Subdiffractional focusing and guiding of polaritonic rays in a natural hyperbolic material

TL;DR

This paper demonstrates that natural hyperbolic material hexagonal boron nitride can be used for subdiffractional focusing and guiding of phonon-polaritons, enabling advanced infrared nanophotonics applications.

Contribution

It provides experimental evidence of subdiffractional focusing and waveguiding in a natural hyperbolic material, highlighting its potential for infrared nanophotonics.

Findings

Hexagonal boron nitride acts as a hyper-focusing lens for phonon-polaritons.

hBN functions as a multi-mode waveguide in the infrared.

Natural hyperbolic materials can outperform artificial counterparts in certain applications.

Abstract

Uniaxial materials whose axial and tangential permittivities have opposite signs are referred to as indefinite or hyperbolic media. In such materials light propagation is unusual, leading to novel and often non-intuitive optical phenomena. Here we report infrared nano-imaging experiments demonstrating that crystals of hexagonal boron nitride (hBN), a natural mid-infrared hyperbolic material, can act as a "hyper-focusing lens" and as a multi-mode waveguide. The lensing is manifested by subdiffractional focusing of phonon-polaritons launched by metallic disks underneath the hBN crystal. The waveguiding is revealed through the modal analysis of the periodic patterns observed around such launchers and near the sample edges. Our work opens new opportunities for anisotropic layered insulators in infrared nanophotonics complementing and potentially surpassing concurrent artificial hyperbolic materials with lower losses and higher optical localization.