  Negative reflection occurs when light is reflected towards the same side of
the normal to the boundary from which it is incident. This exotic optical
phenomenon, which provides a new avenue towards light manipulation, is not only
yet to be visualized in real space but remains largely unexplored both at the
nanoscale and in natural media. Here, we directly visualize nanoscale-confined
polaritons negatively reflecting on subwavelength mirrors fabricated in a
low-loss van der Waals crystal. Our near-field nanoimaging results unveil an
unconventional and broad tunability of both the polaritonic wavelength and
direction of propagation upon negative reflection. Based on these findings, we
introduce a novel device in nano-optics: a hyperbolic nanoresonator, in which
hyperbolic polaritons with different momenta reflect back to a common point
source, enhancing its intensity. These results pave the way to realize
nanophotonics in low-loss natural media, providing a novel and efficient route
to confine and control the flow of light at the nanoscale, key for future
optical on-chip nanotechnologies.

Optics