Observation of antibound states for exciton pairs by four-wave-mixing experiments in a single ZnSe quantum well

We have employed spectrally resolved, subpicosecond transient four-wave-mixing techniques to identify and to study the four-particle “antibound states” associated with excitonic molecules in a binary ZnSe single quantum well. Previously, the existence of robust bound biexcitons have been observed in this wide band-gap semiconductor heterostructure, including a density regime where optical gain is dominated by the molecular states. We employ selective circular polarization excitation to suppress the bound (ground state) exciton-pair state contribution and find a spectrally distinct and isolated feature emerging approximately 3.5 meV higher in photon energy than the n=1 heavy-hole exciton transition. The new resonance is distinct only in the negative time delay regime, and is interpreted as a manifestation of coherent four-particle correlated states within the biexciton continuum.