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...

As any physical particle or 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 into a curved nanoscale plasmonic waveguide. The helicity of the impinging optical wave controls the power distribution between the two counter-propagating subwavelength guided modes including unidirectional waveguiding. We found experimentally that up to 95% of the incoupled light can be selectively directed into one of the two propagation directions of a nanoscale waveguide. This approach offers new degrees of freedom in the manipulation of subdiffraction optical modes and thus appealing new prospects for the development of advanced, deeply subwavelength optical functionalities.

Colloidal Quantum dots (CQDs) are nowadays one of the cornerstones of modern photonics as they have led to the emergence of new optoelectronic and biomedical technologies. However, the full characterization of these quantum emitters is currently restricted to the visible wavelengths and it remains a key challenge to optically probe single CQDs operating in the infrared spectral domain which is targeted by a growing number of applications. Here, we report the first experimental detection and imaging at room temperature of single infrared CQDs operating at telecommunication wavelengths. Imaging was done with a doubly resonant bowtie nano-aperture antenna (BNA) written at the end of a fiber nanoprobe, whose resonances spectrally fit the CQD absorption and emission wavelengths. Direct near-field characterization of PbS CQDs reveal individual nanocrystals with a spatial resolution of 75 nm (λ/20) together with their intrinsic 2D dipolar free-space emission properties and exciton dynamics (blinking phenomenon). Because the doubly resonant BNA is strongly transmissive at both the CQD absorption and emission wavelengths, we are able to perform all-fiber nano-imaging with a standard 20 % efficiency InGaAs avalanche photodiode (APD). Detection efficiency is predicted to be 3000 fold larger than with a conventional circular aperture tip of the same transmission area. Double resonance BNA fiber probes thus offer the possibility of exploring extreme light-matter interaction in low band gap CQDs with current plug-and-play detection techniques, opening up new avenues in the fields of infrared light emitting devices, photodetectors, telecommunications, bio-imaging and quantum information technology.