Spatiotemporal Superfocusing

Abstract

Superfocusing enables the confinement of electromagnetic waves within subwavelength‐scale structures, thereby breaking the diffraction limit. Structures with spatial singularities, such as metallic wedges, are crucial for achieving nanoscale focusing, leading to significant advancements in the sensing and imaging of subwavelength structures. In this study, the spatiotemporal analogue of the wedge structure, i.e. a dielectric medium sandwiched between two subluminal interfaces with distinct velocities, is exploited to focus propagating waves to deep subwavelength scales, thereby achieving the spatiotemporal superfocusing. In this configuration, an incident pulse undergoes cascaded compression due to the Doppler effects, accumulating to extreme focusing and enhancement as it approaches the spatiotemporal vertex. Remarkably, unlike the field localization in conventional superfocusing, the compressed light in spatiotemporal wedges experiences significant amplification and then couple to the far field in free space. The findings represent an indispensable paradigm for extreme concentration and amplification of propagating waves in space‐time dimensions.