The dynamics of photoluminescence due to biexcitons and exciton-exciton scattering (M and P emissions, respectively) has been investigated in the layered-type semiconductor PbI 2 by using the optical Kerr gate method. We simultaneously observed P and M emissions under high-density excitation. The M emission emerges instantaneously, whereas the P emission shows a delayed onset whose latency increases as the excitation photon energy increases. The latency to onset indicates that the P emission takes place after the relaxation of excitons with excess energy toward the bottleneck region via exciton-longitudinal optical (LO) phonon scattering processes. Based on the time-dependent peak energy shift of the P emission and a line-shape analysis of the M emission, we evaluated the effective temperatures of both photogenerated excitonic and biexcitonic systems as well as the self-energy due to the collisions among biexcitons. We conclude that these systems are separately formed in space owing to potential fluctuations between PbI 2 layers, and independently reach thermal equilibrium after ∼30 ps with different cooling processes. The exciton-exciton and exciton-LO phonon scattering processes play an important role in cooling the excitonic system, whereas the biexciton-biexciton and biexciton-exciton collisions are dominant in cooling the biexcitonic system.

We have investigated the photoluminescence (PL) dynamics of a ZnO thin film under intense excitation conditions at 10 K. In the ZnO thin film, a PL band due to exciton‐exciton scattering, the so‐called P emission, appears. The optical‐Kerr‐gating method with the ultrashort gating time of 0.6 ps enabled us to obtain precise information of the temporal profiles including the peak energy, the bandwidth and the intensity of the P emission, at various detection energies. We have found that the decay time of the P emission gradually shortens with decreasing detection energy. The change in the decay time can be reasonably attributed to the photon‐like characteristics of the lower polariton branch that is the final state of the exciton‐exciton scattering. Furthermore, we have shown that the decay time of the P emission is phenomenologically determined by the group velocity of the final state although the origin of the long decay time remains to be elucidated (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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