We have performed spectrally resolved degenerate four-wave mixing (FWM) spectroscopy on exciton systems in -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 1s free exciton and the bound exciton. Dephasing times of the 1s 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.
Exciton states in wide-gap materials CaF 2 and BaF 2 have been studied with the technique of two-photon spectroscopy using synchrotron radiation and laser. The two-photon excitation spectrum of CaF 2 , and that of BaF 2 as well, exhibited a clear peak on the high-energy side of the 1S exciton energy. The peaks are attributed to the 2 P states of the valence excitons. According to this assignment, the binding energies of the excitons have been determined to be 0.8 and 0.9 eV for CaF 2 and BaF 2 , respectively. The signal intensities were larger than that in the case of one-beam excitation by a factor of about 500, which is attributed to a resonant effect on two-photon absorption. ͓S0163-1829͑99͒52836-1͔ RAPID COMMUNICATIONS
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