Quantum-mechanical interference in two-photon absorption: A nonlinear analog of the Hanle effect

When an atom is irradiated by two light beams whose sum frequency corresponds to an allowed two-photon transition between two ${}_{}{}^1{}_{}{}^{}\mathrm{S}$ states, no radiation is absorbed if the electric vectors of the two beams are perpendicular to each other. Application of a magnetic field in a direction perpendicular to the plane defined by the two electric vectors permits two-photon absorption to take place. According to quantum theory the effect is interpreted as an interference phenomenon. That is, if no field is applied, the intermediate states are degenerate and the interference is destructive; when a magnetic field is applied, two different paths through intermediate states are possible, permitting population of the upper state. The effect is related to the Hanle effect and likewise provides the basis for an experimental method of determining Landé factors.