The interaction of light with mechanical vibrations is at the heart of many fundamental experiments, including ground-state cooling, mechanical squeezing of light and optomechanically induced transparency. Marcelo Wu and co-workers from Canada have now harnessed the interactions necessary to create an optomechanical torque detector with a sensitivity of 1.3 × 10−21 N m Hz−1/2 operating in a vacuum pressure of 2 Torr. The torque detector — composed of a Si-based photonic crystal split-beam nanocavity operating at a wavelength of 1530 nm — was created by using a pair of suspended nanobeam waveguides that were separated by two small (60 nm) air gaps that allowed mechanical movement. The nanobeams were patterned with periodic holes serving as mirrors. When the nanobeams deflect, the cavity's optical resonance and transmission changes and this can be used to determine the magnitude of torque. The optomechanical coupling between the optical mode and the nanocavity mechanical resonances was studied by measuring the optical power transmitted through the split-beam nanocavities. The researchers observed interference between dissipative and dispersive coupling mechanisms and claim that this can be used to further enhance detection sensitivity.
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Horiuchi, N. Nanocavity torque sensor. Nature Photon 8, 675 (2014). https://doi.org/10.1038/nphoton.2014.201