Optical transverse spin coupling through a plasmonic nanoparticle for particle-identification and field-mapping

Visualizing lateral optical force through surface plasmon-coupled emission

In this Letter, we report the intrinsic relationship among surface plasmon polaritons, lateral optical force, and surface plasmon-coupled emission. The spin-orbit coupling in the near field through circularly polarized beams would lead to the unidirectional excitation of surface plasmon polaritons, where the symmetry state of the electromagnetic field on the surface is broken. This asymmetric scattering would generate the counter-intuitive lateral optical force due to momentum conservation. As the inverse process of surface plasmon polaritons, surface plasmon-coupled emission enables the guide of the near-field surface plasmon polariton signal to the far field. We found that the lateral optical force produced by the unidirectional excitation of surface plasmon polaritons can be observed in the surface plasmon-coupled emission patterns. The elliptical dipole model was used to demonstrate these coupling processes. The magnitude and direction of lateral optical force can be a dipole, respectively. Moreover, the intensity convergence degree and direction of the surface plasmon-coupled emission distribution can reflect the magnitude and direction of lateral optical force, respectively. This work has great potential in the applications of weak force measurement, dynamic optical sorting, and light-matter interaction research.

Propagation of noninteger cylindrical vector vortex beams in a gradient-index fiber

The characteristics of two noninteger cylindrical vector vortex beams (NCVVBs) propagating through a radial gradient-index (GRIN) fiber are analyzed on the basis of the generalized Huygens-Fresnel principle. The NCVVBs exhibit periodic and stable transmission characteristics in the radial GRIN fiber. Polarization changes, the presence of spin angular momentum (SAM), and changes in the orbital angular momentum (OAM) of the NCVVBs are observed at the focal plane of the radial GRIN fiber. Spin-orbit interactions of NCVVBs are verified in the radial GRIN fiber for the first time, to the best of our knowledge.

Measuring the magnetic topological spin structure of light using an anapole probe

Topological spin structures of light, including the Skyrmion, Meron, and bi-Meron, are intriguing optical phenomena that arise from spin-orbit coupling. They have promising potential applications in nano-metrology, data storage, super-resolved imaging and chiral detection. Aside from the electric part of optical spin, of equal importance is the magnetic part, particularly the H-type electromagnetic modes for which the spin topological properties of the field are dominated by the magnetic field. However, their observation and measurement remains absent and faces difficult challenges. Here, we design a unique type of anapole probe to measure specifically the photonic spin structures dominated by magnetic fields. The probe is composed of an Ag-core and Si-shell nanosphere, which manifests as a pure magnetic dipole with no electric response. The effectiveness of the method was validated by characterizing the magnetic field distributions of various focused vector beams. It was subsequently employed to measure the magnetic topological spin structures, including individual Skyrmions and Meron/Skyrmion lattices for the first time. The proposed method may be a powerful tool to characterize the magnetic properties of optical spin and valuable in advancing spin photonics.

Mapping the near-field spin angular momenta in the structured surface plasmon polariton field

Optical spin angular momenta in a confined electromagnetic field exhibit a remarkable difference with their free space counterparts; in particular, the optical transverse spin that is locked with the energy propagating direction lays the foundation for many intriguing physical effects such as unidirectional transportation, quantum spin Hall effects, photonic Skyrmions, etc. In order to investigate the underlying physics behind the spin-orbit interactions as well as to develop the optical spin-based applications, it is crucial to uncover the spin texture in a confined field, yet it faces challenges due to their chiral and near-field vectorial features. Here, we propose a scanning imaging technique which can map the near-field distributions of the optical spin angular momenta with an achiral dielectric nanosphere. The spin angular momentum component normal to the interface can be uncovered experimentally by employing the proposed scanning imaging technique and the three-dimensional spin vector can be reconstructed theoretically with the experimental results. The experiment is demonstrated on the example of surface plasmon polaritons excited with various vector vortex beams under a tight-focusing configuration, where the spin-orbit interaction emerges clearly. The proposed method, which can be utilized to reconstruct the photonic Skyrmion and other photonic topological structures, is straightforward and of high precision, and hence it is expected to be valuable for the study of near-field spin optics and topological photonics.

