Reactive cavity optical force on microdisk-coupled nanomechanical beam waveguides.

We demonstrate spectrally tuned dispersive and reactive optical force in a cavity optomechanics system that comprises a microdisk and a vibrating nanomechanical beam waveguide. The waveguide coupled to the microdisk acts as a bosonic dissipation channel and its motion modulates the cavity's damping rate. As a result a reactive optical force arises in addition to the normal dispersive force. The cavity enhanced force is not observed at zero detuning but shows asymmetric behavior with a maximum at red-detuned offset. Such reactive cavity backaction force points to new avenues in cavity optomechanics.