White light generation and trapping in yttria nanopowders

Summary form only given. Recent experiments have shown the occurrence of new physical phenomena (cw laser action mediated by scattering and stationary light) in structureless nanopowders consisting of rare earth doped dielectric particles smaller than 30 nm. These unexpected radiative properties result from strong localization of light and raise questions as to whether novel non-radiative properties or dynamics in these materials might similarly manifest themselves in unexpected ways. Here we present initial evidence that this may indeed be the case, through observations of spectral differences between single crystal and powdered yttria, high temperature material processing by low power incident light, and white light generation at room temperature. Er,Yb:Y/sub 2/O/sub 3/ nanopowders were irradiated with < 200 mW of laser light tuned to the Yb absorption region near 980 nm and luminescence from the focal region was collected and analyzed with a grating spectrometer. A scan is shown of the resulting upconversion emission in the visible spectral region, compared with a spectrum obtained from a single crystal specimen of Er,Yb:Y/sub 2/O/sub 3/ doped with identical concentrations of erbium and ytterbium ions. While the near infrared emission is similar, the visible emission is dramatically different. In the powder, the energy level responsible for green emission is the /sup 2/H/sub 11/2/ whereas in the crystal emission is dominated by the /sup 4/S/sub 3/2/ level. Since the host media and doping levels are the same in these two samples, one is led to infer that the marked difference in their emission spectra may result from changes in decay rates other than the radiative relaxation rates of key levels involved in emission processes. We examine this hypothesis with direct measurements of these rates.