Anomalous R-Line Behaviour in Nanocrystalline Al2O3:Cr3+

The photoluminescence spectra of Al2O3:Cr3+ nanopowder under excitation by a frequency-doubled YAG:Nd laser (λex = 532 nm) and a nitrogen-vapour pulsed laser (λex = 337.1 nm) have been studied. A considerable difference between the spectra of nanocrystalline and macrocrystalline samples has been observed. The nanocrystalline samples reveal a broad asymmetric line with the maximum at 690 nm under the YAG:Nd excitation, whereas under the nitrogen-vapour laser excitation, the broad line becomes more symmetric, and its maximum shifts by 45 nm to the longer-wavelength region. The above shift is shown to be due to a drastic change in the Huang-Rhys parameter of the 2E–4A2 transition. A decay of the broad line with the increase of the duration of UV illumination has been observed.