  In this article, we rigorously analyze the effects of the dispersion forces
(Casimir and van der Waals forces) on a nano-optomechanical device based on a
silicon waveguide and a silicon dioxide substrate, surrounded by air and driven
by optical forces. The dispersion forces are calculated using a modified
Lifshitz theory, in order to take into account the device thickness and
material s dielectric permittivities, which are obtained from experimental
optical data and validated by means of a rigorous 3D FDTD simulation. We also
take into account the mechanical nonlinearity of the waveguide, which is caused
by its large deflection relative to its thickness, due to the nanoscale device
dimensions. The nonlinear mechanical analytical model is also validated using a
3D FEM simulation. Our results show that, under appropriate design conditions,
it is possible to attain a no pull in critical point due only to the optical
force; therefore, in principle, it would be possible to control the device
total deflection just by controlling the optical power. However, the dispersion
forces usually impose a pull in critical point to the device and establish a
minimal initial gap between the waveguide and the substrate. Furthermore, we
show that the geometric nonlinearity effect may be exploited in order to avoid
or minimize the pull in and, therefore, the device collapse.
