arXiv subjects

Shubo Wang

Publications and source records attributed to Shubo Wang.

At least 19 recordsLinked to original sources

arXiv2026

Janus Dipoles: Fundamentals, Realizations, and Emerging Applications

The Janus dipole - featuring orthogonally oriented electric and magnetic dipoles with a 90-degree phase difference - has emerged as a powerful paradigm for wave manipulation. Unlike traditional Huygens dipoles used for directional control, this unique configuration exhibits strongly asymmetric, face-selective near-field behavior while maintaining a quasi-isotropic far-field radiation pattern. These remarkable properties make the Janus dipole an essential platform for directional wave shaping, with wide-ranging applications in on-chip photonics, quantum interactions, and wireless power transfer. This review systematically traces the rapid development of the Janus dipole from its foundational theoretical inception to its diverse implementation platforms across optical, microwave, and acoustic frequencies. In this paper, we explore the governing principles, classify realization strategies into passive Janus dipoles, active Janus dipoles, and advanced near-field coupling control, and highlight emerging frontiers. By bridging foundational electrodynamics with advanced device engineering, this paper serves as an essential reference and roadmap for researchers designing next-generation, highly integrated, and compact wave-manipulation systems.

physics.app-ph↗

arXiv2026

Dynamic Chirality in Photonic Time Crystals

Temporal modulation offers a fundamentally distinct degree of freedom for active wave control beyond static spatial structuring. Photonic time crystals (PTCs), based on periodic modulation of electromagnetic parameters in time, have expanded photonic band engineering from space to time by enabling controlled energy exchange between light and the modulation. Yet, the use of PTCs to synthesize rotational dynamics and thereby control chirality and circular dichroism (CD) remains largely unexplored. Here, we propose a spatiotemporal PTC whose central cylindrical element is driven by an azimuthally traveling-wave permittivity modulation. Although the structure is geometrically static, its dielectric profile evolves as an effectively rotating pattern in time. This synthetic rotation lifts a static modal degeneracy and produces two nondegenerate counter-rotating states with opposite orbital angular momenta. These chiral modes selectively couple to left- and right-circularly polarized light, giving rise to tunable CD. In addition, the spatiotemporal modulation induces orbital angular-momentum conversion between the Floquet replica bands. Our work reveals the microscopic origin of dynamic chiral response and establishes a strategy for reconfigurable chiral photonics without mechanical motion.

physics.optics↗

arXiv2026

Spatiotemporal Vortex Rings Induced by Spatiotemporal Coupling

Vortices and vortex rings are topological structures that arise in various physical systems. However, the generation of spatiotemporal vortices (STVs) and vortex rings (STVRs) has so far relied on complex, often active wavefront modulation. We theoretically and experimentally demonstrate that spatiotemporal coupling can drive unstructured wave packets to form vortices upon scattering from simple obstacles. The resulting STVs and STVRs possess controllable topological charges and excellent propagation stability. These findings reveal a fundamental mechanism for spatiotemporal singularity formation and provide a universal route to structured-wave generation.

physics.class-ph↗

arXiv2026

Chiral switching of elastic spin via dynamic encirclement of exceptional points

Dynamically encircling exceptional points (EPs) enables chiral state conversion in classical wave systems. However, whether this mechanism can be extended to chiral spin conversion has remained elusive. Here we demonstrate chiral switching of elastic spin via dynamic encirclement of EPs in a non-Hermitian micropolar (Cosserat) metamaterial. The interplay between micropolar chirality and anisotropic loss generates EPs with a nontrivial Riemann-sheet topology. Encircling these EPs converts the elastic spin, with the final spin sign dictated solely by the handedness of the encircling trajectory. Our results establish a fundamental route for the selective manipulation of elastic spin, opening avenues for non-Hermitian spin phononics and broader applications in other wave systems.

