The PINQUED project aims to develop an all-optical diagnostic technique for measuring electric fields in plasmas. This detection scheme uses the effect of Rydberg electromagnetically induced transparency (Ry-EIT) in Rb atomic vapor, which is added in trace amounts to Ar plasma.

Electric fields are the driving force behind plasma dynamics. In plasmas, they determine the strength and nature of plasma-surface interactions within the plasma sheaths, layers of plasma depleted of electrons, where electric fields dictate the motion of gas ions. Additionally, they play a key role in self-organizing dusty plasmas. Therefore, understanding the distribution of electric fields within plasmas is crucial for both fusion and material processing applications.

In our experiment, Rb atoms are excited to high-lying states known as Rydberg states. Rydberg atoms are highly polarizable in external electric fields because their highly excited valence electrons are relatively weakly bound, allowing the external electric field to induce a significantly larger atomic dipole moment. To excite the atoms to Rydberg states, we used a bichromatic laser field that effectively transformed the atoms into a three-level system.

An infrared laser field (780 nm) excited ⁸⁵Rb atoms between the ground 5S_1/2 and intermediate 5P_3/2 states, while a blue laser (480 nm) induced quantum transitions between the intermediate and Rydberg nD or nS levels, where the principal quantum number n>>1. When the infrared and blue lasers are at resonance with their respective atomic transitions, quantum interference induces a dark state in the atoms, preventing absorption and creating a narrow transmission window in the infrared laser absorption profile and reducing resonance fluorescence. In the presence of an external electric field, these transmission peaks or fluorescence dips are shifted and split owing to the Stark effect. By spatially tracking the positions of these peaks, we can reconstruct the distribution of the electric-field magnitude.

We constructed a setup for generating Ar plasma using a heated cathode. A diffuse electron beam, thermally emitted from the cathode, is accelerated through ultrapure Ar gas at pressures below 10 mTorr to energies that exceed the Ar ionization potential. These accelerated electrons induce Ar ionization, leading to electrical breakdown and the establishment of a glow discharge between the electrodes. At ionization ratios typical for low-temperature plasmas, approximately 10_-6 to 10_-3, a sufficient number of Rb atoms in Rydberg states remain to produce a detectable Ry-EIT signal. The image below compares the Ry-EIT Stark maps for two scenarios: when the electron beam energies are below and above the Ar ionization potential. As observed, below the ionization threshold, the Ry-EIT provides a clear depiction of the Stark effect in the 44D Rydberg levels of ⁸⁵Rb. Above the Ar ionization threshold, the plasma neutralizes the electric field within its bulk through screening, allowing electric fields only at the plasma edges and plasma-surface interfaces, known as sheaths.

While we currently use rubidium to obtain information about the plasma electric field, it is typically not a naturally occurring species in plasmas. To minimize our detector presence in the plasma, we also investigate the possibility of using metastable helium as a species for electromagnetic field sensing in plasmas. Helium contains a metastable ground state (naturally populated in plasmas) that has similar energy level structure as Rb, and has been previously used to demonstrate coherent resonances. A similar process can be used with argon, another common species within plasmas.

So far, we have been able to resolve linear and nonlinear magneto-optical rotation within a helium cell to detect magnetic fields, and are working to characterize atomic properties in metastable helium using resulting experimental spectra to measure electromagnetic fields within plasmas.