Approximate and explicit expression of optical forces and pull-in instability of a silicon nano-optomechanical device

Nano-optomechanical systems actuated by optical forces enable many interesting scientific and technological applications. They are vulnerable to the effects of surface stress and Casimir forces. Therefore, calculation of optical forces is essential for the reliability applications of these advanced devices. In this paper, an approximate and explicit expression is developed for the evaluation of the optical force existing between a waveguide and a substrate through the effective refractive index. The influences of surface stress and Casimir forces on the pull-in instability of a silicon nano-optomechanical device actuated by optical forces are investigated. It is found that if neglecting the effect of surface stress, the maximum size, which indicates the device can be safely fabricated, will be over-predicted. The surface stress reduces the critical optical power and its effect is more significant for a slender waveguide.