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Ultrafast photoluminescence dynamics of biexcitons in a CuCl thin film grown by vacuum deposition M. Nakayama a*, S. Wakaiki a, K. Mizoguchi a, D. Kim a, H. Ichida b, and Y. Kanematsu b a Department of Applied Physics, Graduate School of Engineering, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan b Venture Business Laboratory, Center for Advanced Science and Innovation, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan Cuprous halides have been typical materials for investigations of excitonic processes owing to the large exciton binding energies leading to high stability of excitonic systems under intense excitation conditions. Although there have been numerous reports on the excitonic properties, investigations of ultrafast photoluminescence (PL) dynamics of biexcitons have not been activated mainly because of a lack of time resolution. An optical-Kerr-gating (OKG) method is a powerful technique to measure time-resolved PL spectra with a typical time resolution of ~1 ps. In the present work, we have investigated the PL dynamics of biexcitons in a 500-nm thick CuCl film with the OKG method. Although Akiyama et al. reported a precise study of the radiative decay of biexcitons in a CuCl crystal via two-photon excitation with a time resolution of ~8 ps , we have focused on the biexciton-PL dynamics under one-photon excitation from aspects of the formation process of biexcitons and the cooling process of the effective temperature. The CuCl sample with a thickness of 500 nm was grown on a (0001) Al2O3 substrate by vacuum deposition. The thin film is preferentially oriented along the  direction with highly crystalline quality . Time-resolved PL measurements were performed with the OKG method: the time resolution of 1 ps and the excitation-laser wavelength of 381 nm. Figure 1 -5 shows the image map of time-resolved biexciton- PL spectra of the 500-nm thick CuCl film at 10 K 0 under the excitation power of ~160 J/cm . We 2 notice the following two facts from Fig. 1. One is 5 Laser Time Delay (ps) that the biexciton PL has a rise time of ~2 ps. This pulse rise time reflects the relaxation process of 10 photogenerated excitons and exciton-exciton collisions. The other is that the PL-peak energy 15 shifts toward the high-energy side with an increase in time delay, which is due to cooling of 20 the effective temperature in the decay process. Thus, the time-resolved PL spectra reveal the 25 dynamics of the biexciton-emission process. * corresponding author: 30 firstname.lastname@example.org 3.15 3.20 3.25 3.30 3.10 Photon Energy (eV) References Fig.1: Time-resolved biexciton-PL spectra 1. H. Akiyama et al., Phys. Rev. B 42, 5621 (1990). of the 500-nm thick CuCl film measured 2. M. Nakayama et al., Phys. Rev. B 55, 10099 (1997). with the OKG method at 10 K.
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