Spectroscopy of rubidium Rydberg states in inert buffer gases

Rydberg atoms are well-suited for electric field sensing because of their large electric polarizabilities. Electromagnetically induced transparency (EIT), a coherent quantum optical phenomenon, allows all-optical spectroscopic measurement of Rydberg-atom Stark shifts in plasmas, thereby revealing the plasma electric field in a non-invasive manner. Since there is no need to insert metallic probes into the plasma, the spectroscopic approach complements Langmuir and other plasma probes. Here, we aim to utilize Rydberg-EIT for electric-field diagnostics in low temperature RF plasmas. To this end, we have first studied how common buffer gases used for plasma generation affect Rydberg-EIT in the absence of a plasma. We have measured frequency shifts and line broadening of the EIT spectra in Rb vapor cells with argon, neon or nitrogen buffer gases at pressures ranging from a few mTorr to 5 Torr. The results qualitatively agree with a model that accounts for scattering between the Rydberg electron and buffer-gas atoms as well as for polarization of the buffer-gas atoms within the Rydberg-atom volume. The results indicate the viability of Rydberg-EIT as an electric field probe in low-pressure plasma. We report our progress towards probing electric fields in inductively coupled RF discharge plasma generated in a sub-100 mTorr Argon gas.