Cavity cooling of a levitated nanosphere by coherent scattering


In this work we demonstrate cavity cooling by coherent scattering, a new paradigm of cavity cooling for levitated optomechanics. Genuine three-dimensional cavity cooling of a levitated 140 nm silica nanosphere is provided by coherently scattered tweezer photons into an empty cavity, which in turn modifies the Stokes and Anti-Stokes scattering rates.

We initially show 2D cooling of the nanosphere motion along the tweezer and cavity axes for optimal scattering into the cavity. A rotation of the tweezer polarization (and thus the elliptical trap potential in the transverse plane of the optical tweezer) by 45 degrees provides a full 3D cavity cooling. Positioning of the nanosphere along the cavity standing wave with sub-wavelength precision allows for an optimized cooling of either the axial motion (at the cavity node, i.e. the intensity minimum of the cavity standing wave) or the motion along the tweezer axis (at the cavity antinode, i.e. the intensity maximum of the cavity standing wave). In addition, the axial motion is cooled even at the cavity antinode, which we explain by quadratic coupling.


The determined maximum optomechanical coupling rates gx/2π=60 kHz and gz/2π=120 kHz are significantly higher than the expected coupling rates in the dispersive regime for an equal intracavity photon number (approximately 10e6). Furthermore, laser phase noise is significantly suppressed for a nanosphere positioned at the cavity node due to the destructive interference of the scattered light at the drive laser frequency. We subsequently estimate that this cavity cooling method is not limited by phase noise heating. Although we cool to temperatures of about 1K in this work (limited by the moderate pressure of 10e-1 mbar in the vacuum chamber), we anticipate that operating the system in high vacuum will allow for ground state cooling of the axial motion.


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A tightly focused laser field traps a nanoparticle between two highly reflecting mirrors, i.e. an optical cavity. Preferential scattering along this optical resonator allows to induce cooling of the nanoparticle motion in all three directions. (Copyright/Imgae credits: Aspelmeyer group, University of Vienna")