Measurement of strong electric fields using room-temperature Rydberg-EIT

We present a measurement of strong electric fields using room-temperature Rydberg-EIT. In a vapor cell, we drive microwave transitions between $^{85}$Rb Rydberg states and use the resultant resonances as probes of the applied field. We focus on the 65D$_{5/2}$ - 66D$_{5/2}$ two-photon transition and model the spectra with a non-perturbative Floquet analysis, which provides information about the strong microwave-induced n- and $\ell$-mixing effects on levels and excitation rates. We are able to measure fields up to $\sim$250 V/m to within $\sim$1$\%$ relative uncertainty. This is approximately 50 times higher in intensity than we have previously measured with this technique. We are also able to account for electric field inhomogeneities within the vapor cell by comparing the experimental line strengths to the calculated excitation rates. The accessible field range can be extended to even higher fields by utilizing transitions between lower-lying Rydberg states, where dipole moments generally are smaller and larger fields are needed to enter the strong-field regime. We also discuss how the sensitivity of the method at small fields can be enhanced by analyzing minute changes of the EIT-signatures that occur at fields much lower than where Autler-Townes splitting occurs.