Single Atom Optical Polarization Sensing for a high-NA system

The creation of sub-micron optical features using high-NA optical systems is readily used in AMO experiments, optical lithography etc. However, the optical fields produced in such systems are known to have a spatially varying polarization at the length scale of the optical wavelength. Here, we demonstrate a polarization sensor with sub-wavelength resolution based on a single trapped ¹⁷¹Yb⁺ ion. The sensing is based on the polarization-dependence of optical pumping in the ion. We model the interaction of the ion with light at 369 nm by including all 8 hyperfine energy levels in S_1/2 and P_1/2 manifolds, enabling efficient numerical simulation of the system. Using this technique, we extract the polarization of a 1.5-micron beam shining on a single ion. We verify these results by comparing the measured and predicted Ramsey T2 times for the ion in the presence of the light. We extend our study numerically to successfully reconstruct the optical field at the focus of a 0.8 NA objective lens and find that the spatial resolution of the polarization measurement is limited by the ion wavepacket spread.