Transverse spin dynamics in structured electromagnetic guided waves

Gradient and curl optical torques

Optical forces and torques offer the route towards full degree-of-freedom manipulation of matter. Exploiting structured light has led to the discovery of gradient and curl forces, and nontrivial optomechanical manifestations, such as negative and lateral optical forces. Here, we uncover the existence of two fundamental torque components, which originate from the reactive helicity gradient and momentum curl of light, and which represent the rotational analogues to the gradient and curl forces, respectively. Based on the two components, we introduce and demonstrate the concept of lateral optical torques, which act transversely to the spin of illumination. The orbital angular momentum of vortex beams is shown to couple to the curl torque, promising a path to extreme torque enhancement or achieving negative optical torques. These results highlight the intersection between the areas of structured light, Mie-tronics and rotational optomechanics, even inspiring new paths of manipulation in acoustics and hydrodynamics.

A decomposition of light's spin angular momentum density

Light carries intrinsic spin angular momentum (SAM) when the electric or magnetic field vector rotates over time. A familiar vector equation calculates the direction of light's SAM density using the right-hand rule with reference to the electric and magnetic polarisation ellipses. Using Maxwell's equations, this vector equation can be decomposed into a sum of two distinct terms, akin to the well-known Poynting vector decomposition into orbital and spin currents. We present the first general study of this spin decomposition, showing that the two terms, which we call canonical and Poynting spin, are chiral analogies to the canonical and spin momenta of light in its interaction with matter. Like canonical momentum, canonical spin is directly measurable. Both canonical and Poynting spin incorporate spatial variation of the electric and magnetic fields and are influenced by optical vortices. The decomposition allows us to show that a linearly polarised vortex beam, which has no total SAM, can nevertheless exert longitudinal chiral pressure due to equal and opposite canonical and Poynting spins.

Dynamics of polarization-tuned mirror symmetry breaking in a rotationally symmetric system

Lateral momentum conservation is typically kept in a non-absorptive rotationally symmetric system through mirror symmetry via Noether's theorem when illuminated by a homogeneous light wave. Therefore, it is still very challenging to break the mirror symmetry and generate a lateral optical force (LOF) in the rotationally symmetric system. Here, we report a general dynamic action in the SO(2) rotationally symmetric system, originating from the polarization-tuned mirror symmetry breaking (MSB) of the light scattering. We demonstrate theoretically and experimentally that MSB can be generally applied to the SO(2) rotationally symmetric system and tuned sinusoidally by polarization orientation, leading to a highly tunable and highly efficient LOF (9.22 pN/mW/μm-2) perpendicular to the propagation direction. The proposed MSB mechanism and LOF not only complete the sets of MSB of light-matter interaction and non-conservative force only using a plane wave but also provide extra polarization manipulation freedom.

Optomagnetism with a plasmonic skyrmion

Research at the frontier between optics and magnetism is revealing a wealth of innovative phenomena and avenues of exploration. Optical waves are demonstrating the capacity to induce ultrafast magnetism, while optical analogs of magnetic states, such as magnetic skyrmions, offer the prospect of novel, to the best of our knowledge, spin-optical states. In this Letter, we strengthen the synergy between light and magnetism by exploring the ability of plasmonic Neel skyrmions to create an optomagnetic field, i.e., an opto-induced stationary magnetic field, within a thin gold film. We show that, when generated using a focused radially polarized vortex beam (RPVB), a plasmonic Neel skyrmion emerges as an optimum for inducing optomagnetism in a thin gold film. Optical skyrmions offer new degrees of freedom for enhancing and controlling optomagnetism in plasmonic nanostructures, with direct application in all-optical magnetization switching, magnetic recording, and the excitation of spin waves.

