Modulation of a Fluorescent Light Beam by an Optically Pumped Vapor Undergoing a Double-Quantum Resonance

The high-frequency modulation of a light beam scattered by optically-pumped ${\mathrm{Rb}}^{85}$ vapor undergoing a double-quantum resonance in the ground state has been studied both experimentally and theoretically. The vapor cell used in the experiment was an evacuated 3-in.-diam sphere with paraffin-coated walls. It was subjected simultaneously to a ${\sigma }^{+}$-polarized $D_1$ pumping beam and a $\pi$-polarized unfiltered monitoring beam at right angles to the pumping beam. The scattered light was detected in a third orthogonal direction after passage through a $\sigma$ polarizer and a $D_2$ filter. A superheterodyne amplifier was used to select the component of the scattered light modulated at twice the frequency of the pulsed rf driving field. A theoretical analysis has been developed which predicts the existence of two pairs of nonvanishing off-diagonal elements of the excited-state density matrix oscillating at twice the applied frequency under these conditions, and also predicts a corresponding modulation of the scattered light. Despite the feebleness of the observed signal caused by heavy damping in the excited state of ${\mathrm{Rb}}^{85}$, such a modulation has been observed, and its resonant behavior has been found to agree with theory.