Search for non-Newtonian gravity with optically-levitated microspheres*

The universal law of gravity has undergone stringent tests for a long time over a significant
range of length scale, from an atomic scale to a planetary scale. Of particular interest
is the short distance regime, where modifications to Newtonian gravity may arise from
axion-like particles and extra dimensions. We have constructed an ultra-sensitive force sensor
based on optically-levitated microspheres with a force sensitivity of ∼ 10−17 N/√Hz for the
purposed of investigating non-Newtonian forces in the 1-100 μm range. Microspheres interact
with a variable-density attractor mass made by alternating silicon and gold segments with
periodicity of ∼ 50 μm. The attractor can be located as close as a few μm to a microsphere.
We report on the characterization of this system, its sensitivity, and preliminary
results. Further technological developments to reduce background are investigated to provide
orders of magnitude improvement in the sensitivity, going beyond current constraints on
non-Newtonian interactions.