Microwave field imaging inside an atomic cell by fluorescence thermography

Rydberg atom-based electromagnetic field sensors are currently the focus of intense research efforts, promising high sensitivity, tunability, and compactness with SI-traceable measurements. However, the presence of a cell around the atoms alters the microwave field distribution, particularly due to reflection by materials like quartz. To characterize these sensors, measuring how the presence of the cell affects the microwave field distribution is therefore essential. Electro-optical probes offer high spatial resolution but require time-consuming scanning. Infrared electromagnetic thermography provides direct field mapping but is limited by the opacity of glass to infrared radiation. In contrast, fluorescence thermography allows for electric field mapping through optically transparent media. In this paper, we report the application of this technique to map the microwave electric field inside a quartz cell intended for atomic cooling and trapping. This provides a direct observation of the standing waves inside the cell, in good agreement with numerical simulations. This technique could be a valuable tool for the design and optimization of atomic cells for applications requiring good control of the microwave field distribution inside the cell, such as atomic clocks or electric field sensors based on hot or cold Rydberg atoms.