Exciton-light coupling in quantum wells: From motional narrowing to superradiance

We show numerically that spatial delocalization of excition-polariton states in multiple quantum wells leads to efficient narrowing of exciton resonances with respect to the single-quantum-well (QW) case. This is a manifestation of a motional narrowing effect, i.e., averaging of short-range potential fluctuations by extended exciton-polariton modes. Increasing the number of QW's or decreasing the degree of disorder, one recovers the superradiance regime where the exciton linewidth is governed by the splitting between the strongest exciton-polariton modes and now increases with an increasing number of QW's. Vertical disorder, in which the different wells are no longer equivalent, suppresses both effects.