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

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.

Spin-Manipulated Photonic Skyrmion-Pair for Pico-Metric Displacement Sensing

Photonic spin skyrmions with deep-subwavelength features have aroused considerable interest in recent years. However, the manipulation of spin structure in the skyrmions in a desired manner is still a challenge, while this is crucial for developing the skyrmion-based applications. Here, an approach of optical spin manipulation by utilizing the spin-momentum equation is proposed to investigate the spin texture in a photonic skyrmion-pair. With the benefit of the proposed approach, a unique spin texture with spin angular momentum varying linearly along the line connecting the two skyrmion centers is theoretically designed and experimentally verified. The optimized spin texture is then applied in a displacement-sensing system, which is capable of attaining pico-metric sensitivity. Compared with the conventional polarization and phase schemes, the spin-based manipulation mechanism provides a new pathway for optical modulation, which is of great value in nanophotonics from both fundamental and application.

Quantification of the Real Plasmonic Field Transverse Distribution in a Nanocavity Using the Vibrational Stark Effect

Quantifying the real plasmonic field strength experimentally has been long pursued in expanding the applications related to plasmonic enhancement. However, it is still an enormous challenge to determine the inhomogeneous plasmonic field distribution. Here, self-assembled monolayers (SAMs) of 4-mercaptobenzonitrile (MBN) are sandwiched as a gap spacer in a nanoparticle-on-mirror (NPoM) structure, effectively forming ultrahigh field enhancement to observe Stark shifts of the chemical bond. Transverse position-dependent Stark shifts of ν(C═C) and ν(C≡N) in the individual nanocavity measured by surface-enhanced Raman scattering (SERS) experiment combined with the Stark tuning rate by density functional theory (DFT) simulation accurately revealed the inhomogeneous plasmonic field transverse distribution and quantified the transverse plasmonic field strength up to ∼1.9 × 10⁹ V/m, which matches the value predicted by finite element method (FEM) simulation. This work deepens the insight into plasmon-based technologies and will coordinate high-resolution techniques such as tip-enhanced Raman spectroscopy (TESR) to reveal the real plasmonic field distribution.

Spin-orbit interactions of a circularly polarized vortex beam in paraxial propagation

Spin-orbit interactions (SOIs) of circularly polarized beam and circularly polarized vortex beam during paraxial propagation in a radial gradient-index (GRIN) fiber are analyzed using the generalized Huygens-Fresnel principle and the GRIN fiber's ABCD matrix. SAM is only associated with polarized light helicity and OAM is only associated with topological charge m. SAM and OAM do not crosstalk or convert between each other; SOIs did not occur at the GRIN fiber's focal plane. SOIs of partially coherent circularly polarized beam and partially coherent circularly polarized vortex beam in the GRIN fiber are also studied and show the same characteristics as the perfectly polarized beam.

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.

Intrinsic Spin-Momentum Dynamics of Surface Electromagnetic Waves in Dispersive Interfaces

Intrinsic spin-momentum locking is an inherent property of surface electromagnetic fields and its study has led to the discovery of phenomena such as unidirectional guided waves and photonic spin lattices. Previously, dispersion was ignored in spin-momentum locking, resulting in anomalies contradicting the apparent physical reality. Here, we formulate four dispersive spin-momentum equations, revealing in theory that transverse spin is locked with kinetic momentum. Moreover, in dispersive metal or magnetic materials spin-momentum locking obeys the left-hand screw rule. In addition to dispersion, structural features can affect substantially this locking. Remarkably, an extraordinary spin originating from coupling polarization ellipticities is uncovered that depends on the symmetry of the field modes. We further identify the properties of this spin-momentum locking with diverse photonic topological lattices by engineering their rotational symmetry akin to that in solid-state physics. The concept of spin-momentum locking based on photon flow properties translates easily to other classical wave fields.

Spin-orbit periodic conversion in a gradient-index fiber

The characteristics of the cylindrical vector beam (CVB) and the cylindrical vector vortex beam (CVVB) in a radial gradient-index (GRIN) fiber are analyzed on the basis of the generalized Huygens-Fresnel principle. The CVB and CVVB exhibit periodic and stable transmission characteristics in the radial GRIN fiber. In the beam with a vortex phase (CVVB), the polarization changes and the spin angular momentum (SAM) is detected at the focal plane of the radial GRIN fiber. A spin-orbit periodic conversion is observed in the radial GRIN fibers. Finally, the SAM expression of partially coherent light is deduced and verified via a simulation.

Mapping the spin angular momentum distribution of focused linearly and circularly polarized vortex fields

Using the previously proposed spin-resolved near-field scanning optical microscopy (NSOM) technique, we mapped the spin angular momentum (SAM) axial component (S_z) distributions of tightly focused linearly and circularly polarized vortex beams. The system's effectiveness was confirmed in our previous article by mapping various tightly focused cylindrical vector vortex beams. The SAM of different focused vortex light fields is essential in the research of near-field spin optics and topological photonics. The SAM distributions of different orders of linearly and circularly polarized vortex beams were mapped by separating their right spin (I_RCP) and left spin component (I_LCP) using the relationship S_z∝I_RCP-I_LCP.

Photonic Spin Lattices: Symmetry Constraints for Skyrmion and Meron Topologies

Symmetry and topology govern many electronic, magnetic, and photonic phenomena in condensed matter physics and optics, resulting in counterintuitive skyrmion, meron, and other phenomena important for modern technologies. Here we demonstrate photonic spin lattices as a new topological construct governed by the spin-orbit coupling in an optical field. The symmetry of the electromagnetic field in the presence of the spin-orbit interaction may result in only two types of photonic spin lattices: either hexagonal spin-skyrmion or square spin-meron lattices. We show that these spin structures correspond to the lowest energy of the electromagnetic field configuration, therefore, energetically stable. We further show that in the absence of spin-orbit coupling these spin topologies are degenerated in dynamic field skyrmions, unifying the description of electromagnetic field topologies. The results provide a new understanding of electromagnetic field topology and its transformations as well as new opportunities for applications in quantum technologies, spin optics, and topological photonics.