  On-chip manipulating and controlling the temporal and spatial evolution of
light is of crucial importance for information processing in future planar
integrated nanophotonics. The spin and orbital angular momentum of light, which
can be treated independently in classical macroscopic geometrical optics,
appear to be coupled on subwavelength scales. We use spin-orbit interactions in
a plasmonic achiral nano-coupler to unidirectionally excite surface plasmon
polariton modes propagating in seamlessly integrated plasmonic slot waveguides.
The spin-dependent flow of light in the proposed nanophotonic circuit allows
on-chip electrical detection of the spin state of incident photons by
integrating two germanium-based plasmonic-waveguide photodetectors.
Consequently, our device serves as a compact ($\sim$ 6 $\times$ 18 $\mu$m$^2$)
electrical sensor for photonic spin Hall dynamics. The demonstrated
configuration opens new avenues for developing highly-integrated
polarization-controlled optical devices that would exploit the spin-degree of
freedom for manipulating and controlling subwavelength optical modes in
nanophotonic systems.
