Optical anisotropy induced by two intense laser fields in sodium vapor

Optical anisotropy of an atomic sodium system driven by two intense laser fields has been studied with polarization spectroscopy. The anisotropy has revealed various aspects of higher order optical processes such as a resonant three-photon transition and an ac Stark effect, and it could have been regarded as due to one of the transitions between "dressed states". We have measured the optical anisotropy which appears at frequencies where 2ω₁-ω₂ or 2ω₂-ω₁ are close to the sodium D lines. The peak anisotropy was shifted by an ac Stark effect caused by both laser fields. Signal lineshapes due to the anisotropy have been calculated in the vicinity of the D₁ line by using the modified Mollow theory. Agreement between the experiment and the calculations is satisfactory.