physics.class-ph↗

arXiv2026

Real-space Hybrid Topological Singularities in Structured Elastic Waves

Real-space singularities govern a broad spectrum of wave phenomena, yet they remain largely unexplored in elastic wave systems. Here, we report hybrid topological singularities that emerge on the surfaces of finite-sized solids due to the full vectorial character of elastic waves. These textures fuse spin-field singularities with displacement-field singularities and exhibit unique non-pairwise topological charge dynamics. Moreover, a subset of these singularities imprint dislocation lines onto the radiated acoustic field, generating robust acoustic vortices in free space from an otherwise achiral source and structure. Our results establish elastic waves as a powerful platform for engineering real-space singularities and open avenues for singular phononics and the exploration of rich topological defects in elastic media.

physics.app-ph↗

arXiv2026

Strong optical nonreciprocity in a photonic crystal composed of spinning cylinders

Moving media break time-reversal symmetry and exhibit intriguing optical nonreciprocity. This nonreciprocity is usually weak due to the much lower moving speed of media relative to the speed of light. We demonstrate that strong optical nonreciprocity can emerge in a two-dimensional photonic crystal composed of spinning dielectric cylinders. The photonic crystal supports two types of chiral modes at the Brillouin zone center: hybridized multipole modes and symmetry-protected bound states in the continuum (BICs), both of which carry intrinsic spin angular momentum. For finite wavevectors near the zone center, the BICs transform into quasi-bound states in the continuum (QBICs). Under oblique incidence of circularly polarized plane waves, the photonic crystal exhibits nonreciprocal transmission and absorption that are significantly enhanced at the frequencies of these hybridized multipole modes and QBICs. Furthermore, the high quality factors of the QBICs enable sharp transitions in nonreciprocity. Our work uncovers strong chiral light-matter interactions in periodic moving structures, with potential applications in nonreciprocal light manipulation. The mechanism may also be generalized to other classical wave systems, such as phononic crystals.

physics.optics↗

arXiv2026

Deep learning assisted inverse design of nonreciprocal multilayer photonic structures

Nonreciprocal structures play an important role in optical physics and applications. Conventional approaches for designing nonreciprocal optical structures rely heavily on extensive numerical simulation and parameter tuning, leading to high computational cost and low efficiency. Here, we apply deep learning to the design of nonreciprocal multilayer photonic structures. Three neural-network models-a forward neural network (FNN), an inverse design network (IDN), and a variational autoencoder (VAE)-are employed to learn the complex mapping between structural/material parameters and nonreciprocal spectral characteristics. We show that the FNN can rapidly and accurately predict the nonreciprocal electromagnetic response of a given structure, while the IDN can directly generate suitable structural parameters for target spectral responses. Both approaches substantially reduce computational cost and design time while improving nonreciprocal performance. Furthermore, the VAE can generate band-limited inverse design under practical performance constraints, facilitating efficient exploration of multiple feasible structures that meet different threshold requirements within specified frequency bands. Our work highlights the potential of deep learning for the advanced design of nonreciprocal optical structures and devices.

physics.optics↗

arXiv2025

Acoustic helical dichroism enhanced by chiral quasi-bound states in the continuum

Acoustic helical dichroism (HD) arises from the interaction between vortex beams carrying orbital angular momentum (OAM) and chiral media, yet such chiral sound-matter interactions are typically weak. Here, we employ quasi-bound states in the continuum (QBICs) in acoustic meta-cavities composed of coupled Helmholtz resonators to enhance acoustic HD. We design both achiral and chiral meta-cavities that support QBICs in the form of vortex states with high Q-factors. Using full-wave numerical simulations, we show that the QBICs in the achiral meta-cavities cannot enhance acoustic HD due to the absence of a chiral wavefront. In contrast, the chiral meta-cavity exhibits a pronounced HD enhancement through the QBICs with a 3D helical wavefront, which can be excited by incident waves either with or without OAM. Our work identifies two essential requirements for enhancing acoustic HD effect via QBICs: a high Q-factor of the states and 3D chirality of the state fields, which usually compromise each other in conventional acoustic resonators. The findings open new avenues for achieving strong chiral sound-matter interactions, with potential applications in acoustic chiral sensing and acoustic OAM manipulation.

