  Using a boundary element method to calculate the electromagnetic fields and
the Maxwell stress tensor method to compute the electromagnetic forces, we
investigate electromagnetic wave induced forces acting on a pair of identical
metal plates that forms an electromagnetic resonance cavity. Different
frequency regimes are considered, from infrared frequencies with micron scale
structures down to the microwave regime which involves millimeter scale
structures. We found that at both length scales, the electromagnetic wave
induced forces can be significantly stronger than the usual photon pressure
exerted by a laser beam if the cavity is excited at resonance although the
mechanisms that underlie the strong force are different at different length
scales. In the infrared frequency regime, the strong force is induced by field
penetration into the metal, whereas in the microwave regime, the
electromagnetic force is induced by the leakage of electric field at the edges.
At both frequency scales, we compare the results we obtained for Au metal
plates with fictitious perfect electric conductor plates so as to understand
the effect of field penetration. We also showed that a transmission line model
can give simple expressions that can capture the essence of the physics. The
effect of surface corrugation and surface roughness is also investigated, and
we find that corrugation/roughness generally induces attraction between the
plates.
