  Half a century ago, Veselago proposed left-handed materials with negative
permittivity and permeability, in which waves propagate with phase and group
velocities in opposite directions. Significant work has been undertaken to
attain this left-handed response, such as establishing a negative refractive
index in so-called metamaterials, which consist of periodic sub-wavelength
structures. However, an electronic counterpart has not been demonstrated owing
to difficulties in creating repeated structures smaller than the electronic
Fermi wavelength (\lambda_F) of the order ~ 10 nm. Here, without needing to
engineer sub-wavelength structures, we demonstrate negative refractive
behaviour of Dirac fermions in graphene, exploiting its unique relativistic
band structure. Analysis of both electron focusing through a n-p-n flat lens
and negative refraction across n-p junctions confirms left-handed behaviour in
the electronic system. This new approach to electronic optics is of particular
relevance to the on-going efforts to develop novel quantum devices with
emerging layered materials.
