Non-Hermitian metasurface with non-trivial topology

Frank Yang,Gururaj V Naik,Ciril S Prasad,Rosemary Lach,Henry O Everitt,Weijian Li

doi:10.1515/nanoph-2021-0731

Abstract

Abstract The synergy between topology and non-Hermiticity in photonics holds immense potential for next-generation optical devices that are robust against defects. However, most demonstrations of non-Hermitian and topological photonics have been limited to super-wavelength scales due to increased radiative losses at the deep-subwavelength scale. By carefully designing radiative losses at the nanoscale, we demonstrate a non-Hermitian plasmonic–dielectric metasurface in the visible with non-trivial topology. The metasurface is based on a fourth order passive parity-time symmetric system. The designed device exhibits an exceptional concentric ring in its momentum space and is described by a Hamiltonian with a non-Hermitian Z 3 ${\mathbb{Z}}_{3}$ topological invariant of V = −1. Fabricated devices are characterized using Fourier-space imaging for single-shot k-space measurements. Our results demonstrate a way to combine topology and non-Hermitian nanophotonics for designing robust devices with novel functionalities.

Highlights

In recent years, intense research efforts have been undertaken to understand the roles of symmetry and topology in non-Hermitian physics, with photonics acting as a convenient testbed [1, 2]
By carefully designing radiative losses at the nanoscale, we demonstrate a non-Hermitian plasmonic–dielectric metasurface in the visible with nontrivial topology
Our results demonstrate a way to combine topology and non-Hermitian nanophotonics for designing robust devices with novel functionalities

Summary

Introduction

Intense research efforts have been undertaken to understand the roles of symmetry and topology. Non-Hermitian Hamiltonians are not guaranteed to have real eigenvalues, orthogonal eigenmodes, or physically meaningful observables – unlocking a host of physical effects without Hermitian counterparts One such unique feature is the exceptional point. Passive PT-symmetric systems are a subset of non-Hermitian Hamiltonians that can exhibit fully real eigenvalues [45], exceptional points, and phase transitions without the introduction of gain [16]. The bandgap restricts radiative losses, resulting in highly localized modes in the sub-wavelength resonators This photonic mode couples to its image charges in the aluminum ground plane under normally incident input excitation (Figure 1(b), left). Horizontal and vertical photonic modes are spatially and spectrally overlapped in single silicon resonators (coupled by κθ) by design. Coupling between horizontal and vertical modes (κθ) can be non-zero and is controlled by the out-of-plane angle of excitation. Exceptional lines hint at the possibility of non-trivial topology in the metasurface

Results and discussion

Conclusions

Simulations

Device fabrication

Optical characterization