Vortex Chirality Research Articles

Moiré polar vortex, flat bands, and Lieb lattice in twisted bilayer BaTiO3.

Through first-principles calculations based on density functional theory, we investigate the crystal and electronic structures of twisted bilayer BaTiO3. Our findings reveal that large stacking fault energy leads to a chiral in-plane vortex pattern that was recently observed in experiments. We also found nonzero out-of-plane local dipole moments, indicating that the strong interlayer interaction might offer a promising strategy to stabilize ferroelectric order in the two-dimensional limit. The vortex pattern in the twisted BaTiO3 bilayer supports localized electronic states with quasi-flat bands, associated with the interlayer hybridization of oxygen pz orbitals. We found that the associated bandwidth reaches a minimum at ∼19∘ twisting, configuring the largest magic angle in moiré systems reported so far. Further, the moiré vortex pattern bears a notable resemblance to two interpenetrating Lieb lattices and the corresponding tight-binding model provides a comprehensive description of the evolution the moiré bands with twist angle and reveals the topological nature.

Mechanical Writing of Polar Skyrmionic Topological States via Extrinsic Dzyaloshinskii-Moriya-like Flexoelectricity in Ferroelectric Thin Films.

Exploring complex topological structures in condensed matter has shown promising applications in nanotechnology. Although polar topologies such as chiral vortices and skyrmions have been observed in ferroelectric heterostructures, their existence in simple systems has posed challenges due to the absence of intrinsic noncollinear interaction (like Dzyaloshinskii-Moriya interaction in ferromagnetics). Here, we demonstrate that a nanoindentation mechanically switches local polarizations to stable polar topologies, including skyrmions, within a room-temperature PbTiO3 thin film via the flexoelectric effect as a noncollinear (Dzyaloshinskii-Moriya-like) driving force using phase-field simulations. In addition, by moving the indenter, the continuous polarization switching leads to the "writing" of arbitrary polar patterns (such as donut-like skyrmionium). Furthermore, the written topologies can be "erased" by applying a voltage with the same conducted indenter. Therefore, this study shows the writing and erasing process of room-temperature polar topologies in a ferroelectric thin film, which significantly advances their potential applications.

Current-controlled periodic double-polarity reversals in a spin-torque vortex oscillator

Micromagnetic simulations are used to study a spin-torque vortex oscillator excited by an out-of-plane dc current. The vortex core gyration amplitude is confined between two orbits due to periodical vortex core polarity reversals. The upper limit corresponds to the orbit where the vortex core reaches its critical velocity triggering the first polarity reversal which is immediately followed by a second one. After this double polarity reversal, the vortex core is on a smaller orbit that defines the lower limit of the vortex core gyration amplitude. This double reversal process is a periodic phenomenon and its frequency, as well as the upper and lower limit of the vortex core gyration, is controlled by the input current density while the vortex chirality determines the apparition of this confinement regime. In this non-linear regime, the vortex core never reaches a stable orbit and thus, it can be of interest for neuromorphic application as a leaky integrate-and-fire neuron for example.

Polar and quasicrystal vortex observed in twisted-bilayer molybdenum disulfide.

We report the observation of an electric field in twisted-bilayer molybdenum disulfide (MoS2) and elucidate its correlation with local polar domains using four-dimensional scanning transmission electron microscopy (4D-STEM) and first-principles calculations. We reveal the emergence of in-plane topological vortices within the periodic moiré patterns for both commensurate structures at small twist angles and the incommensurate quasicrystal structure that occurs at a 30° twist. The large-angle twist leads to mosaic chiral vortex patterns with tunable characteristics. A twisted quasicrystal bilayer, characterized by its 12-fold rotational symmetry, hosts complex vortex patterns and can be manipulated by picometer-scale interlayer displacement. Our findings highlight that twisting 2D bilayers is a versatile strategy for tailoring local electric polar vortices.

Electric Field-Manipulated Optical Chirality in Ferroelectric Vortex Domains.

Manipulating optical chirality via electric fields has garnered considerable attention in the realm of both fundamental physics and practical applications. Chiral ferroelectrics, characterized by their inherent optical chirality and switchable spontaneous polarization, are emerging as a promising platform for electronic-photonic integrated circuits applications. Unlike organics with chiral carbon centers, integrating chirality into technologically mature inorganic ferroelectrics has posed a long-standing challenge. Here, the successful introduction of chirality is reported into self-assembly La-doped BiFeO3 nanoislands, which exhibit ferroelectric vortex domains. By employing synergistic experimental techniques with piezoresponse force microscopy and nonlinear optical second-harmonic generation probes, a clear correlation between chirality and polarization configuration within these ferroelectric nanoislands is established. Furthermore, the deterministic control of ferroelectric vortex domains and chirality is demonstrated by applying electric fields, enabling reversible and nonvolatile generation and elimination of optically chiral signals. These findings significantly expand the repertoire of field-controllable chiral systems and lay the groundwork for the development of innovative ferroelectric optoelectronic devices.

