Polarization rotation effects in atomic sodium vapor

Polarization rotation effects in atomic sodium vapor are studied both experimentally and theoretically. Two pulsed dye laser beams propagate collinearly through a cell containing sodium vapor. The direction of polarization of a linearly polarized dye laser beam of frequency ${\omega }_1$ is found to be rotated by a circularly polarized beam of frequency ${\omega }_2$. This rotation is due to the difference in refractive index at ${\omega }_1$ for right and left circularly polarized light which is produced by the circularly polarized beam. When ${\omega }_1+{\omega }_2$ is tuned near the $3S-5S$ two-photon transition of atomic sodium, this difference is primarily due to the dispersion associated with the two-photon transition. Other sources of polarization rotation which were identified include (i) real transfers of population to excited atomic levels due to collisionally induced transitions and (ii) optically induced atomic energy-level shifts. The theory includes both the effects of two-photon dispersion and optically induced energy shifts. The application of the two-photon polarization rotation effect as a fast, optically controlled shutter or modulator and as a sensitive detection technique for two-photon spectroscopy are briefly discussed.