  We have developed a process to fabricate suspended graphene devices with
local bottom gates, and tested it by realizing electrostatically controlled pn
junctions on a suspended graphene mono-layer nearly 2 micrometers long.
Measurements as a function of gate voltage, magnetic field, bias, and
temperature exhibit characteristic Fabry-Perot oscillations in the cavities
formed by the pn junction and each of the contacts, with transport occurring in
the ballistic regime. Our results demonstrate the possibility to achieve a high
degree of control on the local electronic properties of ultra-clean suspended
graphene layers, a key aspect for the realization of new graphene
nanostructures.
