Abstract
We have performed spectrally resolved degenerate four-wave mixing (FWM) spectroscopy on exciton systems in β-ZnP 2 using a femtosecond laser. The FWM signal clearly exhibits a periodic ultrafast modulation as a function of the delay time between two excitation pulses, which indicates the quantum beats due to the interference of two closely spaced transitions. From FWM and reflection spectra, the transitions are assigned to the 1 s free exciton and the bound exciton. Dephasing times of the 1 s exciton polariton are measured at various energies between the longitudinal and transverse excitons. The dephasing time is found to be longer as the polariton energy is lower, which suggests that the scatterings by acoustic phonons mainly govern the polariton dephasing. The difference in the modulation behavior of the quantum beats for the free and bound excitons is briefly discussed based on a simple model with a three-level system.
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