Sub-natural linewidths in excited-state spectroscopy

TL;DR

This paper demonstrates that in a coherently pumped thermal vapor, the absorption linewidth of an excited-state transition can be reduced below the natural linewidth, with experimental and theoretical analysis confirming sub-natural linewidths.

Contribution

It provides the first combined theoretical and experimental demonstration of sub-natural linewidths in excited-state atomic transitions in thermal vapor.

Findings

Measured minimum linewidth of 6.6 MHz in cesium vapor

Theoretical model matches experimental spectra well

Sub-natural linewidths achieved in excited-state transitions

Abstract

We investigate, theoretically and experimentally, absorption on an excited-state atomic transition in a thermal vapor where the lower state is coherently pumped. We find that the transition linewidth can be sub-natural, i.e. less than the combined linewidth of the lower and upper state. For the specific case of the 6P_{3/2} -> 7S_{1/2} transition in room temperature cesium vapor, we measure a minimum linewidth of 6.6 MHz compared with the natural width of 8.5 MHz. Using perturbation techniques, an expression for the complex susceptibility is obtained which provides excellent agreement with the measured spectra.