Anomalous carrier-induced dispersion in quantum-dot active media

The complex optical susceptibility in a quantum-dot active medium is investigated theoretically. It is found that the Coulomb coupling of the localized discrete states in a quantum dot to extended continuum states in the surrounding quantum well region strongly influence optical properties under high excitation conditions. As a result, the behavior of the carrier-induced refractive index change $\delta n$ differs significantly from that expected from the often used atomlike description, where the interaction between a quantum dot and the surrounding region involves only the transfer of electrons and holes. Furthermore, there is the possibility of increasing $\delta n$ with increasing carrier density under conditions where laser gain is present, which is a distinct departure from bulk and quantum well behavior. Therefore, quantum-dot lasers may not show beam filamentation tendency, which has been a long-standing problem in semiconductor lasers.