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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