  Negative refraction is such a prominent electromagnetic phenomenon that most
researchers believe it can only occur in artificially engineered metamaterials.
In this article, we report negative refraction for all incident angles for the
first time in a naturally existing material. Using ellipsometry measurement of
the equifrequency contour in the deep-ultraviolet frequency region (typically
254 nm), obvious negative refraction was demonstrated in monocrystalline
graphite for incident angles ranging from 20o to 70o. This negative refraction
is attributed to extremely strong anisotropy in the crystal structure of
graphite, which gives the crystal indefinite permeability. This result not only
explores a new route to identifying natural negative-index materials, but it
also holds promise for the development of an ultraviolet hyperlens, which may
lead to a breakthrough in nanolithography, the most critical technology
necessary for the next generation of electronics.
