Theory of double refraction in a gas due to spin alignment: observations of birefringence in optically pumped helium

Abstract

Transverse birefringence has been observed in an aligned spin assembly consisting of helium 4 atoms in the metastable state 2 ³S₁. The alignment of the spins was produced by optical pumping with unpolarized light. Using a cross-beam of off-resonant radiation, for example from a helium 3 lamp, together with a suitable combination of polarizer and analyser, birefringence signals could be displayed analogous to the absorption signals conventionally used for the optical monitoring of magnetic resonance. An analysis of the phenomenon is given, showing quite generally that birefringence monitors separately the degree of alignment of the spin assembly, even when orientation is also present; the latter can be monitored by the magnetic rotation of the plane of polarization for axial light. The analysis also includes the effect of the spectral characteristics of the monitoring lamp, i.e. the central frequencies of emission and the Doppler width, on the strengths of absorption and birefringence signals displayed by optically thin samples.