Anisotropic nonlinear optics based on quantum interference

Weak coherent radiation interacting with multilevel atomic systems can excite a family of higher-order multiphoton transitions in contravention to conventional perturbative analysis. Monochromatic excitation leads to a strong anisotropic response arising from the interference of numerous underlying multiphoton pathways. The interference can be controlled through the polarization content of the input radiation. This anisotropic response is subnatural, quite unlike the anisotropy arising from spontaneous-emission–assisted optical pumping. Its sensitivity to external influences such as magnetic field suggests its utility in magnetometry. Specifically, we discover the presence of an elemental closed loop involving series of absorption and emission cycles that is central to the total response, and is universal to all atomic configurations. However, as not all atomic configurations support such anisotropy, we present a general classification of atomic transitions that exhibit anisotropic nonlinear response through quantum interference.