Numerical Derivation of Dual Vortex Beam Generation using Polarization‐Sensitive Dielectric Metasurfaces

AbstractMetasurfaces, composed of arranged nanoscale particles, manipulate electromagnetic waves for tailored physical properties. Recently vortex beams, carrying orbital angular momentum, have been generated through metasurfaces to realize diverse applications. Here, the study introduces a metasurface capable of generating dual‐mode vortex beams, which combines the functionalities of chiral metalenses and vortex beam generation. These dual‐mode vortex beams exhibit varying characteristics depending on the polarization state of the incident light, offering improved control over orbital angular momentum. This advancement holds promise for enhancing applications such as optical communication, optical tweezers, and imaging for overcoming diffraction limit. By employing titanium dioxide (TiO2) for its efficiency, the design concept is validated through simulations and discuss considerations for fabrication. The proposed approach paves the way for compact optical systems with heightened adaptability.

Generation of chiral optical vortex lattice for controlled aggregation of particles

The chiral light field has attracted great attention owing to its interaction with chiral matter. The generation of chiral light fields with rich structures has become crucial as it can expand application scenarios. Herein, we introduce a chiral optical vortex lattice. As a whole, the optical vortex lattice has a chiral intensity distribution, with each spiral arm having sub-vortices (chiral phase). By using an expansion factor to adjust the involute of a circular lattice, this helical optical vortex lattice can be continuously varied from a circular lattice. The chirality of intensity and phase can be controlled independently. Furthermore, the optical tweezers using the lattice demonstrate the capability of sub-vortices to manipulate particle movement, with the chiral intensity determining the trajectory of particle motion. As the lattice possesses both intensity and phase chirality, it may also find potential applications in tasks such as chiral structure microfabrication.

Reconfigurable Chiral Spintronic THz Emitters

AbstractCollective spin arrangements manifest diverse spin textures, encompassing ferromagnetism, antiferromagnetism, and chiral vortices. However, mapping these spin textures in ultrathin magnetic multilayers has remained elusive. A reconfigurable chiral spintronic terahertz emission method is introduced through investigations on a model system of synthetic antiferromagnet and the spin information of individual ferromagnet (FM) layers is extracted. Upon femtosecond photoexcitation of the synthetic antiferromagnet (FM1/Ru/FM2), the ferromagnets generate a pair of linearly polarized ultrafast spin currents, which, after relaxation at the FM/Ru interface, emit corresponding THz pulses. The Ruderman–Kittel–Kasuya–Yosida interactions between the two FMs give rise to magnetic‐field‐controlled spin textures causing a spin relaxation imbalance at the interfaces and induce a phase shift between the orthogonal components of the emitted terahertz fields, consequently reconfiguring the polarization of the emitted terahertz field from linear to circular. This approach offers a novel means of investigating electronic and magnetic states in ultrathin spintronic heterostructures, low‐dimensional quantum materials, topological insulators, and Weyl semimetals. Furthermore, it enables the exploration of spin textures, including skyrmions, merons, and solitons.

Robot-assisted chirality-tunable acoustic vortex tweezers for contactless, multifunctional, 4-DOF object manipulation.

Robotic manipulation of small objects has shown great potential for engineering, biology, and chemistry research. However, existing robotic platforms have difficulty in achieving contactless, high-resolution, 4-degrees-of-freedom (4-DOF) manipulation of small objects, and noninvasive maneuvering of objects in regions shielded by tissue and bone barriers. Here, we present chirality-tunable acoustic vortex tweezers that can tune acoustic vortex chirality, transmit through biological barriers, trap single micro- to millimeter-sized objects, and control object rotation. Assisted by programmable robots, our acoustic systems further enable contactless, high-resolution translation of single objects. Our systems were demonstrated by tuning acoustic vortex chirality, controlling object rotation, and translating objects along arbitrary-shaped paths. Moreover, we used our systems to trap single objects in regions with tissue and skull barriers and translate an object inside a Y-shaped channel of a thick biomimetic phantom. In addition, we showed the function of ultrasound imaging-assisted acoustic manipulation by monitoring acoustic object manipulation via live ultrasound imaging.

