Picosecond quantum-beat spectroscopy of bound excitons in CdS

We report the first observation of quantum beats in the resonance fluorescence of a bound exciton in a semiconductor, the ionized-donor exciton complex in CdS. Measurements of the polarization behavior and magnetic-field dependence of the beats permitted a detailed investigation of strain effects and magneto-optical parameters of the states. From the damping of the beats, coherence times of the order of several hundreds of picoseconds are deduced. For the neutral-donor and -acceptor bound exciton states, very rapid dephasing with coherence times of the order of 20 ps is found.