Inversion of the excitonic spectrum in semiconductor microcavities

Abstract

We show that coupling of odd photonic modes in microcavities with odd excitied exciton states in wide quantum wells (WQWs) situated at the nodes of the electric field of the cavity mode may lead to the inversion of the excitonic spectrum: an odd exciton-polariton state (X2) may lie at lower energy than even (X1) exciton state. The kinetics of exciton-polariton relaxation is expected to be very peculiar in this case: the excitons are accumulated in the optically inactive even state at the beginning, and then scatter to the upper and lower odd polariton modes. This regime is favourable for Bose-Einstein condensation of X1 excitons whose life-time becomes much longer because of their decoupling from the cavity mode. Experimentally, the evidence for condensation of “dark” X1 excitons would come from the coherent emission of light from the ground odd exciton-polariton state. If the WQW is shifted from the node of the field, X1 exctions get coupled to light, and the anticrossing between even and odd polaritons can be distinguished in reflection spectra.