Dynamics of high-density excitons in PbI2 studied by two-photon absorption

Relaxation and diffusion processes of high-density excitons in layered-type semiconductor PbI2 have been investigated through femtosecond transient grating and time-resolved luminescence measurements under two-photon absorption. We observed two luminescence bands at low temperature; one is due to an exciton-exciton collision process, while the other originates from localized excitonic states. By utilizing the transient grating spectroscopy, the lifetime and diffusion coefficient of the high-density excitons in PbI2 are estimated to be 72 ps and 4.6 cm2/s, respectively. The lifetime observed is almost equal to the values of the decay time of the luminescence due to the exciton-exciton collision process and the rise time of the luminescence due to the localized excitons. This strongly shows that the decay of the exciton-exciton collision process is dominated by the exciton trapping to the localized states arising from potential fluctuations due to imperfection of crystal.