  Using light to measure an object's motion is central to operating mechanical
sensors that probe forces and fields. Cavity optomechanical systems embed
mechanical resonators inside optical resonators. This enhances the sensitivity
of optomechanical measurements, but only if the mechanical resonator does not
spoil the properties of the optical cavity. For example, cavity optomechanical
detection of resonators made from optically absorbing materials, or whose
geometry does not possess suitable spatial symmetry, is challenging. Here we
demonstrate a system that overcomes challenges in measuring high-frequency
twisting motion of a nanodisk by converting them to vibrations of a photonic
crystal cavity. Optomechanical readout of the cavity then enables measurement
of the nanodisk's torsional resonances with sensitivity $5.1\times
10^{-21}-1.2\times 10^{-19}\,\text{Nm}/\sqrt{\text{Hz}}$ for a mechanical
frequency range of 5--800 MHz. The nanodisk can be outfitted with magnetic
nanostructures or metasurfaces without affecting the optical properties of the
cavity, making the system suitable for torque magnetometry and structured light
sensing.
