Quantum interference in two-photon absorption: Polarization and magnetic field effects in the (7s${)}^1$S←(5$s^2$${)}^1$S transition of atomic Sr

Quantum-mechanical interference is found in the two-photon excitation of an atomic ${}_{}{}^1{}_{}{}^{}\mathrm{S}$ through the nonresonant sequence ${}_{}{}^1{}_{}{}^{}\mathrm{}_{}^1{}_{}{}^{}$P←${\mathrm{}}^1$S, since the magnetic sublevels of the intermediate ${}_{}{}^1{}_{}{}^{}\mathrm{P}$ state provide multiple pathways for absorption. Depending on the relative polarization of the two incident light beams, the interference can be either constructive or destructive. When the electric vectors of the light beams are linearly polarized and orthogonal, the interference is purely destructive and two-photon transitions are forbidden. Application of an external magnetic field, however, lifts the degeneracy of the intermediate state giving a two-photon absorption rate proportional to the square of the magnetic field strength. These effects are reported for the (7s${)}^1$S←(5${\mathrm{s}}^2$ ${)\mathrm{}}^1$S transition of atomic Sr excited by two dye lasers operating at different wavelengths.