Generation of pure transverse spin and nontrivial polarization structures of beams by dielectric metasurface

Transverse spin, a spin component with unique characteristics, provides a new dimension for plenty of applications, such as optical trapping, imaging, and communication. Here, we analyze the pure transverse spin in the Bessel beam, which is solely present in the azimuthal direction. Based on a single layer dielectric metasurface, we efficiently generate Bessel beams with pure transverse spin in a compact optical system. As designed, the transverse spin is flexibly tunable by converting the polarization of the incident light. Furthermore, in the scattered Bessel beam, the local electromagnetic field oscillates around the transverse axis, which is perpendicular to the beam propagation. At certain positions, the local polarization ellipse degenerates into a perfect circle, resulting in a ring-periodic distribution of circularly polarized points (C points) in the beam. This suggests that the local polarization demonstrates a nontrivial periodic structure. This work deepens our understanding of spin-related physics and opens a new avenue for the study and application of transverse spins in ultracompact, flat, multifunctional nanophotonic platforms.

On-chip optical wavefront shaping by transverse-spin-induced Pancharatanam-Berry phase

Pancharatnam-Berry (PB) metasurfaces can be applied to manipulate the phase and polarization of light within subwavelength thickness. The underlying mechanism is attributed to the geometric phase originating from the longitudinal spin of light. Here, we demonstrate, to the best of our knowledge, a new type of PB geometric phase derived from the intrinsic transverse spin of guided light. Using full-wave numerical simulations, we show that the rotation of a metallic nano-bar sitting on a metal substrate can induce a geometric phase covering 2π full range for the surface plasmons carrying an intrinsic transverse spin. Especially, the geometric phase is different for the surface plasmons propagating in opposite directions due to spin-momentum locking. We apply the geometric phase to design metasurfaces to manipulate the wavefront of surface plasmons to achieve steering and focusing. Our work provides a new mechanism for on-chip light manipulations with potential applications in designing ultra-compact optical devices for imaging and sensing.

Exciting space-time surface plasmon polaritons by irradiating a nanoslit structure

Space-time (ST) wave packets are propagation-invariant pulsed optical beams that travel freely in dielectrics at a tunable group velocity without diffraction or dispersion. Because ST wave packets maintain these characteristics even when only one transverse dimension is considered, they can realize surface-bound waves (e.g., surface plasmon polaritons at a metal-dielectric interface, which we call ST-SPPs) that have the same unique characteristics as their freely propagating counterparts. However, because the spatiotemporal spectral structure of ST-SPPs is key to their propagation invariance on the metal surface, their excitation methodology must be considered carefully. Using finite-difference time-domain simulations, we show that an appropriately synthesized ST wave packet in free space can be coupled to an ST-SPP via a single nanoscale slit inscribed in the metal surface. Our calculations confirm that this excitation methodology yields surface-bound ST-SPPs that are localized in all dimensions (and can thus be considered as plasmonic "bullets"), which travel rigidly at the metal-dielectric interface without diffraction or dispersion at a tunable group velocity.

Topological state transitions of skyrmionic beams under focusing configurations

The recent emerging appearance of optical analogs of magnetic quasiparticles, i.e., optical skyrmions constructed via spin, field, and Stokes vectors, has garnered substantial interest from deep-subwavelength imaging and quantum entanglement. Here, we investigate systematically the topological state transitions of skyrmionic beams constructed by the Stokes vectors in the focusing configuration. We theoretically demonstrated that in the weak focusing, the skyrmion topological number is protected. Whereas, in the tight focusing, a unique topological transformation with skyrmion number variation is exhibited for the optical skyrmion, anti-skyrmion, and 2nd-order skyrmion structures. The significant difference between the topological state transitions of these two cases originates from the transformation from the paraxial optical system to the nonparaxial optical system, and the approximate two-dimensional polarization structure to the three-dimensional polarization structure. The results provide new insights into the topological state transitions in topological structures, which promote applications in information processing, data storage, and free-space optical communications.

Polarization characteristics and transverse spin of Mie scattering

Complicated polarization states in the near field of Mie scattering have aroused wide interest due to their broad potential applications. In this work, we investigated polarization properties, including polarization dimension, degree of nonregularity, and transverse electric-field spin, of scattering of a partially polarized plane wave by a dielectric nanosphere based on the rigorous Mie scattering theory. It is shown that with the decrease of the correlation coefficient, the polarization dimension and degree of nonregularity generally increase. In the limit of unpolarized incident light, a nearly-perfect nonregular polarization state (PN = 0.928) appears in the near field and the spin is transverse to the radial direction everywhere. The rich structure contained by the partially polarized scattered light offers an approach to manipulating the interaction between light and nanoparticles, which may lead to novel designs of nanoantenna, optical trap and sensing.

