TY - JOUR AB - 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 α-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 λp/4.5 = λ0/50 were achieved (λp is the polariton wavelength and λ0 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. AU - Martín-Sánchez, Javier AU - Duan, Jiahua AU - Taboada-Gutiérrez, Javier AU - Álvarez-Pérez, Gonzalo AU - Voronin, Kirill V. AU - Prieto Gonzalez, Ivan AU - Ma, Weiliang AU - Bao, Qiaoliang AU - Volkov, Valentyn S. AU - Hillenbrand, Rainer AU - Nikitin, Alexey Y. AU - Alonso-González, Pablo ID - 10177 IS - 41 JF - Science Advances TI - Focusing of in-plane hyperbolic polaritons in van der Waals crystals with tailored infrared nanoantennas VL - 7 ER -