Planar nano-optics in anisotropic media: anomalous refraction and diffraction-free lensing of highly confined polaritons

TL;DR

This paper demonstrates anomalous refraction and diffraction-free lensing of highly confined polaritons in natural anisotropic media, revealing new ways to control nanoscale light propagation.

Contribution

It provides the first visualization and analysis of polariton refraction in strongly anisotropic media, including the development of an in-plane hyperlens for nanoscale imaging.

Findings

Refracted polaritons exhibit non-intuitive propagation directions.

Bending-free refraction observed in alpha-MoO3 nanoprisms.

First in-plane hyperlens achieves focus as small as lambda/6.

Abstract

As one of the most fundamental optical phenomena, refraction between isotropic media is characterized by light bending towards the normal to the boundary when passing from a low- to a high-refractive-index medium. However, in anisotropic media, refraction is a much more exotic phenomenon. The most general case of refraction between two anisotropic media remains unexplored, particularly in natural media and at the nanoscale. Here, we visualize and comprehensively study refraction of electromagnetic waves between two strongly anisotropic (hyperbolic) media, and, importantly, we do it with the use of polaritons confined to the nanoscale in a low-loss natural medium, alpha-MoO3. Our images show refraction of polaritons under the general case in which both the direction of propagation and the wavevector are not collinear. As they traverse planar nanoprisms tailored in alpha-MoO3, refracted polaritons exhibit non-intuitive directions of propagation, enabling us to unveil an exotic optical effect: bending-free refraction. Furthermore, we succeed in developing the first in-plane refractive hyperlens, which yields foci as small as lamdap/6, being lamdap the polariton wavelength (lamda0/50 with respect to the wavelength of light in free space). Our results set the grounds for planar nano-optics in strongly anisotropic media, with potential for unprecedented control of the flow of energy at the nanoscale.