Demonstration of a high-contrast optical switching in an atomic Delta system

We present an experimental study of a Δ system in a room temperature, dilute sample of ⁸⁵Rb atoms. A Δ system in the D2 manifold of ⁸⁵Rb is formed by connecting the two lower hyperfine energy levels of a Λ system by a microwave drive field at 3.0357 GHz. We show that when the Rabi frequency of a microwave drive field exceeds that of the optical probe field, a three-wave mixing nonlinear interaction is established. This nonlinear interaction changes the enhanced transmission of the optical probe to a deep absorption for a relative phase difference of π between all the three fields. We establish through our experiment and our numerical simulation, the phase-sensitive and nonlinear nature of the Δ system's response to the applied microwave and optical fields. Using this feature, we demonstrate a high-contrast microwave phase controlled switch for the optical probe field. In comparison with intensity and phase switching demonstrated with all-optical fields so far, our experiment opens up the possibility of obtaining the highest contrast optical switching with room temperature atoms.