Parametric Frequency Conversion of Resonance Radiation in Optically PumpedRb87Vapor

This paper reports an investigation of parametric frequency conversion in a vapor of ${\mathrm{Rb}}^{87}$ atoms in which nearly resonant optical waves are coupled with microwaves at the ground-state hyperfine-structure (hfs) frequency of ${\mathrm{Rb}}^{87}$ by the nonlinear susceptibilities of ${\mathrm{Rb}}^{87}$ vapor. These nonlinear susceptibilities are greatly enhanced by optical pumping of the ground-state hfs levels of ${\mathrm{Rb}}^{87}$ and by using nearly resonant light. The frequency-conversion process generates optical sideband waves which are separated in frequency from the carrier light waves by the ground-state hfs frequency of ${\mathrm{Rb}}^{87}$ (6.835 GHz). These sidebands have been directly observed in the spectral profiles of ${\mathrm{Rb}}^{85}$ resonance light which had passed through ${\mathrm{Rb}}^{87}$ vapor. The conversion efficiency of power from the carrier waves to the sideband waves in a 45-cm length of vapor was greater than 1%. The observed behavior of the sidebands is in excellent qualitative agreement with the predictions of a semiclassical theory.