physics.app-ph↗

arXiv2025

Riemann-Silberstein geometric phase in 4D polarization space

Geometric phase is a far-reaching concept in quantum and classical physics. The first discovered geometric phase, the Pancharatnam-Berry (PB) phase, has profoundly shaped nanophotonics through metasurfaces. However, the PB phase arises from SU(2) polarization evolution and is constrained to a 2D polarization space, failing to capture the full polarization degrees of freedom. We generalize geometric phase to the 4D Riemann-Silberstein (RS) space that simultaneously describes electric, magnetic, and hybrid electric-magnetic polarizations. We show that SU(4) polarization evolution can generate a new geometric phase, the RS phase, alongside the PB phase. Unlike the PB phase that typically manifests in circularly polarized light, the RS phase can emerge in arbitrarily polarized light. Together, they enable a high-dimensional geometric framework for light propagation across general interfaces. We reveal that the phase shifts governed by Fresnel equations are direct manifestations of the RS-space geometric phases, integrating a century-old wave theory into this paradigm. We experimentally validate the framework using metasurfaces and achieve high-dimensional wavefront manipulation. Our work offers fundamental insights into the geometric nature of light-matter interactions, with implications for topological and non-Abelian physics in classical wave systems.

physics.optics↗

arXiv2025

Optical force and torque on a spinning dielectric sphere

Optical force can enable precise manipulations of small particles for various applications. It is well known that an isotropic lossless dielectric sphere is only subject to forward optical force under the illumination of an electromagnetic plane wave. By using rigorous full-wave simulations, we show that such a sphere can experience a lateral optical force and an optical torque besides the conventional longitudinal force, if it spins with a constant angular velocity. The emergence of the unusual optical force and torque is attributed to the breaking of mirror and cylindrical symmetries by the spinning motion. Using the multipole expansion in source representation, we illustrate how the spinning-induced effective bi-anisotropy generates the lateral force and torque on the sphere through the interference of electric and magnetic multipoles. We also uncover the effect of Sagnac frequency splitting on the optical force and torque. The results contribute to the understanding of the optical force and torque in moving media and can be applied to realize unconventional optical manipulations of small particles.

physics.optics↗

arXiv2024

Deep learning assisted SERS detection of prolines and hydroxylated prolines using nitrilotriacetic acid functionalized gold nanopillars

Proline (Pro) is one kind of proteinogenic amino acid and an important signaling molecule in the process of metabolism. Hydroxyproline (Hyp) is a product on Pro oxygen sensing post-translational modification (PTM), which is efficiently modulated tumor cells for angiogenesis. Distinguishing between Pro and Hyp is crucial for diagnosing connective tissue disorders, as elevated levels of Hyp can indicate abnormal collagen metabolism, often associated with diseases like osteogenesis imperfecta or fibrosis. However, there is a very small difference between molecular structures of Pro and Hyp, which is a big challenge for current detection technologies to distinguish them. For surface-enhanced Raman scattering (SERS) sensors, the similar molecule structure leads to similar Raman spectra that are difficult to distinguish. Furthermore, another problem is the weak affinity between amino acids sample and SERS-active substrates by physical adsorption. The selecting capturing of Pro and Hyp in the mixture of amino acids is not easy to achieve. In this work, we designed a new method for Pro and Hyp specifical detection and recognition by using gold nanopillars as the SERS substrate and combing nitrilotriacetic acid (NTA) with nickel (Ni) to form NTA-Ni structure as a specifical affinity agent. One side of NTA-Ni was attached to gold nanopillars through thiol binding. Another side captured the amino acids using reversible binding by receptor-ligand interaction between Ni and amino acids. Because of the different binding time with NTA-Ni and amino acids, the sensor can recognize Pro and Hyp from amino acids mixture. Then we used automatic peak assignment program for data analysis and machine learning model to distinguish between Pro and Hyp. The label-free SERS detection of amino acids PTM using gold nanopillars provides a potential method to further biomolecule detection and specifical capture.

