  We have realized a Dirac fermion reflector in graphene by controlling the
ballistic carrier trajectory in a sawtooth-shaped npn junction. When the
carrier density in the inner p-region is much larger than that in the outer
n-regions, the first straight np interface works as a collimator and the
collimated ballistic carriers can be totally reflected at the second zigzag pn
interface. We observed clear resistance enhancement around the np+n regime,
which is in good agreement with the numerical simulation. The tunable
reflectance of ballistic carriers could be an elementary and important step for
realizing ultrahigh-mobility graphene field effect transistors utilizing Dirac
fermion optics in the near future.
