Polarization Effects in STIRAP Between Triplet He States

We studied the effect of laser polarization on the excitation of helium atoms from the metastable 2$^3$S state to Rydberg $n$S states via the intermediate 3$^3$P states. For STIRAP, an infrared laser connects the $3^3$P$_{2,1,0}$ and $n$S levels, and then $389$ nm uv light drives the $2^3$S$_1$ to $3^3$P$_{2,1,0}$ transition. The light polarizations determine the nature of the couplings between the different Zeeman sublevels, and interference may arise in those different excitation paths. Rydberg atoms are suitable for this experiment because of their relatively long life times so the $n$S populations are readily measured. The correspondence between polarizations and $n$S level populations have been studied both theoretically and experimentally, and the interference patterns are revealed.