Abstract
The optical Bloch equations which incorporate the phenomenological population (T1) and dipole dephasing (T2) times have been tested recently by optical free induction decay (FID) measurements on an impurity-ion crystal Pr3+:LaF3 at 1.6°K. At low optical fields, the observed Pr3+ optical linewidth is dominated by magnetic fluctuations arising from pairs of fluorine nuclear flip-flops where the condition T1>>T2 prevails. At elevated fields, this nuclear broadening mechanism is quenched and the Bloch equations are violated with T2→T1. A microscopic theory appropriate for a low temperature impurity solid is developed which reveals the above features both for optical and radio frequencies, and a simple physical interpretation of this line narrowing phenomenon is given. Modified Bloch equations of a novel form are derived to second order and yield analytic FID solutions over the entire range of optical field strength. A discussion of the earlier NMR theories will be given pointing out similarities and differences.
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