  We show that bulk rhenium diselenide, ReSe2 is characterized by record high
value of the refractive index exceeding 5 in near-infrared frequency range. We
use back focal plane reflection spectroscopy to extract the components of the
ReSe2 permittivity tensor and reveal its extreme biaxial anisotropy. We also
demonstrate the good agreement between the experimental data and the
predictions of the density functional theory. The combination of the large
refractive index and giant optical anisotropy makes ReSe2 a perspective
material for all-dielectric nanophotonics in the near-infrared frequency range.
