Optical traps for dielectric particles have become an exceptional tool in testing optomechanics as well as fundamental physics. We report on our use of optical trapping in a program searching for non-Newtonian gravity, as well as recent tests that demonstrate control of the rotational degrees of freedom of optically trapped particles via electrostatic interactions with the dipole moments of test particles. The technique levitates individual micrometer-sized dielectric spheres and measures their three-dimensional position by optical heterodyne detection, making use of a single laser for both trapping and imaging. The two radial degrees of freedom are measured by interfering light transmitted through the microsphere with a reference wavefront, while the axial degree of freedom is measured from the phase of the light reflected from the surface of the microsphere. This method pairs the simplicity and accessibility of single-beam optical traps to a measurement of displacement that is intrinsically calibrated by the wavelength of the trapping light and has exceptional immunity to stray light. A theoretical shot noise limit of 1.3e-13 m/rt{Hz} for the radial degrees of freedom, and 3.0e-15 m/rt{Hz} for the axial degree of freedom can be obtained in the system described. The measured acceleration noise in the radial direction is 7.5e-5 (m/s^2)/rt{Hz}.

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Charles Blakemore, Alexander Rider, Akio Kawasaki, and Giorgio Gratta, "Single-beam dielectric-microsphere trapping with optical heterodyne detection: investigating torsional optomechanics and testing fundamental physics (Conference Presentation)," Proc. SPIE 10723, Optical Trapping and Optical Micromanipulation XV, 107230I (Presented at SPIE Nanoscience + Engineering: August 20, 2018; Published: 17 September 2018); https://doi.org/10.1117/12.2320749.5836016052001.