  We show that charge carrier transport in graphene exhibit sharp resonances in
the presence of spatially and temporarily modulated scattering. Resonances
occur when the period of an applied a-c field corresponds to the time taken by
quasi-ballistic carriers to drift over a spatial scattering period, provided
the latter is shorter than the distance taken by carriers to emit an optic
phonon. We show that such system can be achieved with interdigitated gates
energized with an a-c bias on graphene layers. Gate separation and fields to
achieve ballistic transport would result in resonances in the terahertz range,
with the generation of higher harmonics characterized by large Q-factors, which
are tunable with gate spacing.
