Laser phosphors.
Williams, Guy Raymond
1999
Abstract
Continuous-wave random laser action has been observed experimentally in the ultraviolet and visible spectral regions at room temperature in delta-alumina nanopowders doped with Nd<super>3+</super> and Ce<super>3+</super>, and in beta<super> '</super><super>'</super>-alumina nanopowders doped with Pr<super> 3+</super> impurities. These are the first observations of stimulated emission from electrically-pumped phosphors with scattering lengths and (virtual) cavity dimensions shorter than a wavelength, in the strong localization regime for electromagnetic radiation. In the experiments described here, control of the electron voltage and current permitted independent tuning of the location of the gain volume with respect to the surface of the sample and its excitation rate. This in turn permitted us to differentiate between continuous-wave laser action and amplified spontaneous emission in the optical properties of the nanophosphors. Continuous-wave laser action was achieved on the 5d → 4f inter-configurational transition in Ce<super>3+</super>:delta-Al<sub>2</sub>O<sub>3</sub> at 357 nm for the first time. Modest line-narrowing and abrupt changes in the slope of output intensity accompanied the onset of stimulated emission and the transport mean free path was measured to be 115 nm, which is less than half the emission wavelength. Dramatic thresholds measured at 632.8nm for various voltages in Pr<super>3+</super>:beta<super>'</super><super>'</super>-Al<sub> 2</sub>O<sub>3</sub> also demonstrated laser action in the visible region and laser action was obtained in Nd<super>3+</super>:delta-Al<sub>2</sub>O<sub> 3</sub> simultaneously on the <super>2</super> F<sub>5/2</sub> →<super> 4</super> F<sub>3/2</sub> (373nm) and the <super>2</super>P<sub>1/2</sub> → <super> 4</super>I<sub>1/2</sub> transitions (467nm). No evidence of mode structure was observed on any laser phosphor transition, consistent with subwavelength cavity dimensions. Laser phosphor emission was shown to be speckle-free or incoherent, in the case of the ultraviolet light generated by Ce-doped powders. The coherence length was found to be less than half a wavelength. In order for stimulated emission to be incoherent, however, it must originate from an evanescent source. Since evanescent waves cannot propagate, our results appear to contradict the simplest observation reported here, namely that our phosphors emit light. The resolution of this surprising situation is that total internal reflection can occur in highly scattering powders, but that frustration of the reflection takes place at the surface. The results are therefore consistent with Anderson localization. Many applications for evanescent laser sources emitting incoherent light can be imagined in conventional lighting and displays, since existing phosphors in television, fluorescent lights, plasma and field emission displays all rely exclusively on spontaneous rather than stimulated emission.Subjects
Alumina Nanopowders Laser Phosphors Rare Earth-doped Strong Localization
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