  Gears are rotating machines, meshing with each other by teeth to transmit
torque. Interestingly, the rotating directions of two meshing gears are
opposite, clockwise and counterclockwise. Although this opposite handedness
motion has been widely investigated in machinery science, the analogue behavior
of photons remains undiscovered. Here, we present a simple nanophotonic
directional coupler structure which can generate two meshing gears of angular
momentum (AM) of light, optical gears. Due to the abrupt phase shift effect and
birefringence effect, the AM states of photons vary with the propagation
distance in two adjacent waveguides of the coupler. Thus, by the choice of
coupling length, it is able to obtain two light beams with opposite handedness
of AM, confirming the appearance of optical gears. The full control in the
handedness of output beams is achieved via tuning the relative phase between
two orthogonal modes at the input ports. Optical gears thus offer the
possibility of exploring light-matter interactions in nanoscale, opening up new
avenues in fields of integrated quantum computing and nanoscale bio-sensing of
chiral molecules.
