability distribution of the disorder potential causing the broadening. This is because the mass of the exciton is large, so its motion in the potential is essentially classical. We show that in a microcavity, where polariton mixing leads to much smaller masses, quantum mechanical effects are expected to cause significant spatial averaging over the disorder potential and hence motional narrowing of the spectral lines. Our prediction is verified by experimental measurements of linewidths near resonance in a high quality microcavity structure.