  As one of the most fundamental optical phenomena, refraction between
isotropic media is characterized by light bending towards the normal to the
boundary when passing from a low- to a high-refractive-index medium. However,
in anisotropic media, refraction is a much more exotic phenomenon. The most
general case of refraction between two anisotropic media remains unexplored,
particularly in natural media and at the nanoscale. Here, we visualize and
comprehensively study refraction of electromagnetic waves between two strongly
anisotropic (hyperbolic) media, and, importantly, we do it with the use of
polaritons confined to the nanoscale in a low-loss natural medium, alpha-MoO3.
Our images show refraction of polaritons under the general case in which both
the direction of propagation and the wavevector are not collinear. As they
traverse planar nanoprisms tailored in alpha-MoO3, refracted polaritons exhibit
non-intuitive directions of propagation, enabling us to unveil an exotic
optical effect: bending-free refraction. Furthermore, we succeed in developing
the first in-plane refractive hyperlens, which yields foci as small as
lamdap/6, being lamdap the polariton wavelength (lamda0/50 with respect to the
wavelength of light in free space). Our results set the grounds for planar
nano-optics in strongly anisotropic media, with potential for unprecedented
control of the flow of energy at the nanoscale.
