Two--photon absorption

Abstract

Two--photon absorption is theoretically analyzed within the semiclassical formalism of radiation--matter interaction. We consider an ensemble of inhomogeneously broadened three--level atoms subjected to the action of two counterpropagating fields of the same frequency. By concentrating in the limit of large detuning in one--photon transitions, we solve perturbatively the Bloch equations in a non-usual way. In this way we derive an analytical expression for the width of the two--photon resonance that makes evident sub-Doppler two--photon spectroscopy. We also derive an analytical expression for the Stark shift of the two--photon resonance.