Abstract

We report the observation of coherent two-photon free-induction decay (FID) in a three-level Λ system excited by a low-intensity square-modulated laser pulse. Using electromagnetically induced transparency in an 85Rb cold atomic cloud, for what we believe to be the first time, we observe FID signals with coherence time exceeding the relevant atomic excited-state natural lifetime. Because of the on-resonance enhancement, the two-photon FID signal can be observed at the falling edge without using heterodyne means. © 2010 Optical Society of America OCIS codes: 190.4180, 020.1670. Transient optical phenomena associated with coherent interaction of radiation field with matter have been a fas-cinating aspect of the field of atomic, molecular, and op-tical physics. These fundamental studies have led to the development of many powerful analytical tools, such as nuclear magnetic resonance (NMR) spectroscopy and the atomic clock. Following the invention of the laser, there was much interest in optical coherent transitions from a two-level system, which is formally equivalent