Collective near-field coupling and nonlocal phenomena in infrared-phononic metasurfaces for nano-light canalization

Peining Li,Andrea Alu,Saül Vélez,Rainer Hillenbrand,Guangwei Hu,Mykhailo Tymchenko,Francisco Javier Alfaro-Mozaz,Cheng-Wei Qiu,Irene Dolado,Luis E Hueso,Fèlix Casanova,James H Edgar,Song Liu

doi:10.1038/s41467-020-17425-9

Abstract

Polaritons – coupled excitations of photons and dipolar matter excitations – can propagate along anisotropic metasurfaces with either hyperbolic or elliptical dispersion. At the transition from hyperbolic to elliptical dispersion (corresponding to a topological transition), various intriguing phenomena are found, such as an enhancement of the photonic density of states, polariton canalization and hyperlensing. Here, we investigate theoretically and experimentally the topological transition, the polaritonic coupling and the strong nonlocal response in a uniaxial infrared-phononic metasurface, a grating of hexagonal boron nitride (hBN) nanoribbons. By hyperspectral infrared nanoimaging, we observe a synthetic transverse optical phonon resonance (strong collective near-field coupling of the nanoribbons) in the middle of the hBN Reststrahlen band, yielding a topological transition from hyperbolic to elliptical dispersion. We further visualize and characterize the spatial evolution of a deeply subwavelength canalization mode near the transition frequency, which is a collimated polariton that is the basis for hyperlensing and diffraction-less propagation.

Highlights

Polaritons – coupled excitations of photons and dipolar matter excitations – can propagate along anisotropic metasurfaces with either hyperbolic or elliptical dispersion
We demonstrate that the strong collective nearfield coupling of subwavelength elements in an infrared-phononic metasurface yields a synthetic optical phonon resonance and subsequently a topological transition
We provide real-space images of deeply subwavelength canalization polaritons, which experimentally demonstrate that these modes are the consequence of the strong collective polaritonic near-field coupling of the nanoribbons and the associated strong nonlocal response of the metasurface

Summary

Introduction

Polaritons – coupled excitations of photons and dipolar matter excitations – can propagate along anisotropic metasurfaces with either hyperbolic or elliptical dispersion. On the other hand, when εeff,x and εeff,y have opposite signs, polaritons possess a so-called hyperbolic dispersion (k describes hyperbolic IFCs in k-space)[1,4,5,6,7,8,9,12,13], exhibiting increased polariton confinement and ray-like anisotropic propagation along the surface These two types of anisotropic metasurfaces can be applied, for example, to enhance optical birefringence[14,15], to control light polarization[16], for nanoscale directional polariton guiding[5,8,9,17,18], and for subwavelength-scale optical imaging[9,19]. A interesting regime arises when uniaxial metasurfaces exhibit a topological transition of the IFCs upon frequency variation[8,9,20], changing from hyperbolic to elliptical It offers unique opportunities in nanophotonics, for example, for enhancing the local photonic density of states[8,9,20] and for supporting deeply subwavelength canalization modes[17,21]. We provide real-space images of deeply subwavelength canalization polaritons, which experimentally demonstrate that these modes are the consequence of the strong collective polaritonic near-field coupling of the nanoribbons and the associated strong nonlocal response of the metasurface

Methods

Results

Conclusion