  Massless Dirac fermions (MDFs) emerge as quasiparticles in various novel
materials such as graphene and topological insulators, and they exhibit several
intriguing properties, of which Veselago focusing is an outstanding example
with a lot of possible applications. However, up to now Veselago focusing
merely occurred in p-n junction devices based on the isotropic MDF, which lacks
the tunability needed for realistic applications. Here, motivated by the
emergence of novel Dirac materials, we investigate the propagation behaviors of
anisotropic MDFs in such a p-n junction structure. By projecting the
Hamiltonian of the anisotropic MDF to that of the isotropic MDF and deriving an
exact analytical expression for the propagator, precise Veselago focusing is
demonstrated without the need for mirror symmetry of the electron source and
its focusing image. We show a tunable focusing position that can be used in a
device to probe masked atom-scale defects. This study provides an innovative
concept to realize Veselago focusing relevant for potential applications, and
it paves the way for the design of novel electron optics devices by exploiting
the anisotropic MDF.
