  The relativistic charge carriers in monolayer graphene can be manipulated in
manners akin to conventional optics (electron-optics): angle-dependent Klein
tunneling collimates an electron beam (analogous to a laser), while a Veselago
refraction process focuses it (analogous to an optical lens). Both processes
have been previously investigated, but the collimation and focusing efficiency
have been reported to be relatively low even in state-of-the-art ballistic
pn-junction devices. These limitations prevented the realization of more
advanced quantum devices based on electron-optical interference, while
understanding of the underlying physics remains elusive. Here, we present a
novel device architecture of a graphene microcavity defined by
carefully-engineered local strain and electrostatic fields. We create a
controlled electron-optic interference process at zero magnetic field as a
consequence of consecutive Veselago refractions in the microcavity and provide
direct experimental evidence through low-temperature electrical transport
measurements. The experimentally observed first-, second-, and third-order
interference peaks agree quantitatively with the Veselago physics in a
microcavity. In addition, we demonstrate decoherence of the interference by an
external magnetic field, as the cyclotron radius becomes comparable to the
interference length scale. For its application in electron-optics, we utilize
Veselago interference to localize uncollimated electrons and characterize its
contribution in further improving collimation efficiency. Our work sheds new
light on relativistic single-particle physics and provides important technical
improvements toward next-generation quantum devices based on the coherent
manipulation of electron momentum and trajectory.