q-bio.BM↗

arXiv2024

Optical polarization singularities in metallic cavities excited by electric dipole sources

Optical polarization singularities (PSs) in real space carry rich topological properties and can enable highly precise manipulations of light fields. Conventional studies focus on the PSs in the open space of optical systems. The properties of PSs inside optical cavities remain largely unexplored. By using full-wave finite-element simulations, we investigate the optical PSs inside metallic cavities excited by electric dipole sources. We determine the topological indices, morphology, and spatial evolutions of the singularities inside both spherical and torus cavities. We uncover the relationship between spatial symmetries and the PSs, as well as the mechanism underlying the emergence of polarization Mobius strips in the spherical cavity. In addition, we investigate the topological transitions of the PSs connecting two geometries (i.e., sphere and torus) with distinct topologies. The results provide insight into the singular and topological properties of light fields in optical cavities and can find applications in optical sensing and optical manipulation of small particles.

physics.optics↗

arXiv2024

Nonreciprocal optical metasurface based on spinning cylinders

Optical systems breaking Lorentz reciprocity have attracted broad attention due to their intriguing physics and applications. Nonreciprocal metasurfaces can enable one-way light transmission and reflection with essential applications in optical communication. Conventional nonreciprocal metasurfaces rely on using magneto-optic or nonlinear materials to induce nonreciprocal optical properties. Here, we propose and demonstrate a new mechanism for realizing nonreciprocal metasurfaces based on the relativistic effect of a moving medium. The metasurface is composed of periodic spinning dielectric cylinders located above a dielectric substrate. The spinning motion breaks the time-reversal symmetry and induces bi-anisotropic Tellegen-type response of the meta-atoms. We show that the metasurface can realize both asymmetric and nonreciprocal manipulations of the incident plane wave. The underlying mechanism is attributed to the Sagnac effect associated with the chiral multipole modes of the coupled spinning cylinders. By introducing dielectric pillars to modulate the phase profile, the metasurface can enable nonreciprocal wavefront manipulations. Our work offers a new mechanism for realizing nonreciprocal light manipulation in free space. The proposed metasurface can serve as a platform to explore the interesting physics of nonreciprocal optics, non-Hermitian optics, and topological photonics.

physics.optics↗

arXiv2024

Directional sources realised by toroidal dipoles

Directional optical sources can give rise to the directional excitation and propagation of light. The directionality of the conventional directional dipole (CDD) sources are attributed to the interference of the electric and/or magnetic dipoles, while the effect of the toroidal dipole on optical directionality remains unexplored.} Here, we numerically and analytically investigate the directional properties of the toroidal dipole. We show that the toroidal dipole can replace the electric dipole in the CDD sources to form the pseudo directional dipoles (PDDs), which can be applied to achieve analogous near-field directional coupling with a silicon waveguide. Moreover, the directionality of the PDDs can be flexibly controlled by changing the geometric parameters of the toroidal dipole, leading to tunable asymmetric coupling between the sources and the waveguide. These new types of directional sources provide more degrees of freedom for tailoring the optical directionality compared to the conventional sources. The results open new possibilities for directional light manipulation and can find applications in on-chip optical routing, waveguiding, and nanophotonic communications.

