  Negative reflection and negative refraction are exotic phenomena that can be
achieved by platforms such as double-negative metamaterial, hyperbolic
metamaterial, and phase-discontinuity metasurface. Recently, natural biaxial
van der Waals (vdW) materials, which support extremely anisotropic, low-loss,
and highly confined polaritons from infrared to visible regime, are emerging as
promising candidates for planar reflective and refractive optics. Here, we
introduce three degrees of freedom, namely interface, crystal direction, and
electric tunability to manipulate the reflection and refraction of the
polaritons. With broken in-plane symmetry contributed by the interface and
crystal direction, distinguished reflection and refraction such as negative and
backward reflection, positive and negative refraction could exist
simultaneously and exhibit high tunability. The numerical simulations show good
consistency with the theoretical analysis. Our findings provide a robust recipe
for the realization of negative reflection and refraction in biaxial vdW
materials, paving the way for the polaritonics and interface nano-optics.