Collective control of a vortex array in a ferroelectric ultrathin film

Recently, the observation of ferroelectric vortex arrays has triggered the investigation of topological domain structures and their characteristics. Vortices are typical topological domain structures with chirality in nanoscale ferroelectric materials. The chirality of a single vortex in a nanodot can be easily manipulated, but the collective control of a vortex array is exceptionally difficult and has not yet been realized. This Letter proposes an effective scheme for the collective control of a vortex array and investigates it via phase-field simulations. The results indicate that the collective control of a vortex array with bidirectional switching can be realized by introducing a bending film with periodic large curvatures under alternative electric fields. Furthermore, a general rule for determining the electrically controllable chirality of ferroelectric vortices is proposed. This Letter demonstrates the feasibility of the collective control of vortex arrays and provides insights for developing ferroelectric nanoelectronic devices based on vortex arrays.

Controllable Chiral Light Generation and Vortex Field Investigation Using Plasmonic Holes Revealed by Cathodoluminescence.

Control of the angular momentum of light is a key technology for next-generation nano-optical devices and optical communications, including quantum communication and encoding. We propose an approach to controllably generate circularly polarized light from a circular hole in a metal film using an electron beam by coherently exciting transition radiation and light scattering from the hole through surface plasmon polaritons. The circularly polarized light generation is confirmed by fully polarimetric four-dimensional cathodoluminescence mapping, where angle-resolved spectra are simultaneously obtained. The obtained intensity and Stokes maps show clear interference fringes as well as almost fully circularly polarized light generation with controllable parities by the electron beam position. By applying this approach to a three-hole system, a vortex field with a phase singularity is visualized in the middle of three holes.

Response to Pitkanen’s Solar System Model: Towards Gross-Pitaevskiian Description of Solar System and Galaxies and More Evidence of Chiral Superfluid Vortices

Response to Pitkanen’s Solar System Model: Towards Gross-Pitaevskiian Description of Solar System and Galaxies and More Evidence of Chiral Superfluid Vortices

Observation of Magnetic Quadrupole Endowed Helical Dichroism in Artificial Propeller Meta‐Molecules

AbstractThis study demonstrates that the magnetic quadrupole (MQ) can make an indispensable contribution to helical dichroism (HD), which is distinct from HD caused by electric quadrupole (EQ). Specifically, the established multipole expansion theory of the light–matter interaction reveals that EQ‐induced HD belongs to the pure electric dipole–quadrupole (E1‐E2) excitation, while newly discovered MQ‐induced HD belongs to the pure magnetic dipole‐quadrupole (M1‐M2) excitation. Metallic propeller meta‐molecules are used to elaborate on this novel physical mechanism and demonstrate that HD can be significantly enhanced and flexibly tuned by increasing stereo twist of propeller meta‐molecules or slightly focusing the linearly polarized vortex beam to improve the orbital angular momentum number. Importantly, the aforementioned intriguing phenomena are validated by fabricating stereo twisted propeller meta‐molecules via nano‐kirigami method. The findings reveal a paradigm‐shift approach to manipulate the chiroptical response of a single metallic nanostructure via photonic orbital angular moment, opening prospects for optical encryption, communication, next‐generation chiroptical spectroscopy, and chiral vortex optical devices.

Vortex switching in epitaxial nanodot under uniform electric field: The effect of misfit strain

Epitaxial strain plays an extraordinary role in the formation, evolution, and phase transition of topological domain structures in nanoscale ferroelectrics. Unfortunately, how vortex switching reacts to misfit strain in epitaxial nanodots remains unclear. Based on phase-field simulations, the reversal of vortex chirality in an epitaxial triangular nanodot induced by a uniform electric field applied along various directions is systematically investigated as a function of misfit strain. The results indicate that three basic types of vortex switching exist in an epitaxial triangular nanodot: type I under compressive strain, type II under tensile strain, and a narrow transition type with the characteristics of types I and II. The results show that misfit strain plays a crucial role in determining the type of vortex switching. This work, thus, clarifies the role of misfit strain and could be conducive to facilitating the mechanical manipulation of ferroelectric nanoelectronic devices.

A numerical solver for active hydrodynamics in three dimensions and its application to active turbulence

We present a higher-order convergent numerical solver for active polar hydrodynamics in three-dimensional domains of arbitrary shape, along with a scalable open-source software implementation for shared- and distributed-memory parallel computers. This enables the computational study of the nonlinear dynamics of out-of-equilibrium materials from first principles. We numerically solve the nonlinear active Ericksen–Leslie hydrodynamic equations of three-dimensional (3D) active nematics using both a meshfree and a hybrid particle-mesh method in either the Eulerian or Lagrangian frame of reference. The solver is validated against a newly derived analytical solution in 3D and implemented using the OpenFPM software library for scalable scientific computing. We then apply the presented method to studying the transition of 3D active polar fluids to spatiotemporal chaos, the emergence of coherent angular motion in a 3D annulus, and chiral vortices in symmetric and asymmetric 3D shapes resembling dividing cells. Overall, this provides a robust and efficient open-source simulation framework for 3D active matter with verified numerical convergence and scalability on parallel computers.

