Advancing Three-Photon-Excited Rydberg RF Sensing: Fluorescence Readout for Wide-Dynamic-Range Characterization and Spatial Resolution Beyond Transmission.

We theoretically and experimentally investigated the fluorescence and transmission readouts of radio-frequency (RF) electrometry based on three-photon-excited Rydberg atoms. We developed a theoretical model for the fluorescence and transmission readout processes of a three-photon-excited Rydberg atom electrometer and performed a qualitative comparative analysis of fluorescence versus probe transmission readouts. Theoretical calculations revealed that while both fluorescence and probe transmission readouts can employ Autler-Townes (AT) splitting to measure strong RF fields, probe transmission readouts become ineffective in weak-field regimes, whereas fluorescence readouts remain sensitive to weak RF fields. Experimentally, we comprehensively characterize the fluorescence response across a wide range of RF field strengths: from the weak-field regime (exhibiting scaling of fluorescence peak amplitude with RF field strength), through the intermediate-field regime (where fluorescence spectral linewidth scales proportionally with RF field strength), to the strong-field regime (characterized by traditional A-T splitting). Furthermore, by adding a narrow slit in front of the photomultiplier tube (PMT) and scanning the slit together with the PMT along the light beam propagation, we exploit fluorescence's inherent spatial information to directly map the Rydberg excitation profile and local RF field strength. This overcomes the transmission readout's inherent limitation of providing only path-integrated signals along the probe beam, even by imaging the probe beam with a CCD camera. Our results establish fluorescence readouts as a superior technique for three-photon Rydberg electrometry, offering enhanced wide-range RF field sensing and direct spatial field mapping.