  Optical orbital angular momentum (OAM) provides an additional dimension for
photons to carry information in high-capacity optical communication. Although
the practical needs have intrigued the generations of miniaturized devices to
manipulate the OAM modes in various integrated platforms, the on-chip OAM
detection is still challenging to match the newly-developed compact OAM emitter
and OAM transmission fiber. Here, we demonstrate an ultra-compact device, i.e.,
a single plasmonic nanohole, to efficiently measure an optical beam's OAM state
in a nondestructive way. The device size is reduced down to a few hundreds of
nanometers, which can be easily fabricated and installed in the current OAM
devices. It is a flexible and robust way for in-situ OAM monitoring and
detection in optical fiber networks and long-distance optical communication
systems. With proper optimization of the nanohole parameters, this approach
could be further extended to discriminate the OAM information multiplexed in
multiple wavelengths and polarizations.
