Ultrasensitive optical spectroscopy of rare earth solids

In this paper, we report recent work wherein f → f transitions have been observed in samples either too dilute or too opaque to have been studied previously. Ultrasensitive reflection and fluorescence spectra are discussed together with techniques for the fabrication of thin film samples with enhanced rare earth absorption. Results described include, the observation of absorption lines in the spectra of epitaxial films containing less than one layer of absorbing ions, the observation of sharp lines in the spectra of rare earth semiconductors and metals, and the preliminary observation of frequency dependent structure in the inhomogeneously broadened fluorescence of Eu3+ at 10-100 ppb concentration in a silica glass fiber. © 1994.