Maximal free-space concentration of electromagnetic waves

Abstract

We derive upper bounds to free-space concentration of electromagnetic waves, mapping out the limits to the maximal intensity for any spot size and optical-beam-shaping device. For sub-diffraction-limitedoptical beams, our bounds suggest the possibility for orders-of-magnitude intensity enhancements compared with existing demonstrations, and we use inverse design to discover metasurfaces operating near these new limits. We also demonstrate that our bounds may surprisingly describe maximal concentration defined by a wide variety of metrics. Our bounds require no assumptions about symmetry, scalar waves, or weak scattering, instead relying primarily on the transformation of a quadratic program via orthogonal-projection methods. The bounds and inverse-designed structures presented here can be useful for applications from imaging to three-dimensional printing.