Focusing of in-plane hyperbolic polaritons in van der Waals crystals with tailored infrared nanoantennas

TL;DR

This paper demonstrates the first focusing of in-plane hyperbolic phonon polaritons in van der Waals crystals using tailored nanoantennas, achieving highly confined foci and advancing planar polariton optics.

Contribution

It introduces a novel method for focusing in-plane hyperbolic phonon polaritons with metallic nanoantennas in van der Waals materials, enabling nanoscale infrared light manipulation.

Findings

Achieved sub-wavelength focusing of hyperbolic polaritons

Enhanced near-field confinement and absorption at the focus

Foci sizes as small as lambda/50

Abstract

Phonon polaritons (PhPs),light coupled to lattice vibrations,with in-plane hyperbolic dispersion exhibit ray-like propagation with large wavevectors and enhanced density of optical states along certain directions on a surface. As such, they have raised a surge of interest as they promise unprecedented possibilities for the manipulation of infrared light with planar circuitry and at the nanoscale. Here, we demonstrate, for the first time, the focusing of in-plane hyperbolic PhPs propagating along thin slabs of MoO3. To that end, we developed metallic nanoantennas of convex geometries for both the efficient launching and focusing of the polaritons. Remarkably, the foci obtained exhibit enhanced near-field confinement and absorption compared to foci produced by in-plane isotropic PhPs. More intriguingly, foci sizes as small as lamdap/5 =lamda0/50 were achieved (lamdap is the polariton wavelength and lamda0 the photon wavelength). Focusing of in-plane hyperbolic polaritons introduces a first and most basic building block developing planar polariton optics utilizing in-plane anisotropic van der Waals materials and metasurfaces.