Conversion between electromagnetically induced transparency and absorption in a three-level lambda system

TL;DR

This paper demonstrates how to switch between electromagnetically induced transparency and absorption in a three-level system by adding a second control beam, enabling control over the dispersion properties in atomic vapors.

Contribution

It introduces a method to convert EIT to EIA in a three-level lambda system using a second control beam, supported by density-matrix analysis.

Findings

Switch from EIT to EIA observed in Rb vapor.

Control of dispersion sign through beam configuration.

Supported by density-matrix theoretical analysis.

Abstract

We show that it is possible to change from a {\it subnatural} electromagnetically induced transparency (EIT) feature to a {\it subnatural} electromagnetically induced absorption (EIA) feature in a (degenerate) three-level $\Lambda$ system. The change is effected by turning on a second control beam counter-propagating with respect to the first beam. We observe this change in the $D_2$ line of Rb in a room-temperature vapor cell. The observations are supported by density-matrix analysis of the complete sublevel structure including the effect of Doppler averaging, but can be understood qualitatively as arising due to the formation of $N$-type systems with the two control beams. Since many of the applications of EIT and EIA rely on the anomalous dispersion near the resonances, this introduces a new ability to control the sign of the dispersion.