Figure 1: Low-temperature optomechanics.

(a) Tilted-view scanning electron micrograph of the optomechanical torque sensor used here: a 10 μm diameter optical microdisk evanescently coupled to a torsional nanomechanical resonator by a vacuum gap of 60 nm. The torsional mode twists about the torsion axis, as indicated by the yellow arrow. Scale bar, 2 μm. A 1.1 μm diameter ‘landing pad’ on the torsion rod allows for deposition of secondary test samples. (b) Finite element model of the microdisk optical resonance, which couples dispersively to the mechanical resonator. Scale bar, 2 μm and the red dashed box shows the same area as in a. (c) The sample chip is clamped into a gold-plated copper mount, on a stack of low-temperature-compatible nanopositioners, with thermal braids linking it to the base-plate of the dilution refrigerator²⁵. A dimpled-tapered optical fibre is held on a positionable Invar fork above the sample stage and the chip-fibre system can be imaged by a low-temperature endoscope²⁵. The dimpled-tapered fibre is used to excite the microdisk’s optical mode and read out the optomechanical signal resulting from the Brownian motion of the mechanical resonances. (d) Optomechanically measured thermal noise voltage spectrum of the five lowest-order mechanical modes, at 4.2 K, with finite element simulations of the mode shapes colour coded by their total displacements. The second resonance, at 14.5 MHz, corresponds to the torsion mode with optimized signal to noise.