Directional and Singular Surface Plasmon Generation in Chiral and Achiral Nanostructures Demonstrated by Leakage Radiation Microscopy

Abstract

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...

Ultracompact Broadband Plasmonic Polarimeter

“…It is possible to achieve polarizers with high extinction ratio with help of the polarization‐selective property of SPPs and anisotropic scatterers. Polarization‐sensitive plasmonic devices, which show large controllability with high contrast, are successfully demonstrated, directional launching and switching of SPPs, multiplexed plasmonic lenses, polarization‐controlled resonators, SPP beam steering, and caustic beam generation, for instances. Other reports of polarization generators and converters, optical routers, and multiplexed holography show strong potential for wide variety of applications.…”

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].…”

“…The spin Hall (SH) effect [3][4][5][6], which couples the internal momentum of light to its orbital angular momentum, can be applied to tune the scattering of a nanoparticle towards a confined propagating wave [7]. This type of SH effect has been reported for surface plasmon polaritons and guided waves [7][8][9][10][11]. The spin manipulation of guided modes is of particular interest for integrated optics [12], quantum optics [13], optoelectronics, biosensors [14][15][16].…”

Vortex Beam Generation by Spin-Orbit Interaction with Bloch Surface Waves

“…However, this approach can be hardly adopted when the input field has a limited spatial extension (as for focused beams or localized coherent sources such as optical antennas or cavities), unless some mode coupling is intervening [25]. This is indeed the case in structured metallic films, wherein the generation of free-space vortex beam carrying OAM occurs upon spin-orbit coupling and scattering/diffraction of plasmonic modes by means of nano-slits [26][27][28], properly arranged nano-apertures [29], possibly combined with circular diffraction gratings [30,31]. Such results rely on the fact that OAM possessed by surface plasmons can be further manipulated and transferred to freely propagating radiation [28,32].…”