Highly monochromatic fluorescence via quantum interference

Abstract

In this article we consider a four-level atom, where two closely lying levels are coupled via a laser to a lower state. Apart from the direct spontaneous emission, the upper pair relaxes to the ground level via an intermediate non-metastable state. We show that coherent population trapping may induce fluorescence along the driven transitions with an incoherent component consisting of a single ultranarrow line. This line is out of resonance with the driving field and its intensity is comparable to the intensity of the fluorescence in the absence of quantum interference. We develop an analytic description leading to explicit formulas for the lineshapes and linewidths in the strong-field limit, which accounts for the features of the fluorescence that emerge from the numerical work.