Selected Topics in Laser Spectroscopy

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.