Optical Coherence in Semiconductors: Strong Emission Mediated by Nondegenerate Interactions

The presence of interactions between carriers in differing states, nondegenerate interactions, is experimentally demonstrated to result in strong, new contributions to the optical coherent response of semiconductors. These interactions are manifest in a new, state-selective, two-pulse four-wave-mixing technique as emission from a state that is excited by only one pulse. The emission arises due to interactions with another state that is excited by both pulses. These interactions are observed between the $1s$ exciton and continuum states and also between distinct magnetoexciton states. The resulting contributions must be included to properly understand the coherent response.