Maximum helical dichroism enabled by an exceptional point in non-Hermitian gradient metasurfaces

Helical dichroism (HD) utilizing unbounded orbital angular momentum degree of freedom, has provided an important means of exploring chiral effects in diverse wave systems, surpassing the two-state constraint in circular dichroism that relies on intrinsic spin. However, the naturally feeble chiral signals that arise during wave-matter interactions pose significant challenges to the effective enlargement of HD. Here, we introduce a new paradigm for realizing maximum HD through non-Hermitian gradient metasurfaces by engineering a chiral exceptional point (EP) in intrinsic topological charge. The non-Hermitian gradient metasurfaces are empowered by the asymmetric coupling feature at the EP, enabling flexible construction to realize versatile chirality control in extreme circumstances where one chiral vortex is totally reflected and the opposite counterpart is completely absorbed or transmitted into the customized vortex modes. As the manifestation of the maximum HD, we present the first experimental demonstration of perfect chirality-selected vortex transmission in acoustics. Our findings open new venues to achieve maximum chirality and explore chiral physics of wave-matter interactions, which can boost many vortical applications in asymmetric chirality manipulation, one-way propagation, and information multiplexing.

Axis-symmetric Onsager clustered states of point vortices in a bounded domain

We study axis-symmetric Onsager clustered states of a neutral point vortex system confined to a two-dimensional disc. Our analysis is based on the mean field of bounded point vortices in the microcanonical ensemble. The clustered vortex states are specified by the inverse temperature β and the rotation frequency ω, which are the conjugate variables of energy E and angular momentum L, respectively. The formation of the axis-symmetric clustered vortex states (azimuthal angle independent) involves the separating of vortices with opposite circulation and the clustering of vortices with the same circulation around the origin and edge. The state preserves SO(2) symmetry while breaking symmetry. We find that, near the uniform state, the rotation-free clustered state (ω = 0) emerges at particular values of L 2/E and β. At large energies, we obtain asymptotically exact vortex density distributions, whose validity condition gives rise to the lower bound of β for the rotation-free states. Noticeably, the obtained vortex density distribution near the edge at large energies provides a novel exact vortex density distribution for the corresponding chiral vortex system.

Double-banded topology valley transport

The advent of topology theory has facilitated the emergence of unique physical properties in engineered structures, garnering significant interest and investigation. This paper presents a phonon crystal design approach predicated on the Valley Hall effect, capable of achieving energy band inversion and topological valley transport across various frequency ranges. By merely rotating the scatterer, multiple coexisting linear Dirac points at the Brillouin zone K point can be disrupted, giving rise to bandgaps exhibiting distinct topological characteristics. Reversing the rotation direction simultaneously alters the vortex chirality. This inherent property lays the foundation for topological valley transport. Furthermore, this paper examines the two-band valley transport attributes of phonon crystals with diverse transmission routes. To explore the applicability of the designed phonon crystal in topological acoustics, a two-band acoustic splitter is employed to investigate topological transport behavior. Simulation outcomes demonstrate exceptional transport performance. This study offers novel insights for sound wave control design and bears potential applicability to multifunctional acoustic devices such as acoustic sensors, acoustic signal processing, and acoustic communication.

Customized optical chirality of vortex structured light through state and degree-of-polarization control

We show how both the ellipticity $\eta$ and degree of polarization $\textit{P}$ influences the extraordinary optical chirality properties of non-paraxial vortex beams. We find that, in stark contrast to paraxial optics and non-vortex modes, extremely rich and tuneable spatial distributions of optical chirality density can be produced by an optical vortex beam under tight focussing. We develop a theoretical description of how the optical chirality can be tailored for purpose by altering both the state $\eta$ and degree of polarization $\textit{P}$ of the input vortex mode, along with the magnitude and sign of optical orbital angular momentum via the pseudoscalar topological charge $\ell$. We expect that the results will have a significant role in both producing novel techniques and improving existing methods in chiral nano-optics and structured light photonics.

Asymmetric scattering behaviors of spin wave dependent on magnetic vortex chirality

We investigate asymmetric spin wave scattering behaviors caused by vortex chirality in a cross-shaped ferromagnetic system by using the micromagnetic simulations. In the system, four scattering behaviors are found: (i) asymmetric skew scattering, depending on the polarity of vortex core, (ii) back scattering (reflection), depending on the vortex core stiffness, (iii) side deflection scattering, depending on structural symmetry of the vortex circulation, and (iv) geometrical scattering, depending on waveguide structure. The first and second scattering behaviors are attributed to nonlinear topological magnon spin Hall effect related to magnon spin-transfer torque effect, which has value for magnonic exploration and application.