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

Sapam Ranjita Chanu, Kanhaiya Pandey and Vasant Natarajan^a

Department of Physics, Indian Institute of Science - Bangalore 560 012, India

^a vasant@physics.iisc.ernet.in

Received: 13 February 2012
Accepted: 26 April 2012

Abstract

We show that it is possible to change from a subnatural electromagnetically induced transparency (EIT) feature to a subnatural electromagnetically induced absorption (EIA) feature in a (degenerate) three-level Λ 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 D₂ 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.