Optomechanics of colloidal microparticles has wide applications in biological analysis and sensing. For colloidal microspheres or microdroplets, the optomechanical force can be magnified through the cavity enhancement effect. However, it is difficult to analyze the force since the colloidal microspheres are suspended in liquid, and addressing a movable microsphere at a specific position in three-dimensional space is also challenging. An on-chip integrated operating platform comprising waveguides and microelectromechanical systems is employed to study the cavity-enhanced optical gradient force on colloidal microspheres, owing to the ability to precisely control the distance between a suspended microsphere and a waveguide through dielectrophoretic force. We introduce two kinds of optomechanical coupling mechanisms at resonance, depending on the initial coupling gap without inclusion of the optical gradient force. One is self-adjusted coupling, where the coupling gap of a suspended microsphere continuously varies with the optical input power, and the other is bistable coupling, where the coupling gap hops from one state to the other as the input power exceeds over a threshold value, which is caused by the nature of nonlinear gap-dependent optical gradient force.
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