  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.
