Nonabsorption resonances by nonlinear coherent effects in a three-level system

Abstract

The behaviour of a three-level system consisting of two ground-state sublevels optically coupled to a common excited state is analysed. An exact steady solution of the semi-classical equations of motion for the density matrix is obtained (within the RWA) and then numerically investigated. When the frequency difference of the two applied fields becomes equal to the frequency splitting of the lower levels, a mechanism of transverse optical pumping takes place that accumulates all the atoms in a coherent superposition state of the ground sublevels, in which they are no longer able to absorb the pumping fields. The effect is due to nonlinear effects of coherence and is responsible for the appearance of a very strong and narrow resonance in the system response. Moreover, if the interaction energies of the two transitions have different magnitude, another resonance appears which corresponds to the power-shifted two-photon transition between the lower sublevels. The reported results are in agreement with experimental phenomena recently observed.