Focusing Of In-Plane Hyperbolic Polaritons In Van Der Waals Crystals With Tailored Infrared Nanoantennas

Phonon polaritons (PhPs)-light coupled to lattice vibrations-with in-plane hyperbolic dispersion exhibit ray-like propagation with large wave vectors and enhanced density of optical states along certain directions on a surface. As such, they have raised a surge of interest, promising unprecedented manipulation of infrared light at the nanoscale in a planar circuitry. Here, we demonstrate focusing of in-plane hyperbolic PhPs propagating along thin slabs of alpha-MoO3. To that end, we developed metallic nanoantennas of convex geometries for both efficient launching and focusing of the polaritons. The foci obtained exhibit enhanced near-field confinement and absorption compared to foci produced by in-plane isotropic PhPs. Foci sizes as small as lambda(p)/4.5 = lambda(0)/50 were achieved (lambda(p) is the polariton wavelength and lambda(0) is the photon wavelength). Focusing of in-plane hyperbolic polaritons introduces a first and most basic building block developing planar polariton optics using in-plane anisotropic van der Waals materials.