B. Min, T.J. Kippenberg, L. Yang, K.J. Vahala, J. Kalkman and A. Polman, Erbium-implanted high-Q silica toroidal microcavity laser on a silicon chip, Phys. Rev. A 70, (Article number: 33803), 1-12 (2004)
O. Muñoz, H. Volten, J.W. Hovenier, B. Veihelmann, W.J. van der Zande, L.B.F.M. Waters and W.I. Rose, Scattering matrices of volcanic ash particles of Mount St. Helens, Redoubt, and Mount Spurr Volcanoes, J. Geophys. Res. 109, (2004)