Comment on "Electromagnetic force on structured metallic surfaces"

In [Phys. Rev. B 92, 115416 (2015)], Velzen and Webb proposed that by structuring a metal film suspended in free space with a periodic array of nanoscale slots, the steady-state optical pressure on the film resulting from an incident plane wave can be substantially enhanced compared to the standard radiation-pressure limit. This result appears to be in violation of the law of momentum conservation, given that the momentum of free-space electromagnetic plane waves is well established. We recalculated the optical fields and forces for the same configuration as Velzen and Webb and found that, although the calculated optical fields and force densities agree with the published result, the resulting total optical forces were not enhanced and, instead, matched the standard radiation-pressure relation. Our calculations imply that the diverging results arise due to different choices of the region used for integration of the force density.