Electromagnetically-induced-transparency spectra of Rydberg atoms dressed with dual-tone radio-frequency fields

We examine spectral signatures of Rydberg atoms driven with near-resonant dual-tone radio-frequency (rf) fields in the regime of strong driving. We experimentally demonstrate and theoretically model a variety of nonlinear and multiphoton phenomena in the atomic Rydberg response that manifest in the electromagnetically-induced-transparency spectra. Our results echo previous studies of two-level atoms driven with bichromatic optical fields. In comparison to optical studies, the rf-driven Rydberg system utilizes a more complex excitation pathway and electromagnetic fields from two different spectral regimes: a two-photon optical excitation continuously creates highly excited Rydberg atoms, while rf fields drive resonant coupling between the Rydberg levels and generate strong mixing. However, our spectra reflect nearly identical effects of the dual-tone rf fields on the atomic Rydberg observables, showing detuning-dependent splittings and Rabi-frequency-dependent peak numbers and relative strengths, and avoided crossings at subharmonic resonances. We thus validate previous two-state models in this more complex physical system. In the context of Rydberg electrometry, we use these investigations to explore a technique in which we tune a known rf field to observe spectra which give the frequency and power of an unknown rf field using the complex dual-tone spectra.