Broadly tunable and coherent microwave-to-optical conversion in a thermal atomic vapor

Flexible frequency tunability is a desired feature for frequency conversion schemes. Here, we experimentally realize nonlinear sum-frequency generation (SFG) between input 6.8 GHz microwave and 384 THz (780 nm) optical fields, mediated by room temperature 87Rb atoms inside an enclosed microwave cavity. We demonstrate coherence in this microwave-to-optical conversion and show that the output optical frequency can be tuned across more than 500 MHz due to the Doppler broadening of the atomic sample. We characterize this system in terms of bandwidth, tunability, and efficiency. Additionally, using amplitude-modulated microwave fields, we demonstrate that multimode signals can be transduced to the generated optical light, paving the way for frequency bin encodings. Finally, we characterize the temporal characteristics of the conversion process and explore how the non-linearities of this configuration may lead to new opportunities for microwave-controlled signal manipulation.