Ultraviolet laser action in strongly scattering Ce:alumina nanopowders

Summary form only given. Laser action in the multiple-scattering regime has been reported previously by several researchers from powders containing rare earth and transition metal ions. Lossless powders in which gain is encountered despite very short scattering mean free paths are sometimes referred to as "laser paints" and these media have interesting properties which may be useful for speckle-free lithography at submicron dimensions or applications in which bright, omnidirectional output is desired for displays or light sources of arbitrary shape. However, highly-scattering powders are difficult to pump and study optically because the very scattering which provides the feedback for laser action causes pump light to be scattered backwards very efficiently as it enters the medium. We have used electron pumping instead to demonstrate that high-energy, short optical pulses are not necessary to generate "laser paint" emission in dielectric powders. We also investigated strong scattering conditions to search for evidence that the onset of recurrent scattering trajectories which begin and end on the same site facilitate laser action and strong Anderson localization. Indeed, as we show here, weak electron excitation in doped oxide nanocrystals does readily generate stimulated emission at room temperature and coherent backscattering measurements confirm that our particles are lossless on length scales of both the scattering mean free path and the transport mean free path.