Kilohertz-range electric field calibration in an alkali vapor cell using time-averaged Stark shifts

Abstract

We present a model-independent experimental method for calibrating the amplitude of a low-frequency electric field in an alkali vapor cell. Three-photon excitation to a Rydberg state in atomic rubidium produces an electromagnetically induced transparency (EIT) signal that is sensitive to low-frequency fields originating outside the cell. We superpose the measured shape of the EIT resonance in the presence of an electric field with the expected line shape, obtained by numerically time-averaging the effect of the DC-Stark shift. The value of the fractional field amplitude transmission, caused by Faraday screening by the metallic rubidium layer on the cell interior, is a direct result of this comparison. A transmission spectrum is obtained by making measurements between 0 and 100 kHz, which demonstrates high-pass filtering behavior described by two cutoff frequencies. Independently, finite-element simulation provides supporting evidence of the accuracy of the measurement, which is responsive to the nonuniform alkali distribution on the vapor cell wall. In combination with recent advances in the sensitivity of low-frequency electrometry, this may improve absolute, free-space measurements of weak, low-frequency electric fields that are of interest in basic and applied research.