Sharp Spectral Lines of Luminescent Centers in Solids
In the first part of this article we consider the interactions of luminescent centers in solids with radiation and review the basic mechanisms present in spectral lines, such as saturation, and homogeneous and inhomogeneous broadening, and relate them to the lineshapes. The phenomena that affect the width of sharp lines in crystals are due to the interaction of the optically active centers with the thermal vibrations of the solid and to the spatial randomness of these centers. This randomness is present even in nominally ordered solids because the sites occupied by the luminescent centers in such solids are not exactly equal, due to the slight changes in the local crystalline field. In such systems the effects of randomness become evident at very low temperatures where the thermal broadening is drastically reduced, and the spectral lines appear mainly affected by a residual inhomogeneous broadening due to the spatial randomness. In glasses this randomness may represent the prevalent cause of broadening even at room temperature and may produce lines as wide as 100 cm−1for rare earth centers. Inhomogeneous broadening may have relevant effects on the spectral characteristics of a system of luminescent ions when monochromatic laser light is used to excite selectively the ions residing in a particular environment. The technique used in this case is called FLN (Fluorescence Line Narrowing); the conditions for its applications are examined.
In the second part of the article we present a simple theory that, starting from the vibrations of solids, leads to the formulation of the interaction between the optically active ion and the phonon system. The possible mechanisms that may produce the thermal broadening of a sharp line are examined; it is found that the predominant mechanism is the Raman scattering of phonons. The mechanisms by which the spectral lines change their positions with temperature are also examined; it is found that the main mechanism is due to the stationary effects of the ion-vibration interaction, namely the emission and absorption of virtual phonons. The similarities and differences between the thermal shift and the electrmagnetic (Lamb) shift are examined. Finally some experimental findings on sharp spectral lines are reported together with representative examples.
KeywordsSpectral Line Luminescent Center Thermal Vibration Sharp Line Spatial Randomness
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