Dissipative and dispersive optomechanical couplings are experimentally observed in a photonic crystal split-beam nanocavity optimized for detecting nanoscale sources of torque. Dissipative coupling of up to approximately $500$ MHz/nm and dispersive coupling of $2$ GHz/nm enable measurements of sub-pg torsional and cantilever-like mechanical resonances with a thermally-limited torque detection sensitivity of 1.2$\times 10^{ -20} \text{ N} \, \text{ m}/\sqrt{ \text{ Hz}}$ in ambient conditions and 1.3$\times 10^{ -21} \text{ N} \, \text{ m}/\sqrt{ \text{ Hz}}$ in low vacuum. Interference between optomechanical coupling mechanisms is observed to enhance detection sensitivity and generate a mechanical-mode-dependent optomechanical wavelength response.
Categories: cond-mat.mes-hall physics.optics quant-ph