- Once σ is known, the number density (N) of color centers can be obtained from a measurement of transmission (under a non-saturating condition) of a collimated beam of monochromatic light through a specimen of thickness l, by use of the equation
N = [ln(I0/I)]/σl
where I0 is the irradiance of the beam incident on the specimen and I is the irradiance of the light emerging from the specimen.
The laboratory apparatus for the saturation measurements (see Figure 1) includes a laser that generates a precisely characterized beam and a mechanism for translating the focus (more precisely, the beam waist) through the specimen. By moving the beam waist through the specimen, one can easily vary the irradiance over a wide range. To obtain the value of σ, the fluorescence and transmission measurement data thus obtained at a succession of beam-waist positions (and, hence, at a succession of known irradiance vales) are plotted and fitted to theoretical curves of transmission and fluorescence as functions of irradiance (see Figure 2).
This work was done by W. Thomas Roberts of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. NPO-30796
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