Anomalous Refraction and Lensing of Nanoscale-Confined Hyperbolic Polaritons

Refraction between isotropic media is a well understood phenomenon, described by the simplest form of Snell’s law and characterized by light bending towards the normal to the boundary when passing from a low- to a high-refractive-index medium. However, refraction between anisotropic media is a more exotic phenomenon which had remained barely investigated before the research presented in this chapter was carried out, particularly at the nanoscale. In this chapter, we visualize and comprehensively study the general case of refraction of electromagnetic waves between two strongly anisotropic (hyperbolic) media, and we do it by imaging nanoscale-confined polaritons in \(\alpha \text{-MoO}_{3}\) . Upon refraction, these polaritons exhibit non-intuitive directions of propagation as they traverse planar nanoprisms, enabling to unveil an exotic optical effect: bending-free refraction. Furthermore, we develop an in-plane refractive hyperlens, yielding foci as small as \(\lambda _p/6\) , being \(\lambda _p\) the polariton wavelength ( \(\lambda _0/50\) compared to the wavelength of free-space light). These results set the grounds for planar nano-optics in strongly anisotropic media, with potential for effective control of the flow of energy at the nanoscale. The near-field experiments reported in this chapter were carried out in collaboration with Dr. Jiahua Duan (University of Oviedo, Spain). Sample fabrication was performed mainly by Ana Isabel Fernández-Tresguerres Mata (University of Oviedo, Spain), Bingdong Chang and Prof. Sanshui Xiao (Technical University of Denmark, Denmark). The results were published in “Planar refraction and lensing of highly confined polaritons in anisotropic media” by Jiahua Duan, Gonzalo Álvarez-Pérez et al. in Nature Communications 12, 4325 (2021).