Optomechanical effects caused by non-zero field quantities in multiple evanescent waves

Evanescent waves, with their high energy density, intricate local momentum, and spatial distribution of spins, have been the subject of extensive recent study. These waves offer promising applications in near-field particle manipulation. Consequently, it becomes imperative to gain a deeper understanding of the impacts of scattering and gradient forces on particles in evanescent waves to enhance and refine the manipulation capabilities. In this study, we employ the multipole expansion theory to present analytical expressions for the scattering and gradient forces exerted on an isotropic sphere of any size and composition in multiple evanescent waves. The investigation of these forces reveals several unusual optomechanical phenomena. It is well known that the scattering force does not exist in counter-propagating homogeneous plane waves. Surprisingly, in multiple pairs of counter-propagating evanescent waves, the scattering force can arise due to the nonzero orbital momentum (OM) density and/or the curl part of the imaginary Poynting momentum (IPM) density. More importantly, it is found that the optical scattering force can be switched on and off by simply tuning the polarization. Furthermore, optical forces typically vary with spatial position in an interference field. However, in the interference field generated by evanescent waves, the gradient force becomes a spatial constant in the propagating plane as the particle's radius increases. This is attributed to the decisive role of the non-interference term of the electromagnetic energy density gradient. Our study establishes a comprehensive and rigorous theoretical foundation, propelling the advancement and optimization of optical manipulation techniques harnessed through multiple evanescent waves. Specifically, these insights hold promise in elevating trapping efficiency through precise control and manipulation of optical scattering and gradient forces, stimulating further explorations.

Spin-orbit interactions in plasmonic crystals probed by site-selective cathodoluminescence spectroscopy

The study of spin-orbit coupling (SOC) of light is crucial to explore the light-matter interactions in sub-wavelength structures. By designing a plasmonic lattice with chiral configuration that provides parallel angular momentum and spin components, one can trigger the strength of the SOC phenomena in photonic or plasmonic crystals. Herein, we explore the SOC in a plasmonic crystal, both theoretically and experimentally. Cathodoluminescence (CL) spectroscopy combined with the numerically calculated photonic band structure reveals an energy band splitting that is ascribed to the peculiar spin-orbit interaction of light in the proposed plasmonic crystal. Moreover, we exploit angle-resolved CL and dark-field polarimetry to demonstrate circular-polarization-dependent scattering of surface plasmon waves interacting with the plasmonic crystal. This further confirms that the scattering direction of a given polarization is determined by the transverse spin angular momentum inherently carried by the SP wave, which is in turn locked to the direction of SP propagation. We further propose an interaction Hamiltonian based on axion electrodynamics that underpins the degeneracy breaking of the surface plasmons due to the spin-orbit interaction of light. Our study gives insight into the design of novel plasmonic devices with polarization-dependent directionality of the Bloch plasmons. We expect spin-orbit interactions in plasmonics will find much more scientific interests and potential applications with the continuous development of nanofabrication methodologies and uncovering new aspects of spin-orbit interactions.

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.

Observation of phononic skyrmions based on hybrid spin of elastic waves

Skyrmions with topologically stable configuration have shown a promising route toward high-density magnetic and photonic information processing due to their defect-immune and low-driven energy. Here, we experimentally report and observe the existence of phononic skyrmions as new topological structures formed by the three-dimensional hybrid spin of elastic waves. We demonstrate that the frequency-independent spin configuration leads to ultra-broadband feature of phononic skyrmions, which can be produced in any solid structure, including chip-scale ones. We further experimentally show the excellent robustness of the flexibly movable phononic skyrmion lattices against local defects of disorder, sharp corners, and even rectangular holes. Our research opens a vibrant horizon toward an unprecedented way for elastic wave manipulation and structuration by spin configuration and offers a promising lever for alternative phononic technologies, including information processing, biomedical testing, and wave engineering.

