  Valleytronics using two-dimensional materials opens unprecedented
opportunities for information processing with the valley polarizer being a
basic building block. Paradigms such as strain engineering, the inclusion of
line defects, and the application of electrostatic-magnetic fields extensively
explored for creating valley polarization suffer from limitations like smaller
transmission or the lack of polarization directionality. We propose an
all-electrical valley polarizer using zigzag edge graphene nanoribbons in a
multi-terminal device geometry, that can be gate-tuned to operate along two
independent regimes: (i) terminal-specific valley filter that utilizes
bandstructure engineering, and (ii) parity-specific valley filter that exploits
the parity selection rule in zigzag edge graphene. We show that the device
exhibits intriguing physics in the multimode regime of operation that affects
the valley polarization and hence investigate various factors affecting the
polarization in wide device geometries, such as, optical analogs of graphene
Dirac fermions, angle-selective transmission via p-n junctions, and the
localization of edge states. We optimize the geometry of the proposed device to
achieve maximum valley polarization, thereby, paving the way toward a physics
based tunable valleytronic device design using monolayer graphene.
