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We investigate the time-dependent reflectivity of the layered manganite La 0:5 Sr 1:5 MnO 4 at low temperature. We find that the reflectivity oscillates with an anomalously long period, e.g., several tens of hours. The period varies with wavelength and the character of the oscillation depends on the illumination condition. The oscillation is clearly observed below T $ 20 K and almost disappears above this temperature. Furthermore, in the temperature-cycle measurements, the reflectivity oscillation shows a particularly peculiar behavior.

We have investigated how the morphology and diameter of GaN nanocolumn affect the optical properties of exciton and biexciton. We measured photoluminescence (PL) of excitons in GaN nanocolumns at low temperature and found that the PL spectra depend on the column diameter. Changing the distance between the sample and the collimating lens, it was established that this phenomenon is due to the z ‐polarized PL radiated from the side surface. This result proves that the nano‐scale morphology produces a change in the optical properties. In addition, to elucidate the mechanism causing the increase of the biexciton binding energy observed in a previous experiment, we calculated the exciton binding energies by the variational method and also estimated their sizes. From the calculated results we can expect that the only biexcitons experience spatial confinement effect even when the nanocolumn width is much larger than the exciton Bohr radius. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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