Page 304 - Tunable Lasers Handbook

264 Norman P. Barnes

and 4T7 manifold have approximately the same energy. By bringing these mani-
folds together, a significantly larger fraction of the excited Cr atoms resides in
the 4T7 manifold. Hence, the effective stimulated emission cross section is larger,
which in turn, increases the gain. However, the high fraction of the population
residing in the 4T2 manifold does decrease the upper laser level lifetime.
In LiCaAlF, and LiSrAlF,, the active atom resides in a position of near octa-
hedral symmetry. In essence, the crystal structure is formed by planes containing
the Ca or Sr atoms [37]. Sandwiched between these planes are the Li and A1
atoms, each surrounded by six F atoms forming an approximate regular octahe-
dron. However, important deviations from the regular octahedron exist. Referring
to Fig. 20, the planes containing the six F atoms above and below the A1 atom are
brought slightly closer to the plane containing the AI atoms than they would be in
a regular octahedron. Such a distortion tends to produce a trigonal distortion of
the octahedral symmetry. Another distortion of the octahedron consists of a slight
clockwise rotation of the three F atoms in the upper plane while the three F atoms
in the lower plane experience a slight counterclockwise rotation. Such a shift in
the position of the F atoms eliminates the inversion symmetry.
As with the other laser materials, the Cr substitutes for the A1 atoms. Con-
centrations of Cr in excess of 0.05 have been incorporated into the LiCaAlF,
material. Similar concentrations are expected in LiSrAIF,. Quenching effects

FIGURE 20 Configuration of LiCaAlF,. (Courtesy of S. A. ~ayne, Lawrence Livermore
National Laboratory.)