Abstract

We report direct evidence for the presence of a spin-diffusion barrier, or frozen core, in Pr³⁺:LaF₃. The evidence is obtained by examining the cross relaxation between the Pr and F nuclei in a magnetic field chosen so that a pair of the optical ground-state Pr³⁺ hyperfine energy levels matches the F splitting or a multiple of the F splitting. This level-crossing condition permits resonant flip–flop interactions with the nearest-neighbor frozen-core F spins to repopulate Pr³⁺ hyperfine levels emptied by laser hole burning and is detected as enhanced absorption of the laser beam. The coupling of core F spins to bulk F spins during Pr–F cross relaxation is measured by NMR of the bulk F spin magnetization. The rate of cross relaxation between the Pr spins and the bulk F spins measured in this way is at least 3–4 orders of magnitude slower than that expected in the absence of a spin-diffusion barrier. This reduction of coupling indicates nearly complete detuning of the frozen-core F spins immediately surrounding the Pr³⁺ ion, cutting off resonant coupling with the bulk F spins.

© 1992 Optical Society of America

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