Single-exciton optical gain in semiconductor nanocrystals: Positive role of electron-phonon coupling

The possibility of obtaining optical gain in an ensemble of semiconductor nanocrystals without involvement of multiexcitons is very attractive for low-threshold laser applications. Here, we reexamine theoretically the conditions required to reach this single-exciton gain regime in nanocrystals. We show that the electron-phonon interaction can play a very positive role, in addition to the exciton-exciton interaction. In presence of both interactions, the optical gain regime can be reached even when the population of nanocrystals containing single excitons is below 10%. For these reasons, we suggest that ultrasmall nanocrystals, or nanocrystals with deep defects at their surface, could be promising materials for light amplification.