Nonperturbative approach to Casimir interactions in periodic geometries

Due to their collective nature Casimir forces can strongly depend on the geometrical shape of the interacting objects. We study the effect of strong periodic shape deformations of two ideal metal plates on their quantum interaction. A nonperturbative approach which is based on a path-integral quantization of the electromagnetic field is presented in detail. Using this approach, we compute the force for the specific case of a flat plate and a plate with a rectangular corrugation. We obtain complementary analytical and numerical results which allow us to identify two different scaling regimes for the force as a function of the mean plate distance, corrugation amplitude, and wavelength. Qualitative distinctions between transversal electric and magnetic modes are revealed. Our results demonstrate the importance of a careful consideration of the nonadditivity of Casimir forces, especially in strongly nonplanar geometries. Nonperturbative effects due to surface edges are found. Strong deviations from the commonly used proximity force approximation emerge over a wide range of corrugation wavelengths, even though the surface is composed only of flat segments. We compare our results to that of a perturbative approach and a classical optics approximation.