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The universal law of gravitation has undergone stringent tests for many decades over a significant range of length scales, from atomic to planetary. Of particular interest is the short distance regime, where modifications to Newtonian gravity may arise from axion-like particles or extra dimensions. We have constructed an ultra-sensitive force sensor based on optically-levitated microspheres with a force sensitivity of $10^{-17}$ N/$sqrt{rm Hz}$ for the purpose of investigating non-Newtonian forces in the 1-100 $mu$m range. Microspheres interact with a variable-density attractor mass made by alternating silicon and gold segments with periodicity of 50 $mu$m. The attractor can be located as close as 10 $mu$m from a microsphere. I describe the characterization of this system, its sensitivity, and some preliminary results. Further technological developments to reduce background are expected to provide orders of magnitude improvement in the sensitivity, probing beyond current constraints on non-Newtonian interactions.
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