Kavli Affiliate: Giorgio Gratta
| First 5 Authors: Akio Kawasaki, Alexander Fieguth, Nadav Priel, Charles P. Blakemore, Denzal Martin
| Summary:
A high sensitivity force sensor based on dielectric microspheres in vacuum,
optically trapped by a single, upward-propagating laser beam, is described.
Off-axis parabolic mirrors are used both to focus the 1064~nm trapping beam and
to recollimate it to provide information on the horizontal position of the
microsphere. The vertical degree of freedom is readout by forming an
interferometer between the light retroreflected by the microsphere and a
reference beam, hence eliminating the need for auxiliary beams. The focus of
the trapping beam has a 1/e$^2$ radius of 3.2~$mu$m and small non-Gaussian
tails, suitable for bringing devices close to the trapped microsphere without
disturbing the optical field. Electrodes surrounding the trapping region
provide excellent control of the electric field, which can be used to drive the
translational degrees of freedom of a charged microsphere and the rotational
degrees of freedom of a neutral microsphere, coupling to its electric dipole
moment. With this control, the charge state can be determined with single
electron precision, the mass of individual microspheres can be measured, and
empirical calibrations of the force sensitivity can be made for each
microsphere. A force noise of $<1times10^{-17}$~N/$sqrt{rm Hz}$, which is
comparable to previous reports, is measured on all three degrees of freedom for
4.7~$mu$m diameter, 84~pg silica microspheres. Various devices have been
brought within $1.6~mu$m of the surface of a trapped microsphere. Metrology in
the trapping region is provided by two custom-designed microscopes providing
views in the horizontal and one of the vertical planes. The apparatus opens the
way to performing high sensitivity three-dimensional force measurements at
short distance.
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