Parametric amplification in an electromagnetically-induced-transparency medium

In an ordinary electromagnetically-induced-transparency (EIT) experiment using a hot sodium vapor, we have observed a probe gain up to 32 in the presence of a coupling beam in an otherwise totally opaque (transmissivity ∼10⁻⁵) situation, increasing the transmissivity by 6 orders of magnitude. This gain is attributable to parametric amplification (PA), accompanying the Stokes (or anti-Stokes) wave with similar output intensity, and is critically dependent upon the atomic density and the coupling beam power. Experimental and theoretical analyses have revealed that, while the EIT picture is valid for relatively low atomic densities, the PA picture becomes dominant for high atomic densities and high coupling powers. We have also found that the parametric coupling between the probe and Stokes waves can be nullified by the proper choice of the coupling detuning frequency, due to destructive interference of two elementary processes.