Analysis of short range forces in optomechanical devices with a nanogap

We employ the finite-difference time-domain method to calculate the dominant short range forces in optomechanical devices, Casimir and gradient optical forces. Numerical results are obtained for typical silicon optomechanical devices and are compared to metallic reference structures, taking into account geometric and frequency dispersion of silicon. Our results indicate that although a small gap is desirable for operating optomechanical devices, the Casimir force offsets the gradient force in strongly coupled optomechanical devices, which has to be taken into account in the design of optical force tunable devices.