  Negative refraction provides an attractive platform to manipulate
mid-infrared and terahertz radiation for molecular sensing and thermal
radiation applications. However, its implementation based on available
metamaterials and plasmonic media presents challenges associated with optical
losses, limited spatial confinement, and lack of active tunability in this
spectral range. Here, we demonstrate gate-tunable negative refraction at
mid-infrared frequencies using hybrid topological polaritons in van der Waals
heterostructures with high spatial confinement. We experimentally visualize
wide-angle negatively-refracted surface polaritons on {\alpha}-MoO3 films
partially decorated with graphene, undergoing planar nanoscale focusing down to
1.6% of the free-space wavelength. Our atomically thick heterostructures
outperform conventional bulk materials by avoiding scattering losses at the
refracting interface while enabling active tunability through electrical
gating. We propose polaritonic negative refraction as a promising platform for
infrared applications such as electrically tunable super-resolution imaging,
nanoscale thermal manipulation, and molecular sensing.
