Optical Properties and New Functionality of Nanocrystalline CuCl and Ge
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OPTICAL PROPERTIES AND NEW FUNCTIONALITY OF NANOCRYSTALLINE CuCI and Ge YASUAKI MASUMOTO Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305, Japan
ABSTRACT Lasing of nanocrystalline CuCl embedded in a NaCI single crystal was observed for the first time. Lasing takes place at 77 K in nanocrystalline CuCl sandwiched between dielectric mirrors under the pulsed ultraviolet laser excitation. The lasing transition is that from biexciton to exciton. The lasing is observed up to 108 K. The optical gain of nanocrystalline CuCI is almost the same as that of bulk crystals in spite of the low concentration of CuCI in the NaCI matrix. The origin of visible photoluminescence of nanocrystalline Ge in SiO 2 glassy matrix has been studied. Spectroscopic analyses of nanocrystalline Ge indicate that the room-temperature photoluminescence comes from nanocrystalline Gc of diameter of 4 nm or less. Highresolution electron microscopic studies imply that the structure of nanocrystalline Ge of diameter < 4 nm differs from the diamond structure. These data suggest that new nanostructure crystalline Ge having a character of direct optical transition exhibits the visible photoluminescence.
BIEXCITON LASING IN NANOCRYSTALLINE CuCI Semiconductor low-dimensional quantum structures are expected as promising semiconductor laser devices. 1 - 3 As the dimension is lowered, the modified density of states concentrate carriers more in a certain energy range. This concentration is expected to give the system more gain for lasing. Zero-dimensional quantum confinement of carriers makes the density of states to be a set of quantum levels. This is most favorable for the semiconductor laser, because the gain energy region is ultimately concentrated. In addition, spatial carrier confinement is also favorable for high optical gain. However, lasing in semiconductor quantum dots has never been observed so far. Therefore, the quantum dot laser should be realized and its characteristics should be clarified. CuCI quantum dots are a unique and typical system to be studied in the sense that the exciton quantum confinement takes place much stronger than the electron and hole individual quantum confinement. 4 Large binding energy of biexciton in a CuCI crystal, 32 meV, makes biexciton stable and allows us to observe biexciton absorption and luminescence at low temperature. 5 Biexciton luminescence is observed in CuCI quantum dots, too, even at 77 K.6 Exciton and biexciton translational motions are quantized and a continuous density of states becomes a set of discrete quantum levels. In photo-pumped bulk CuCI crystals, optical gain due to the transition from biexciton to longitudinal Z 3 exciton is high ' and lasing takes place at this transition. 9 The laser action takes place in CuCI as a result of the population inversion between excitons and biexcitons. In this sense, it is the 3-level laser, where 3 levels are ground, exciton and biexciton states. In this paper, we report the first observation of lasing in CuCl quantum dots. We found that the biexc
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