Polariton Motional Narrowing in Semiconductor Multiple Quantum Wells

We demonstrate a new fast-scan ultrafast coherent reflectivity technique that tracks the amplitude and phase of excitons in GaAs heterostructures at densities down to $4\times {10}^6{\mathrm{cm}}^{-2\mathrm{}}$. Observed growth in the exciton coherence over the first 1.5 ps is direct evidence for the presence of interfering polariton modes. Polariton theory provides a good account of the data as the number of quantum wells is increased. Polariton motional narrowing of spectral lines from multiple quantum wells is demonstrated both experimentally and theoretically, annealing out the inhomogeneous broadening of the exciton energies.