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 fi
lm samples with enhanced rare earth absorption. Results described incl
ude, the observation of absorption lines in the spectra of epitaxial f
ilms 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 conce
ntration in a silica glass fiber.