  Hyperbolic phonon polaritons (HPhPs) sustained in van der Waals (vdW)
materials exhibit extraordinary capabilities of confining long-wave
electromagnetic fields to the deep subwavelength scale. In stark contrast to
the uniaxial vdW hyperbolic materials such as hexagonal boron nitride (h-BN),
the recently emerging biaxial hyperbolic materials such as {\alpha}-MoO3 and
{\alpha}-V2O5 further bring new degree of freedoms in controlling light at the
flatland, due to their distinctive in-plane hyperbolic dispersion. However, the
controlling and focusing of such in-plane HPhPs are to date remain elusive.
Here, we propose a versatile technique for launching, controlling and focusing
of in-plane HPhPs in {\alpha}-MoO3 with geometrically designed plasmonic
antennas. By utilizing high resolution near-field optical imaging technique, we
directly excited and mapped the HPhPs wavefronts in real space. We find that
subwavelength manipulating and focusing behavior are strongly dependent on the
curvature of antenna extremity. This strategy operates effectively in a
broadband spectral region. These findings can not only provide fundamental
insights into manipulation of light by biaxial hyperbolic crystals at
nanoscale, but also open up new opportunities for planar nanophotonic
applications.
