Transient reflectivity changes of β-ZnP2 exciton bands by pump–probe spectroscopy using a femtosecond laser

Ultrafast reflectivity changes of exciton bands in β-ZnP2 have been investigated by pump–probe spectroscopy using a femtosecond laser. The spectrum of the pump–probe signal shows a significant negative reflectivity change in the energy range between ET and EL of the 1s exciton polariton, where ET and EL are the transverse and longitudinal exciton energies, respectively. The reflection relaxation time is 1.1 ps near EL and 1.7 ps near ET. These relaxation times are related to acoustic-phonon scattering of exciton polaritons. Transient reflectivity changes with a dispersion-type spectral shape are also observed at the energy where the excitonic molecule luminescence appears.