Hyperfine structure and collisions in three-photon Rydberg electromagnetically induced transparency

Multi-photon electromagnetically-induced transparency (EIT) of atomic vapors involves several intermediate atomic levels. The sub-structure of these levels and their collisional interactions can drastically alter experimental EIT signals. Here, we report on hyperfine structure and collision effects in three-photon Rydberg EIT on the cascade $5S_{1/2} \rightarrow$ $5P_{1/2} \rightarrow 5D_{3/2}$ $\rightarrow 25F_{5/2}$ in a room temperature $^{85}$Rb vapor cell. In our measurements of EIT spectra, we identify two types of EIT signatures that correspond with distinct excitation pathways and atomic velocity classes in the atomic vapor. The $5D_{3/2}$ hyperfine structure and Autler-Townes splittings lead to complex patterns in the EIT spectra, which we analyze with the aid of 10-level EIT simulations. Adding 50~mTorr of Ar gas alters the EIT spectra and induces an additional, third EIT mode. Based on our simulation results, we attribute these changes to hyperfine collisions in the Rb $5D_{3/2}$ level. Our study may become useful in quantum technologies involving Rydberg EIT and hyperfine collisions in vapor cells, including non-invasive spatio-temporally resolved electric-field sensing of electric fields in low-pressure plasmas.