Coherent propagation of femtosecond optical pulses in a monoclinicZnP2single crystal

The free induction decay signal from excitons in a ${\mathrm{ZnP}}_2$ single crystal shows an unexpected temporal evolution, polariton beat, and coherent dip. We show that the effects come from a nonzero thickness of the sample and nonzero time width of the excitation pulses. A systematic numerical calculation provides good qualitative agreement with the data. These effects are very important in understanding the experimental results which deal with the coherent interaction between excitons and optical fields.