Feedback Cooling of the Torsional Mode of a Nanofiber

Optical nanofibers (ONFs) are conventional optical fibers that are tapered to be hundreds of nanometers in diameter. They have served as platforms for strong interactions between atoms and light. ONFs also exhibit torsional motion with a Q factor of more than 50,000, which we measure here. When used to trap atoms for strong interactions or metrological tasks, the torsional motion of the ONF may limit experimental outcomes. One may counter this problem by reducing the ONF's motion by cooling the torsional mechanical modes. Here, we follow Su et. al. (Photon. Res. 11, 2179-2184, 2023) to cool the nanofiber's torsional modes with direct feedback. We investigate optical cross-modulation by modulating the amplitude of the coupled light at one wavelength and measuring modulated polarization of light at a different wavelength. Additionally, we investigate mode coupling between distinct torsional modes in the ONF and the possibility of simultaneously cooling multiple modes.