Near-Field Interference for the Unidirectional Excitation of Electromagnetic Guided Modes

Abstract

Wave interference is a fundamental manifestation of the superposition principle with numerous applications. While in conventional optics interference occurs between waves undergoing different phase advances during propagation, we show that the vectorial structure of the near-field of an emitter is essential for controlling its radiation as it interferes with itself on interaction with a mediating object. We demonstrate that the near field interference of a circularly polarized dipole results in the unidirectional excitation of guided electromagnetic modes in the near-field, with no preferred far-field radiation direction. By mimicking the dipole with a single illuminated slit in a gold film, we measured unidirectional surface-plasmon excitation in a spatially symmetric structure. The surface wave direction is switchable with the polarization.Interference is the cornerstone of various phenomena in nature enabling numerous applications. In optics, it is intensively used in microscopy, stellar measurements, spectroscopy, and communication technologies, among many others, and is the basis behind the concepts of reflection, refraction and light bending (1, 2). Typically, interference occurs due to the relative phase lag of different propagating waves. On the other hand, nanophotonics -the branch of optics studying the interaction of light with subwavelength nanoscale structures-deals inherently with phenomena that occur via near-field interactions before appreciable phase lags can be accumulated (3). A radiationless form of interference in the near field (4) is behind new exciting applications such as the focusing of evanescent components to achieve subwavelength resolution in imaging (5-8). Near field interference achieved through the full coherent control of the phase and amplitude of excitation light allows asymmetric spatial field localization (9, 10) and selection of propagation paths at intersections of waveguides (11).We demonstrate near field interference by considering a single source of radiation coupled to a mode with a vectorial structure of electromagnetic field. Using an additional degree of freedom provided by the vectorial character of the field, control over the near-field interference can be achieved. We show that an elliptically polarized dipole can produce destructive or constructive interference of different evanescent components in its near field, and as a result, excite electromagnetic modes in neighbouring material structures, such as dielectric and plasmonic waveguides and diffraction gratings, with a controlled directionality of propagation.

Spin–orbit coupling in surface plasmon scattering by nanostructures

“…In both cases, the light scattered at higher values of k x becomes increasingly linearly polarized. The polarization was not found to be fully circular in accordance with the theoretical modelling and experimental observation of direct PSHE on these nanostructures 7 . The direction of the scattered light with a given elliptical polarization and the polarization of the scattering light in the same direction can be simply controlled by changing the SPP propagation direction.…”

“…As a starting point, Fig. 2a shows the direct PSHE 7,13 , where the scattering of circularly polarized light by a spherical metal nanoparticle leads to the directional excitation of SPP waves dependent on the wavevector and spin of incoming photons. SPPs are excited on a smooth metal film on the left or the right from the nanoparticle corresponding to the right-handed or left-handed polarization state of the illuminating light, respectively (Fig.…”

“…The effect is based on the reciprocity of optical interactions and the peculiar transverse angular momentum specific to surface electromagnetic waves on a metal-dielectric interface. While in the direct photonic SHE (PSHE), the polarization of the incident light controls the direction of the SPP excitation on a surface [2][3][4][5][6][7] , in the reciprocal scenario, in analogy to the spin injection current control in solidstate spintronic devices (inverse SHE) 8 , the direction of the scattered light of a given polarization is determined by the direction of the SPPs impinging at a scatterer.…”

“…In the electronic SHE, electrons with opposite spins undertake different trajectories in a current flow 1 . In optics, photons with opposite spins-left or right circularly polarized (LCP or RCP) light-may propagate in different directions upon interaction with the material environment if spin-orbital coupling is engineered using, for example, metasurfaces [2][3][4] , Berry phase 5 , refractive index gradient 6 or near-field interference 7 .…”

Plasmonic polarization generator in well-routed beaming

“…Recently, plasmoninduced spin-orbital coupling has generated strong interest in the field of photonics [13][14][15][16][17] , primarily due to the possibility of polarization and phase modulation. In fact, the vectorial structure of the surface plasmon field gives rise to unique properties in the conversion of optical fields between propagating light and bounded surface plasmon polaritons (SPPs), where the polarization information of light can be reloaded by special SPPs in a controllable way [18][19][20][21][22] . However, most of these devices offer only limited functions in polarization control.…”