Atomic measurements of high-intensity VHF-band radio-frequency fields with a Rydberg vapor-cell detector

We investigate and employ optical Rydberg resonances in an atomic vapor cell for measurements of high-intensity VHF-band radio frequency (rf) electric fields. An atomic vapor cell with integrated electrodes is used to generate high-intensity 50--500 MHz rf electric fields reaching $\ensuremath{\sim}5$ kV/m in a submillimeter gap. The fields are measured using Rydberg electromagnetically induced transparency as an optical readout of field-sensitive $30{D}_{J}$ and $35{D}_{J}$ Rydberg states of atoms within the gap. The rf electric field is determined by matching observed spectroscopic markers, including ac level shifts, even-harmonic rf sidebands, and rf-induced avoided crossings in the Rydberg manifold to calculated spectra derived from a nonperturbative Floquet theory. In our measurements, rf field frequencies and electric-field amplitudes are determined to an accuracy of $1.0%$ and $1.5%$, respectively. In the atom-field interaction, we observe a transition from a quantum regime, characterized by discrete even-harmonic Floquet states separated by an even multiple of the rf field frequency, into a semiclassical regime at very strong fields, in which the spectrum exhibits unresolved resonances whose strengths are smoothly modulated at a frequency of approximately five times the rf frequency. The underlying physics is explored.