  We demonstrate a novel detection scheme for the orbital angular momentum
(OAM) of light using circular plasmonic lens. Owing to a division-of-amplitude
interference phenomenon between the surface plasmon waves and directly
transmitted light, specific intensity distributions are formed near the
plasmonic lens surface under different OAM excitations. Due to different phase
behaviors of the evanescent surface plasmon wave and the direct transmission,
interference patterns rotate as the observation plane moves away from the lens
surface. The rotation direction is a direct measure of the sign of OAM, while
the amount of rotation is linked to the absolute value of the OAM. This OAM
detection scheme is validated experimentally and numerically. Analytical
expressions are derived to provide insights and explanations of this detection
scheme. This work forms the basis for the realization of a compact and
integrated OAM detection architect that may significantly benefit optical
information processing with OAM states.
