In this paper, we describe the implementation of leakage radiation microscopy (LRM) to probe the chirality of plasmonic nanostructures. We demonstrate experimentally spin-driven directional coupling as well as vortex generation of surface plasmon polaritons (SPPs) by nanostructures built with T-shaped and Λ-shaped apertures. Using this far-field method, quantitative inspections, including directivity and extinction ratio measurements, are achieved via polarization analysis in both image and Fourier planes. To support our experimental findings, we develop an analytical model based on a multidipolar representation of Λ-and T-shaped aperture plasmonic coupler allowing a theoretical explanation of both directionality and singular SPP formation. Furthermore, the roles of symmetry breaking and phases are emphasized in this work. This quantitative characterization of spin-orbit interactions paves the way for developing new directional couplers for subwavelength routing. PACS numbers:Chiral plasmonic nanostructures [1] exhibit unique optical properties, such as asymmetric optical transmission [2] and singular optical signatures, like vortices, visible in both the optical near-field [3,4] and in the far-field of twisted structures [5,6]. Motivated by fundamental questions as well as by their potentials ranging from highly integrated photonic circuits to quantum optics [1,7], interest in the field of chiral plasmonics has subsequently become a topic of intensive research. These peculiar optical effects stem from the spin-orbit interactions of light via plasmonic nanostructures, in which the photon spin couples to its spatial motion [3] and in particular to its orbital angular momentum. This leads to optical spin Hall effects [8,9], i.e., to a polarization-dependent photon shift evidenced with SPPs [3, 10, 11] which can be used for instance for inducing SPP directional coupling [12][13][14]. In this context, recent studies done in the optical near-field demonstrated that spin-controlled SPP directionality and vortex generation can be achieved with chiral nanostructures such as T-shaped aperture arrays, rings or spirals milled in metal films [15,16]. While the additional degree of freedom added by the incident spin enables tunable directionality, enhanced directional coupling is achieved by the broken symmetry of the plasmonic structures. Moreover, since controlling SPP propagation direction and rotational motion is essential for applications in integrated optics and optical trapping, it becomes urgent to develop sensitive imaging techniques to map plasmonic chirality not only in the near-field but also in the far-field. Due to the inherently confined SPP fields, near-field optical detection has been widely employed in the past to image SPP propagation (for a review see [17]). Indirect imaging via scattering of SPPs or via grating have also been used [18][19][20]. Here, we propose a different approach based on leakage radiation microscopy (LRM) [21][22][23]. As a complementary method for direct imaging of SPP propagation, L...
We introduce a new paradigm: the chiral electromagnetic local density of states (LDOS) in a spiral plasmonic nanostructure. In both classical and quantum regimes, we reveal using scanning near-field optical microscopy (NSOM) in combination with spin analysis that a spiral cavity possesses spin-dependent local optical modes. We expect this work to lead to promising directions for future quantum plasmonic device development, highlighting the potentials of chirality in quantum information processing.
We report on the reciprocal spin Hall effect of light in T-shaped nanoaperture arrays. Specifically, we demonstrate that the information tied to surface plasmons trajectories can be encoded into free-space spin-carrying photons. The functionality of the system to act as a circular polarizer is therefore implemented in an interference eraser experiment where the device is used as a which-path marker. Complementarity between the wave-like and particle-like behavior of surface plasmons is verified, hence, further demonstrating the outlook for miniaturized optical elements toward on-chip quantum experiments. This work underscores the high potential of plasmonic devices in the realization of integrated polarization optics, hence, opening promising prospects for nanoscale optical communications and quantum photonic network.
Light manipulation through spin-orbit coupling opens new perspectives in photonics and particularly in integrated optics. The reverse spin Hall effect, where the guided wave direction affects the polarization properties of the light scattered by a nanostructure, is a key effect for the development of new functionalities at the connection between integrated structures and free space, and could find application in chiral quantum optics, modulation, and multiplexing. We show that metasurfaces represent a promising platform for the reverse spin Hall effect. Using a periodic array of-shaped metallic nanoantenna, we control the polarization of the extracted light with the guided wave-propagation direction, but also the number of output directions and the polarization of each one. These results and the versatility of metasurfaces for the spin Hall effect could be extended to various frequencies and materials, such as silicon photonics at telecom wavelength.
We report on the coexistence of planar and bulk chiral effects in plasmonic Λ-shaped nanostructure arrays arising from symmetry breaking defects. The manifestation of bi-(2D) and three-dimensional (3D) chiral effects are revealed by means of polarization tomography and confirmed by symmetry considerations of the experimental Jones matrix. Notably, investigating the antisymmetric and symmetric parts of the Jones matrix points out the contribution of 2D and 3D chirality in the polarization conversion induced by the system whose eigenpolarizations attest to the coexistence of planar and bulk chirality. Furthermore, we introduce a generalization of the microscopic model of Kuhn, yielding to a physical picture of the origins of the observed planar chirality, circular birefringence, and dichroism, theoretically prohibited in symmetric Λ-shaped nanostructures.
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...
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