High-Quality Optical Hotspots with Topology-Protected Robustness

Changxu Liu,Stefan A Maier

doi:10.1021/acsphotonics.1c01445

Abstract

Optical hotspots underpin a wide variety of photonic devices ranging from sensing, nonlinear generation to photocatalysis, taking advantage of the strong light–matter interaction at the vicinity of photonic nanostructures. While plasmonic nanostructures have been widely used for strongly localized electromagnetic energy on surfaces, they suffer from high loss and consequently a low quality factor. Resonance-based dielectric structures provide an alternative solution with a larger quality factor, but there is a mismatch between the maximum values of the light confinement (quality factor) and the leakage (intensity in the near-field). Here, we propose to apply the concept of topological photonics to the formation of hotspots, producing them in both topological edge states and topological corner states. The topology secures strong light localization at the surface of the nanostructures where the underlying topological invariant shows a jump, generating a field hotspot with simultaneous increment of quality factor and light intensity. Meanwhile, it leverages a good robustness to fabrication imperfection including fluctuation in shape and misalignment. After a systematic investigation and comparison of the robustness between 1D and 2D topological structures, we conclude that the hotspots from 1D topological edge states promise a fertile playground for emerging applications that require both enhanced light intensity and high spectral resolution.

Highlights

Optical hotspots with tightly focused electromagnetic fields are underpinning a repertoire of devices requiring a strong interaction between materials and photons
Despite the intense near field enhancement and matured fabrication technologies, the mainstream plasmonic structures composed of metals such as Au and Ag suffer from excessive losses at optical frequencies.[28]
Considering the feasibility of fabrication, we focus on two types of topology-induced hotspots, edge states from 1D structures and corner states from 2D structures, instead of impractical corner states from third-order 3D topological phases

Summary

■ INTRODUCTION

Optical hotspots with tightly focused electromagnetic fields are underpinning a repertoire of devices requiring a strong interaction between materials and photons. Plasmonic nanostructures[1,3,4,7,9,16−18,21,26] have been one of the most successfully used solutions for hotspot generation, through the excitation of surface plasmon resonances, collective and coherent oscillations of free electrons.[27] Despite the intense near field enhancement and matured fabrication technologies, the mainstream plasmonic structures composed of metals such as Au and Ag suffer from excessive losses at optical frequencies.[28] plasmonic structures exhibit local heating and a low quality factor, limiting the performance in many applications. While 1D edge states[59−62] and 2D corner states[63−66] have remained two isolated topics, we provide the first comparison of the two, to the best of our knowledge This comprehensive robustness analysis will be a guide for experimental realization of topological states in optical regime where the fabrication imperfections are inevitable and provide a novel platform for various hotspot-based applications mentioned earlier

■ RESULTS

■ DISCUSSION

■ REFERENCES

■ METHODS