Abstract

The basic feature of electromagnetically induced transparency (EIT) is a strong reduction of the light absorption by an atomic medium at a resonance transition frequency. The central idea of EIT is a creation, through quantum interference, of a superpositional state which is not excited by radiation, and preparation of atoms in this “dark” state. In conventional EIT situations, the dark state may exist for any (mean) laser phases and intensities. The only necessary condition is multiphoton resonance of the atom-radiation interaction. As long as the laser fields do not change or change adiabatically, the atoms can always be prepared in the dark state by optical pumping in the c.w. regime or by adiabatic following in the pulsed regime, independent of the laser phases. This is, however, not the case in situations where the radiation induced transitions form a closed loop. In closed-loop systems, the dark state exists only for specific values of the relative phase ɸ and the relative laser intensities, even if the multiphoton resonance condition is satisfied. Therefore, the response of atoms to the e.m. radiation may be changed dramatically by a change of only the phase, at fixed frequencies of the radiation.

© 1999 Optical Society of America