Superfluorescence polarization: Signature of collisional redistribution

We have studied effects of magnetic sublevel degeneracy on the polarization of superfluorescent pulses generated on the Ca 4s4p¹P₁–3d4s¹D₂ transition at 5.5μm. These pulses were generated from a cell of length 50 cm by optically pumping calcium vapor on the 4s²¹S₀–4s4p¹P₁ transition in the presence of Ar gas. The axis of ellipticity of superfluorescence (SF) polarization is oriented parallel to the axis of the pump-laser polarization at large detunings, and undergoes an abrupt rotation through 90° for detunings close to resonance. The distribution of populations in the magnetic sublevels of the ¹P₁ state can be estimated using a simple model based on previously calculated cross sections for collisionally aided absorption in the presence of an intense (pump) field. For large detunings, these estimates are consistent with the polarized SF intensity measured in the experiment. A direct measurement of the populations of the ¹P₁ magnetic sublevels also supports the collisional redistribution predicted by the calculated cross sections. We therefore suggest that SF polarization can be a useful signature of collisional redistribution. However, the change in ellipticity is unexpected, and probable causes for this effect are discussed.