Unidirectional sub-diffraction waveguiding based on optical spin–orbit coupling in subwavelength plasmonic waveguides

Abstract

Subwavelength plasmonic waveguides show the unique ability of strongly localizing (down to the nanoscale) and guiding light. These structures are intrinsically two-way optical communication channels, providing two opposite light propagation directions. As a consequence, when light is coupled to these planar integrated devices directly from the top (or bottom) surface using strongly focused beams, it is equally shared into the two opposite propagation directions. Here, we show that symmetry can be broken by using incident circularly polarized light, on the basis of a spin-orbital angular momentum transfer directly within waveguide bends. We predict that up to 94 % of the incoupled light is directed into a single propagation channel of a gap plasmon waveguide. Unidirectional propagation of strongly localized optical energy, far beyond the diffraction limit, becomes switchable by polarization, with no need of intermediate nano-antennas/scatterers as light directors.This study may open new perspectives in a large panel of scientific domains, such as nanophotonic circuitry, routing and sorting, optical nanosensing, nano-optical trapping and manipulation.Subwavelength plasmonic waveguiding has drawn a considerable interest during the past years for îts unique ability of controlling light down to the nanometer scale, opening the perspective of highly integrated optical circuits and ultra-compact optical functions [1]. Several plasmon waveguide geometries, such as metallic Vgrooves [2,3], nanostripes [4], nanowires [5,6], nanogaps [7][8][9], wedges [10,11], dielectric-loaded metal films [12] have been proposed for strongly confining and guiding light. Given their intrinsic symmetry, plasmonic waveguides provide two-way propagation channels of opposite directions. Generally, light is coupled into the waveguide mode with end-firing techniques in order to reach unidirectional propagation of light at subwavelength scale. This coupling technique avoids one of the two possible propagation directions: propagation reversal within the waveguide requires two different coupling devices positioned at its two extremities.Recently, reversible unidirectional light propagation has been obtained onto planar metallic surfaces (with surface plasmons) [13][14][15], in photonic crystal waveguides [16], in nanofibers [17,18] and in dielectric stripes [19]. All these studies are based on the coupling of angular momentum between a rotating dipolar nano-emitter and the evanescent surface waves involved in the waveguiding process, on the basis of spin-orbit interaction in localized fields [20]: the intrinsic chirality of the evanescent waves in play makes the connection between the point-like emitter and the waveguide [21,22]. This technique allows for reversing the propagation direction of the waveguide mode by switching circular polarization direction. Reversible unidirectional guiding has also been achieved in dielectric waveguides with incident linear polarization [23]. For all these techniques however, light coupling into the waveguide need...

Chiral Coupling of Valley Excitons and Light through Photonic Spin–Orbit Interactions

“…To achieve the unidirectional coupling, a director/coupler locating above the air–waveguide (metal) interface is needed to couple the incident light into the waveguide or SPP modes. This has been realized by using various emitters or scatterers coupled with a series of photonic nanostructures 11–14,18–20,22–24,164,176–181. As an application of the spin‐direction locking, the field of quantum optics can be extended to chiral quantum optics where the SAM of light can serve as spin qubits.…”

“…g,h) Electric field intensity mapping. a–h) Reproduced with permission 177. Copyright 2015, Optical Society of America.…”

“…This has been realized by using various emitters or scatterers coupled with a series of photonic nanostructures. [11][12][13][14][18][19][20][22][23][24]164,[176][177][178][179][180][181] As an application of the spindirection locking, the field of quantum optics can be extended to chiral quantum optics where the SAM of light can serve as spin qubits. Akin to the conventional single-photon emitters in nanostructures, the single photon emitter with circular polarization (i.e., chiral emitter) can be used to transmit quantum information or entanglement, [176] or be used to create quantum logic gates.…”

“…a-h) Reproduced with permission. [177] Copyright 2015, Optical Society of America. i,j) Simulated directional emission in glide-plane photonic-crystal waveguide with an asymmetric structure.…”

Design and implementation of nanoantennas on integrated guides and their application on polarization analysis and synthesis.

“…This property of evanescent waves can be generally considered as a manifestation of the quantum spin Hall effect of light [65]. Such transverse spin is the ultimately responsible for the spin-controlled unidirectional excitation (SCUE) of guided waves [14,66,68,69,70,71,76,77,78,79], a phenomenon that has its origin in the spin-orbit coupling taking place when a circularly-polarized subwavelength dipole is placed in the evanescent-wave region of a guided mode. The spin-momentum locking inherent to evanescent waves [65,80] enables to completely switch the propagation direction of the excited guided mode by merely changing the spin of the exciting dipole [59,69,77].…”