Abstract

There has recently been considerable interest in optical transverse nonlinear effects (see, for example, the special issue on this subject introduced in Ref. 1). We describe here a sensitive technique for observing in an essentially interferometric manner small transverse modifications to the nonlinear propagation of a laser beam through an atomic vapor, and we give illustrative results for a cw dye-laser near-re sonant to the D₁ line in collisionally broadened atomic sodium. When a cell containing the atomic vapor is placed between high-quality exactly crossed polarizers, arranged to be almost, but not exactly pure circular, a weak (~10⁻⁴ of the 5 mW input intensity) transmitted signal is observed (see Fig. 1) when the laser is tuned to near resonance. A very interesting feature associated with this signal is the appearance of spatial structure in the transmitted beam: distinct ring patterns can be observed at both positive and negative laser detunings (Fig. 2). We understand this phenomenon in terms of competition between the pure-circularly polarized component modes comprising the elliptically polarized input. The intensities of the right- and left-handed circularly polarized components are very different; and optical pumping in the buffer-gas-broadened sodium vapor, combined with beam-reshaping effects, leads to transverse propagation differences between these components after competitive nonlinear interaction in the medium. The components interfere when they are projected into the common output (elliptical) polarization state, which is exactly orthogonal to the input.

© 1994 Optical Society of America