Determination of the sign of the population difference in a two-level system by frequency-modulation spectroscopy

Abstract

ABSTRACTPopulation inversions can occur in two-level systems as a result of chemical kinetic processes that involve pulsed excitation of a system with a large amount of excess energy, for example, multi-photon dissociation. In this work, we report the use of frequency-modulation (FM) spectroscopy to determine, from the phase of the FM signal, the sign of the population difference in a two-level system – specifically, whether the two-level system is inverted. As a proof of principle demonstration of this technique, we show that FM signals from an inverted I transition, prepared by a pulsed laser, have opposite phase with respect to signals from a non-inverted system, the Rb D-line absorption transition.