  As any physical object, light undergoing a circular trajectory features a
constant extrinsic angular momentum. Within strong curvatures, this angular
momentum can match the spin momentum of a photon, thus providing the
opportunity of a strong spin-orbit interaction. Using this effect, we
demonstrate tunable symmetry breaking in the coupling of light in a curved
nanoscale plasmonic waveguide. The helicity of the impinging optical wave
controls the power distribution between the two counter-propagating guided
modes, including unidirectional waveguiding. We found that up to 95 % of the
incoupled light can be directed into one of the two propagation directions of
the waveguide. This approach offers appealing new prospects for the development
of advanced, deeply subwavelength optical functionalities.
