Highly efficient singular surface plasmon generation by achiral apertures

Abstract

We report a highly efficient generation of singular surface plasmon (SP) field by an achiral plasmonic structure consisting of Λ-shaped apertures. Our quantitative analysis based on leakage radiation microscopy (LRM) demonstrates that the induced spin-orbit coupling can be tuned by adjusting the apex angle of the Λ-shaped aperture. Specifically, the array of Λ-shaped apertures with the apex angle 60• is shown to give rise to the directional coupling efficiency. The ring of Λ-shaped apertures with the apex angle 60• realized to generate the maximum extinction ratio (ER=11) for the SP singularities between two different polarization states. This result provides a more efficient way for developing SP focusing and SP vortex in the field of nanophotonics such as optical tweezers. PACS numbers:Spin-driven singular surface plasmon generation, like surface plasmon (SP) vortices, [1][2][3][4][5] has been in the recent years intensively investigated in both the optical near-field [1,6] or far-field of planar metal-dielectric nanostructures [7,8]. In these systems the singular generation of SPs at the metal-dielectric interface stems from the spin-orbit interactions between the incoming light and the SP modes generated. The helicity of the incident wavefront, associated with the intrinsic spin angular momentum (SAM) of photons, couples to its orbital angular momentum (OAM) via plasmonic nanostructures. SP vortices have been generated and observed in a number of planar geometries, such as extended plasmonic Archimedes spiral, [1, 2, 9, 10] and chiral plasmonic nanoapertures [3,[11][12][13][14].Among a variety of designs, the chiral T-shaped [11] and the achiral Λ-shaped [15,16] antennas have attracted particular interest for their ability to support a pair of orthogonal dipoles with a phase delay. This phase shift is very sensitive to incident spin states which leads to optical spin Hall effects [17,18]. The polarization-dependent photon shift evidenced with SP propagation [19,20] can be used for instance for inducing SP directional coupling [21][22][23]. Recently, we used leakage radiation microscopy (LRM) on a thin metal film [24][25][26], in order to image singular SP vortices generated by circular structures made of T-Shaped apertures [27]. This far-field methodology offers an interesting alternative to near-field measurements realized on similar systems [20] by allowing a precise quantitative mapping and polarization analysis of SP vortex generation in both the direct and Fourier space. In the present work, we exploit the potentials of this technique in the optimization of such systems, i.e., for controlling the contrast of the induced spin-Hall effect observed with the singular SP field generation. * Electronic address: aurelien.drezet@neel.cnrs.fr Specifically, we focus on Λ-shaped apertures instead of T-shaped apertures because they offer the possibility for tuning the phase delay by adjusting the apex angle α (we define α as being the half of the total angle between two elementary slits as indicated the inset in Fi...

Metasurface for Reciprocal Spin-Orbit Coupling of Light on Waveguiding Structures

“…This approach rigorously involves the nonparaxial light propagation and geometrical aberration in the microscope, which have to be considered for an accurate description of the imaging process [27,28]. To simulate the reversible SH effect for a metasurface formed by an array of 4 × 4 nanoantennas, we thus consider 16 pairs of orthogonal in-plane dipoles (oriented perpendicularly to each arm of the -shaped antennas) and coherently sum their electric fields as a function of the imaging direction [10,20,29]. The interactions between each dipole are fairly neglected as assumed in [29].…”

“…To simulate the reversible SH effect for a metasurface formed by an array of 4 × 4 nanoantennas, we thus consider 16 pairs of orthogonal in-plane dipoles (oriented perpendicularly to each arm of the -shaped antennas) and coherently sum their electric fields as a function of the imaging direction [10,20,29]. The interactions between each dipole are fairly neglected as assumed in [29]. The intensity and polarization associated with the total electric field is presented in Fig.…”

Manifestation of Planar and Bulk Chirality Mixture in Plasmonic Λ-Shaped Nanostructures Caused by Symmetry Breaking Defects

“…It is worth mentioning that our experimental approach allows the determination of the complex value of β, that is the relative weight of the minor dipole amplitude with respect to the major dipole. Recently, we demonstrated that the aspect ratio of the slit is critical for the design of nanostructures with optimized plasmonic coupling and directivity efficiency. , Notably, we used leakage radiation microscopy on a thin metal film , in order to quantify the contribution of each dipole in the SP generation. However, only partial information on the relative weight of the minor axis dipole, that is its real part, was accessed.…”