We demonstrate an all-optically operated centimeter-milligram-scale torsion pendulum for atto-Newton (aN) level force detection, enabled by an ultrathin silica fiber and optical precooling in ultrahigh vacuum. Ten radiation pressure measurement experiments confirm the system’s excellent linearity and accuracy in responding to external forces. The modulated intensity of the radiation laser ranges from 87.2 to 3.16 nW, achieving a minimum external optical force amplitude of 13.3 aN. Notably, our system delivers a force sensitivity of $3.7\mathrm{fN}/\sqrt{\mathrm{Hz}}$ and a force resolution of 6.6 aN after 91.7 h of integration at 6 mHz. Compared with recent work by Sokhi et al. [Phys. Rev. Lett. 133, 083801 (2024)], this represents approximately one order of magnitude higher force sensitivity and a 60-fold enhancement in force resolution. It not only extends the limits of light field coupling and detectability in optomechanical systems to the sub-nW scale but also establishes a new benchmark for macroscale low-frequency torsion pendulums. Building on these superior metrics, our approach opens a new avenue in the fields of tabletop gravity measurements and new physics searches at the aN level.

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• Laboratory studies of gravity
• Optical forces
• Optomechanics
• Micromechanical & nanomechanical oscillators
• Optical microcavities
• Photons
• Bend & torsion tests
• Cooling & trapping