Selective magnetic responses of silicon nanoparticles modulated by waveguide structures

High-refractive-index nanoparticles (NPs), such as silicon NPs, were considered as effective carriers in their response to a magnetic field at optical frequencies. Such NPs play an important role in many state-of-the-art technologies in nano-optics. Although the resonance properties of these NPs when varying their structural parameters have been studied intensely in the past few years, their interaction with the underlying substrate has seldom been discussed, in particular, when the substrate is a waveguide structure that significantly modulates the optical responses of the NPs. We proposed and studied a selective magnetic coupling system comprising a Si-NP on a metal-dielectric waveguide (MDW). The MDW structure supports either a transverse electric (TE) or a transverse magnetic (TM) mode that induces a large polarization dependence in the magnetic resonance. A new manifestation of the optical spin Hall effect was demonstrated in which a vertical rotating magnetic dipole excites a TE-type waveguide mode with a specific unidirectional emission. Making use of this polarization response, we developed a scanning imaging system that can selectively map the transverse or longitudinal magnetic field component of a focused beam depending on the type of MDW used in the system. This selective magnetic resonance coupling system is expected to be valuable for studying the fundamental interactions between the magnetic field and matter and for developing related nano-applications.

Surface plasmon coupled nano-probe for near field scanning optical microscopy

Near-field scanning optical microscopy (NSOM) is a powerful tool for study of the nanoscale information of objects by measuring their near-field electric field distributions. The near-field probe, which determines NSOM system performance, can be either a scattering-type or an aperture-type. Both types have strengths and weaknesses. Here we propose and study a surface plasmon-coupled type nano-probe, which works as a hybrid scheme and could potentially combine the advantages of the two NSOM probe types. The key element of the proposed probe is a nanoparticle-on-film structure designed on a tapered fiber tip. On the one hand, the probe can yield the signals scattered in the near field by a nanoparticle with a scattering mechanism; on the other hand, the scattered signals can be transmitted by the metal film and coupled into the fiber via surface plasmon coupled emission, thus providing a collection mode similar to an aperture-type NSOM. This will lead to signal enhancement, while greatly suppressing background noise. This surface plasmon-coupled nano-probe thus has great potential for near-field optical microscopy applications.

Mapping the weak plasmonic transverse field by a dielectric-nanoparticle-on-film structure with ultra-high precision

Highly confined electromagnetic fields play a significant role in modern nano-optics, among which surface plasmon polaritons (SPPs) are outstanding because of their subwavelength and enhancement nature. While many state-of-the-art methods have been proposed to uncover the field distribution of SPPs, it still faces challenge to map the weak transverse field component (the field tangential to the interface) of SPPs with high contrast and precision. We propose a direct imaging technique, which employs a dielectric-nanoparticle-on-metal-film (DNP-MF) structure as a near-field probe, to overcome this difficulty. The angular distribution of the scattering radiation from the structure is strongly polarization dependent. By extracting the scattering signals that are mainly induced by the horizontal polarization, the imaging of the weak plasmonic transverse field with high precision can be achieved. The mappings of SPPs distributions excited by various vector beams were performed in experiment, which accord excellent with theory. This technique provides a new approach for near-field imaging with high contrast and reliability, which is expected to be valuable for studying the vectorial features of SPPs such as transverse spin, spin-orbit interactions, etc.

Unusual polarizing effect of cylindrical plasmonic holes

We observe an unusual polarization state conversion in the light that passes through a cylindrical hole in a thick metal film. This phenomenon is related to the helicity locking of the guided mode due to the plasmonic transverse spin-an intrinsic angular momentum of the surface waves. We show how this effect is linked to the generation of the plasmonic vortex inside the hole and can be altered by varying the hole diameter. In addition, the total light transmission through the hole is shown to be partially contributed from the direct transmission, which can further modify the resulting light polarization state.