physics.optics↗

arXiv2024

Acoustic Pancharatnam-Berry Geometric Phase

Geometric phases provide a unified framework for understanding diverse phenomena in quantum and classical physics. The Pancharatnam-Berry (PB) geometric phase, arising from variation of optical transverse polarization, has transformed light manipulation. However, this phase has never been observed in sound waves due to their curl-free longitudinal nature. Here, we theoretically and experimentally demonstrate that the PB phase can emerge in general inhomogeneous sound waves with polarization evolution of velocity field. Using surface sound waves as an example, we uncover the intriguing Janus property of the PB phase arising from spin-momentum locking, and realize acoustic PB metasurfaces for versatile wavefront manipulation. We further extend the mechanism to free-space structured sound and realize acoustic $q$-plate for generating acoustic vortices through spin-orbit interaction. Our work provides new insights into sound wave properties and enables the manipulation of inhomogeneous acoustic fields via the PB phase, with potential applications in acoustic communications and imaging.

physics.class-ph↗

arXiv2024

A novel particle-in-well technology for single-molecule sequencing by surface-enhanced Raman spectroscopy

Single-molecule surface-enhanced Raman spectroscopy based on a particle trapped in a plasmonic nanopores provides a unique method for continued and controlled detection of peptide and DNA oligonucleotides in liquid medium. However, the Brownian motion of the particle and the molecule diffusion acting on the particle hinder single-molecule sequencing. In this study, we developed a method for trapping a gold nanoparticle in an air-filled gold nanowell (particle-in-well) to stabilize the particle and provide a powerful platform for continuous single molecule readout. The unlimited resident time of the particle-in-well device with single-molecule level sensitivity elevates nucleobase detection to a new level. We present a technique capable of detecting and monitoring solid-phase molecule diffusion within the plasmonic hotspot. Furthermore, the measured spectra were employed as input data for the validation of the plasmonic hotspot size and, consequently, the distance between the particle and the well. The obtained results form the statistical and experimental base for molecular translocation and DNA sequencing technologies.

physics.app-ph↗

arXiv2024

Topological Dark Spots of Electric Near Field in Metal Structures

Electric dark spots are point singularities at which the electric field amplitude vanishes. These singularities usually emerge in real space accidentally and are unstable due to the vectorial property of the electric field. In this paper, we show that topologically protected electric dark spots can emerge in metal scatterers under external excitation. The material property of metal imposes a boundary condition that reduces the vectorial electric field on the metal surface to a scalar field. The phase singularity of this scalar field has zero amplitude and carries a well-defined topological charge corresponding to an electric dark spot. The topological electric dark spots give rise to the superoscillation phenomenon with a divergent local wavenumber. We uncover the global charge conservation property of the dark spots on the scatterers' surfaces and demonstrate their stability under different perturbations. We also demonstrate the manipulation of the dark spots' topological charge and spatial location. The results open a new avenue for nanophotonic near-field manipulations and may find applications in optical metrology, optical sensing, and super-resolution imaging.

physics.optics↗

arXiv2024

Extinction of guided light induced by coupled spiral meta-atom resonators at arbitrary order exceptional points

Exceptional points (EPs) in non-Hermitian systems can give rise to intriguing effects not available in conventional Hermitian systems due to their unusual properties. Using full-wave simulations, we investigate the scattering, absorption, and transmission of guided light at arbitrary order exceptional points in a non-Hermitian system consisting of coupled spiral meta-atom resonators sitting on a dielectric waveguide. The EPs are realized by exploiting the unidirectional coupling of the chiral dipole modes in the subwavelength meta-atom resonators. The scattering and absorption of the resonators induce the attenuation of the guided light in the dielectric waveguide. We show that the EPs can give rise to a plateau in the attenuation spectrum of the guided light with near-zero transmission, i.e., the guided light is almost completely dissipated via the resonators in the forms of intrinsic material loss and radiation loss. In addition, the width of this plateau (i.e., the extinction bandwidth) increases with the order of the EP. The phenomena can be understood by employing a coupled mode analysis, with the analytical results quantitatively agree with the numerical results. The results may find applications in designing novel on-chip optical absorbers and sensors.

physics.class-ph↗