Spatiotemporal Superfocusing

TL;DR

This paper introduces a novel spatiotemporal superfocusing method using a dielectric wedge structure, enabling light to be focused beyond the diffraction limit with amplification and far-field coupling.

Contribution

It presents the first demonstration of spatiotemporal superfocusing via a dielectric wedge, combining Doppler effects with wave compression for extreme wave concentration.

Findings

Achieved subwavelength focusing beyond diffraction limit.

Demonstrated wave amplification and far-field coupling.

Established a new paradigm for space-time wave concentration.

Abstract

Superfocusing confines light within subwavelength structures, breaking the diffraction limit. Structures with spatial singularities, such as metallic cones, are crucial to enable nanoscale focusing, leading to significant advancements in nanophotonics, sensing, and imaging. Here, we exploit the spatiotemporal analogue of the wedge structure, i.e. a dielectric medium sandwiched between two subluminal interfaces with distinct velocities, to focus propagating waves beyond the diffraction limit, achieving spatiotemporal superfocusing. Within this structure, an incident pulse undergoes continuous spatial and temporal compression due to Doppler effects, which accumulates and results in an extreme focusing 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. Our findings represent an indispensable paradigm for extreme concentration and amplification of propagating waves in space-time dimensions.