Metastability of photonic spin meron lattices in the presence of perturbed spin-orbit coupling

Photonic skyrmions and merons are topological quasiparticles characterized by nontrivial electromagnetic textures, which have received increasing research attention recently, providing novel degree of freedom to manipulate light-matter interactions and exhibiting excellent potential in deep-subwavelength imaging and nanometrology. Here, the topological stability of photonic spin meron lattices, which indicates the invariance of skyrmion number and robustness of spin texture under a continuous deformation of the field configuration, is demonstrated by inducing a perturbation to break the C₄ symmetry in the presence spin-orbit coupling in an optical field. We revealed that amplitude perturbation would result in an amplitude-dependent shift of spin center, while phase perturbation leads to the deformation of domain walls, manifesting the metastability of photonic meron. Such spin topology is verified through the interference of plasmonic vortices with a broken rotational symmetry. The results provide new insights on optical topological quasiparticles, which may pave the way towards applications in topological photonics, optical information storage and transfer.

Stable optical lateral forces from inhomogeneities of the spin angular momentum

Transverse spin momentum related to the spin angular momentum (SAM) of light has been theoretically studied recently and predicted to generate an intriguing optical lateral force (OLF). Despite extensive studies, there is no direct experimental evidence of a stable OLF resulting from the dominant SAM rather than the ubiquitous spin-orbit interaction in a single light beam. Here, we theoretically unveil the nontrivial physics of SAM-correlated OLF, showing that the SAM is a dominant factor for the OLF on a nonabsorbing particle, while an additional force from the canonical (orbital) momentum is exhibited on an absorbing particle due to the spin-orbit interaction. Experimental results demonstrate the bidirectional movement of 5-μm-diameter particles on both sides of the beam with opposite spin momenta. The amplitude and sign of this force strongly depend on the polarization. Our optofluidic platform advances the exploitation of exotic forces in systems with a dominant SAM, facilitating the exploration of fascinating light-matter interactions.

Transverse spin and the hidden vorticity of propagating light fields

Spatially inhomogeneous fields of electromagnetic guided modes exhibit a complex of extraordinary dynamical properties such as polarization-dependent transverse momentum, helicity-independent transverse spin, spin-associated non-reciprocity and unidirectional propagation, etc. Recently, the remarkable relationship has been established between the spin and propagation features of such fields, expressed through the spin-momentum equations [Proc. Natl. Acad. Sci. USA118, e2018816118 (2021) PNASA60027-842410.1073/pnas.2018816118] connecting the wave spin with the curl of momentum. Here, the meaning, limitations, and specific forms of this correspondence are further investigated, involving physically transparent and consistent examples of paraxial light fields, plane-wave superpositions, and evanescent waves. The conclusion is inferred that the spin-momentum equation is an attribute of guided waves with a well-defined direction of propagation, and it unites the helicity-independent "extraordinary" transverse spin with the spatially inhomogeneous longitudinal field momentum (energy flow) density. Physical analogies with the layered hydrodynamic flows and possible generalizations for other wave fields are discussed. The results can be useful in optical trapping, manipulation, and data processing techniques.

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.

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.

Long-Range Directional Routing and Spatial Selection of High-Spin-Purity Valley Trion Emission in Monolayer WS2

Valley-dependent excitation and emission in transition metal dichalcogenides (TMDCs) have recently emerged as a new avenue for optical data manipulation, quantum optical technologies, and chiral photonics. The valley-polarized electronic states can be optically addressed through photonic spin-orbit interaction of excitonic emission, typically with plasmonic nanostructures, but their performance is limited by the low quantum yield of neutral excitons in TMDC multilayers and the large Ohmic loss of plasmonic systems. Here, we demonstrate a valleytronic system based on the trion emission in high-quantum-yield WS₂ monolayers chirally coupled to a low-loss microfiber. The integrated system uses the spin properties of the waveguided modes to achieve long-range directional routing of valley excitations and also provides an approach to selectively address valley-dependent emission from different spatial locations around the microfiber. This valleytronic interface can be integrated with fiber communication devices, allowing for merging valley polarization and chiral photonics as an alternative mechanism for optical information transport and manipulation in classical and quantum regimes.