Laser Excited Fluorescence Spectroscopy in Glass
Glass, on a microscopic scale, epitomizes an inhomogeneous system. Being an in-inherently disordered medium, the environment of each ion in a glass is not identical as in a crystal. In addition, because of differences in the bonding to nearest-neighbor io
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6.1 Background Glass, on a microscopic scale, epitomizes an inhomogeneous system. Being an ininherently disordered medium, the environment of each ion in a glass is not identical as in a crystal. In addition, because of differences in the bonding to nearestneighbor ions and, in multicomponent glass compositions, in the types and statistical distribution of more distant neighbor ions, the local fields at individual ion sites vary. This results in site-to-site differences in the energy levels and the radiative and nonradiative transition probabilities of paramagnetic ions in glasses. Broadbandexcited optical absorption and emission spectra and excited-state decays consist of a superposition of contributions from individual ions distributed among the entire ensemble of local environments. The spectra exhibit inhomogeneous broadening and the decays do not have a single exponential time dependence. When a narrowband source is used for excitation, only those ions resonant with the excitation quanta to within the homogeneous linewidth are excited. This site-selective excitation effectively reduces the inhomogeneous broadening and a line-narrowed fluorescence spectrum is obtained. In comparison to crystals where local strains and other imperfections produce inhomogeneous broadening of narrow1 cm- 1 , in glasses the sameftransition may be broadened by line transitions of 100 cm -1. Since the radiative lifetimes of some rare-earth ion transitions correspond to homogeneous linewidths as small as "'" 10- 8 cm- 1 , line narrowing of ten orders of magnitude in principle is possible. Fluorescence line narrowing (FLN) in glass is not new. In a pioneering study published in 1967, Denisov and Kizel [6.1] reported FLN and time-resolved spectra of Eu 3 + emission in a borate glass. Narrow lines from a mercury lamp that matched the inhomogeneously broadened Eu 3 + absorption bands were used for excitation. While the Denisov-Kizel report was brief and qualitative, it demonstrated both line narrowing and spectral diffusion and the dependence of these properties on paramagnetic ion concentration. This was done several years before Szabo's report of laser-induced FLN in a ruby crystal [6.2] . Although any monochromatic source can be used for FLN in glass, a laser especially a tunable laser - is the ideal spectroscopic tool. Yet another two years passed after Szabo's paper before the first report of laser-induced FLN in glass appeared. Using a fixed-frequency cw laser, Riieberg excited a Nd 3+ absorption line in a silicate glass, observed nonresonant line-narrowed fluorescence from a
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