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 properties of a lightly alloyed In0.02Ga0.98N thin film at 10 K under intense excitation conditions. A photoluminescence band (P band) peculiar to the intense excitation condition has been clearly observed. The excitation-power dependence of the P-band intensity exhibits an almost quadratic behavior, accompanied by a threshold-like appearance. The threshold-excitation power for the P band is very low: ∼3kW∕cm2. At the threshold excitation power, the energy of the P band is lower than the energy of the n=1 A free exciton by the energy difference between the n=1 and n=2 exciton states. The results described above indicate that the P band originates from exciton-exciton scattering. Furthermore, we have confirmed the existence of optical gain leading to stimulated emission in the energy region of the P band by using transmission-type pump-probe spectroscopy.

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We have investigated photoluminescence (PL) properties of a high quality GaN thin film grown by metal organic vapor phase epitaxy under intense excitation conditions in a high temperature regime from 120K to room temperature. It is found that a PL band peculiar to intense excitation conditions appears with a threshold-like behavior. The energy spacing between the PL band at the threshold excitation power and the A exciton is proportional to temperature. The extrapolation of the linear dependence results in zero value of the energy spacing at absolute zero temperature. These PL profiles are specific to an emission process originating from exciton-electron scattering. Furthermore, we have demonstrated that the exciton-electron scattering process produces optical gain at room temperature from measurements of PL with a variable stripe-length method.

We have investigated photoluminescence (PL) properties of a high quality GaN thin film grown by metalorganic vapor phase epitaxy under intense excitation conditions in a wide temperature range from 10 to 300 K. It is found that there are two types of PL band peculiar to intense excitation conditions. In a low temperature region below 80 K, the exciton-exciton scattering dominates the PL, the so-called P emission.On the other hand, in a high temperature region above ~120 K, a PL band, which is different from the P emission, appears. The energy spacing between the new PL band and the fundamental A exciton linearly increases with an increase in temperature. In addition, the energy spacing is estimated to be zero at absolute zero temperature by extrapolation of the temperature dependence. These PL profiles indicate that the PL band observed in the high temperature regime originates from the exciton-electron scattering. . The well-known emission process is exciton-exciton scattering, the so-called P emission, in which one of two n = 1 excitons is scattered into a high-energy state with n ≥ 2, while the other is scattered into a photon-like state; the energy of which is lower than that of the n = 1 exciton state by the energy difference between the n = 1 and n ≥ 2 states. The P emission has been intensively studied in various wide-gap semiconductors: e.g., ), lightly-alloyed InGaN (Ref.[5]), CdS (Ref.[6]), ZnO (Ref. [7]), and CuI (Ref. [8]). On the other hand, there have been limited reports on the investigation of exciton-carrier scattering. In II-VI semiconductors [7,9,10] and cuprous halides [11], the characteristics of PL due to exciton-electron scattering were studied in a high temperature regime, which is called H emission. The exciton-electron scattering process is explained as follows. One electron around the bottom of the conduction band is scattered into a larger wavevector state, i.e., a hot electron, while one exciton in the first quantum (n = 1) state is scattered into a photon-like state; the energy of which is lower than that of the n = 1 exciton state by the energy difference between the initial and hot electron states. There are two specific features of the PL due to exciton-electron scattering. One is a low energy shift of the PL energy with an increase in excitation power because of an increase of effective temperature, which is similar to the P emission. The other is a linear temperature dependence of the en-