Torsional Optomechanics of a Levitated Nonspherical Nanoparticle

TLDR

An optically levitated nanoparticle in vacuum is a paradigm optomechanical system for sensing and studying macroscopic quantum mechanics, yet its torsional vibration has only been theoretically explored in limited cases. The authors propose a simple yet novel scheme to achieve ground‑state cooling of the torsional vibration using a linearly‑polarized Gaussian cavity mode. They exploit the coupling between photon spin angular momentum and the tensor polarizability of a nonspherical nanoparticle, and employ a linearly‑polarized Gaussian cavity mode to realize ground‑state cooling. The study reports the first experimental observation of torsional vibration in a levitated nonspherical nanoparticle, finds that its frequency can be an order of magnitude higher than center‑of‑mass motion—promising for ground‑state cooling—and demonstrates that it can serve as an ultrasensitive nanoscale torsion balance with torque sensitivity around 10⁻²⁹ N·m/√Hz.

Abstract

An optically levitated nanoparticle in vacuum is a paradigm optomechanical system for sensing and studying macroscopic quantum mechanics. While its center-of-mass motion has been investigated intensively, its torsional vibration has only been studied theoretically in limited cases. Here we report the first experimental observation of the torsional vibration of an optically levitated nonspherical nanoparticle in vacuum. We achieve this by utilizing the coupling between the spin angular momentum of photons and the torsional vibration of a nonspherical nanoparticle whose polarizability is a tensor. The torsional vibration frequency can be one order of magnitude higher than its center-of-mass motion frequency, which is promising for ground state cooling. We propose a simple yet novel scheme to achieve ground state cooling of its torsional vibration with a linearly-polarized Gaussian cavity mode. A levitated nonspherical nanoparticle in vacuum will also be an ultrasensitive nanoscale torsion balance with a torque detection sensitivity on the order of $10^{-29} ~\mathrm{N}\cdot \mathrm{m}/\sqrt{\mathrm{ Hz}}$ under realistic conditions.