  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 EIT
spectra. Our results echo previous studies of two-level atoms driven with
bichromatic optical fields. In comparison to the 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. Yet,
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 where we tune a known RF field to
observe spectra which give frequency and power of an unknown RF field using the
complex dual-tone spectra.
