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.