Torsional optomechanics of a levitated nonspherical nanoparticle

TL;DR

This paper reports the first experimental observation of torsional vibrations in a levitated nonspherical nanoparticle, revealing higher frequency torsional modes and proposing a scheme for ground state cooling, advancing nanoscale sensing and quantum studies.

Contribution

It presents the first experimental detection of torsional vibrations in a levitated nonspherical nanoparticle and introduces a method for ground state cooling of these vibrations.

Findings

Torsional vibration frequency exceeds center-of-mass frequency by an order of magnitude.

Coupling between photon spin angular momentum and nanoparticle torsion was demonstrated.

Potential for ultrasensitive torque detection with 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.