  We study Rydberg atoms modulated by strong radio-frequency (RF) fields with a
frequency of 70 MHz. The Rydberg atoms are prepared in a room temperature
cesium cell, and their level structure is probed using electromagnetically
induced transparency (EIT). As the RF field increases from the weak- into the
strong-field regime, the range of observed RF-induced phenomena progresses from
AC level shifts through increasingly pronounced and numerous RF-modulation
sidebands to complex state-mixing and level-crossings with high-l hydrogen-like
states. Weak anharmonic admixtures in the RF field generate clearly visible
modifications in the Rydberg-EIT spectra. A Floquet analysis is employed to
model the Rydberg spectra, and good agreement with the experimental
observations is found. Our results show that all-optical spectroscopy of
Rydberg atoms in vapor cells can serve as an antenna-free, atom-based and
calibration-free technique to measure and map RF electric fields and to analyze
their higher-harmonic contents.
