Corrected Proximity-Force Approximation for Lateral Casimir Forces

The widely-adopted proximity-force approximation (PFA) to estimate normal Casimir forces is known to be asymptotically exact at vanishing separations. In this letter, we propose a correction to the PFA, which is sufficiently accurate in predicting displacement-induced lateral Casimir forces between a sphere and a grating, for separation-to-radius ratio up to 0.5, far beyond the limit within which the application of PFA is previously restricted. Our result allows convenient estimation of Casimir interactions and thus shall be useful in relevant experimental and engineering Casimir applications. We also study the PFA for gradient gratings, and we find that the inhomogeneity-induced lateral Casimir force is beyond the corrected PFA.