On-Chip Detection of Optical Spin–Orbit Interactions in Plasmonic Nanocircuits

Abstract

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 (~ 618 m 2 ) electrical sensor for photonic spin Hall dynamics.The demonstrated configuration opens new avenues for developing highly-integrated polarizationcontrolled optical devices that would exploit the spin-degree of freedom for manipulating and controlling subwavelength optical modes in nanophotonic systems.Introduction. Light carries both the spin, an intrinsic form of angular momentum, and orbital angular momentum, which determines its polarization and spatial degree of freedom. Interaction between the spin and orbital degrees of freedom of photons has evoked intensive investigations owing to its potential to push the development of technologies, such as chiroptical spectroscopy 1,2 , communication 3 , information processing 4 , topological photonics 5,6 and quantum computing 7 , to their full potential. The limiting factor for groundbreaking developments in those fields refers to the fact that the spin-orbit interactions (SOIs) in optics are usually very weak, akin to the Planckconstant smallness of the electron SOI found in solid-state spintronics 8 . A promising way to significantly enhance spin-controlled optical phenomena is to utilize light-matter interactions on the nanoscale that are especially strong in plasmonic nanostructures. It has been shown that geometrically chiral metallic structures, which do not superimpose onto their mirror image, can strongly enhance chiroptical far-field responses as a consequence of structural chirality 9-12 .Remarkably, even achiral structures exhibit the SOI potential in the near-field due to twisted trajectories of surface plasmons at a nanosphere [13][14][15][16] . This feature enables spin-controlled local manipulation within one nanoscale coupler, which responds equally to both photonic spin states.We utilize the strong SOI in an achiral plasmonic nanostructure to demonstrate for the first time onchip detection of spin-controlled directional routing in a compact plasmonic nanocircuit. We find that a subwavelength semiring can launch gap surface plasmons supported by seamlessly integrated plasmonic slot waveguides preferentially in one direction, depending on the spin state of locally incident radiation. This spin-dependent phenomenon can thus be regarded as a manifestation ...

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