Dark-line in optically-thick vapors: inversion phenomena and line width narrowing

Abstract:

An analysis of the phenomenon of coherent population trapping as observed by means of the dark state in the case of optically thick vapors in alkali metal atoms is presented. Very different behaviors are predicted for the observation of the dark state in the fluorescence and transmission spectra of the same atomic sample when the optical length is not negligible. Among other effects, the dark line observed in the fluorescence signal may look inverted appearing as a bright line, while in the transmission signal a narrowing of the line width resonance line is observed for an increase of the atomic density. In the pure three-level scheme a subnatural width is predicted. These effects, related to the electromagnetically induced transparency phenomenon, are readily observed in experiments, which are found to be in excellent agreement with the theory developed.