Dissipative and dispersive optomechanics in a nanocavity torque sensor

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 cantileverlike mechanical resonances with a thermally limited torque detection sensitivity of 1.2 × 10^-20 Nm/√Hz in ambient conditions and 1.3 × 10^-21 Nm=/√Hz in low vacuum. Interference between optomechanical coupling mechanisms is observed to enhance detection sensitivity and generate a mechanical-mode-dependent optomechanical wavelength response.