Opposite Topological Charges Research Articles

Topological Differential Microscopy Based on the Spin–Orbit Interaction of Light in a Natural Crystal

We experimentally demonstrate a two-dimensional topological differential microscopy based on the spin–orbit interaction of light through a thin uniaxial crystal slab. First, a three-dimensional propagation theory is established for the light propagating through a uniaxial material. Then it is found that, due to the spin–orbit interaction, the two circularly polarized components of incident light are imposed with opposite topological charges, leading to the azimuthal splitting of spin states. Consequently, the cross-polarized field approximately amounts to the second-order spatial differentiation of the incident field. Finally, such an optical differentiation performed by a lithium niobate crystal is successfully applied to edge detection in microscopy. Further experiments validate its good performance in edge enhancement of both amplitude and phase objects. The results provide an economic, simple but robust approach to real-time, colorful, and high-contrast biological imaging in living systems.

Phase-Change Perovskite Microlaser with Tunable Polarization Vortex.

Metasurfaces supporting optical bound states in the continuum (BICs) are emerging as simple and compact optical cavities to realize polarization-vortex lasers. The winding of the polarization around the singularity defines topological charges which are generally set by the cavity design and cannot be altered without changing geometrical parameters. Here, a subwavelength-thin phase-change halide perovskite BIC metasurface functioning as a tunable polarization vortex microlaser is demonstrated. Upon the perovskite structural phase transitions, both its refractive index and gain vary substantially, inducing reversible and bistable switching between distinct polarization vortexes underpinned by opposite topological charges. Dynamic tuning and switching of the resulting vector beams may find use in microscopy imaging, particle trapping and manipulation, and optical data storage.

Room-Temperature Magnetic Skyrmions and Large Topological Hall Effect in Chromium Telluride Engineered by Self-Intercalation.

Room-temperature magnetic skyrmion materials exhibiting robust topological Hall effect (THE) are crucial for novel nano-spintronic devices. However, such skyrmion-hosting materials are rare in nature. In this study, a self-intercalated transition metal dichalcogenide Cr1+ x Te2 with a layered crystal structure that hosts room-temperature skyrmions and exhibits large THE is reported. By tuning the self-intercalate concentration, a monotonic control of Curie temperature from 169 to 333K and a magnetic anisotropy transition from out-of-plane to the in-plane configuration are achieved. Based on the intercalation engineering, room-temperature skyrmions are successfully created in Cr1.53 Te2 with a Curie temperature of 295K and a relatively weak perpendicular magnetic anisotropy. Remarkably, a skyrmion-induced topological Hall resistivity as large as ≈106nΩ cm is observed at 290K. Moreover, a sign reversal of THE is also found at low temperatures, which can be ascribed to other topological spin textures having an opposite topological charge to that of the skyrmions. Therefore, chromium telluride can be a new paradigm of the skyrmion material family with promising prospects for future device applications.

Propagation of Gaussian vortex beams in electromagnetically induced transparency media.

Electromagnetically induced transparency (EIT) is an important phenomenon in quantum optics, and has a wide range of applications in the fields of quantum information processing and quantum precision metrology. Recently, with the rapid progress of the generation and detection of structured light, the EIT with structured light has attracted enormous interests and offers new and novel functionalities and applications. Here, we theoretically study the propagation and evolution of Gaussian vortex beams, a typical type of structured light, in an EIT medium with Λ-type three-level atoms. Based on the generalized Huygens-Fresnel principle, we derive the analytical expressions of fully and partially coherent Gaussian vortex beams propagating in the EIT medium, and study the evolution of the intensity and phase distributions of the beams and their dependencies on parameters such topological charge, coherence length, Rabi frequency, etc. It is shown that both the fully and partially coherent Gaussian vortex beams undergo focusing and diverging periodically during propagation. The phase singularity of the fully coherent beam keeps unchanged, while the phase singularity of the partially coherent beam experiences splitting and recombination periodically. In addition, new phase singularities with opposite topological charge are generated in the latter case. Our results not only advance the study of the interaction between structured light and coherent media, but also pave the avenue for manipulating structured light via EIT.

Simulating anti-skyrmions on a lattice

Magnetic skyrmions are meta-stable spin structures that naturally emerge in magnetic materials. While a vast amount of effort has gone into the study of their properties, their counterpart of opposite topological charge, the anti-skyrmion, has not received as much attention. We aim to close this gap by deploying Monte Carlo simulations of spin-lattice systems in order to investigate which interactions support anti-skyrmions, as well as skyrmions of Bloch and Néel type. We find that the combination of ferromagnetic exchange and Dzyaloshinskii–Moriya (DM) interactions is able to stabilize each of the three types, depending on the specific structure of the DM interactions. Considering a three-dimensional spin lattice model, we provide a finite-temperature phase diagram featuring a stable anti-skyrmion lattice phase for a large range of temperatures. In addition, we also shed light on the creation and annihilation processes of these anti-skyrmion tubes and study the effects of the DM interaction strength on their typical size.

All-angle reflectionless negative refraction with ideal photonic Weyl metamaterials

Negative refraction, an unnatural optical phenomenon in which the incident and the refracted waves reside on the same side of the surface normal, has been demonstrated with the invention of negative index media based on artificially engineered photonic structures called metamaterials. It has received wide attention due to its potential applications in imaging, nonlinear optics, and electromagnetic cloaking. However, it is highly challenging to realize negative refraction operating at all angles and with the perfect transmission. In this work, leveraging the recent development in topological photonics, we propose to realize reflectionless negative refraction for all incident angles with a topological metamaterial. The proposed metamaterial possesses two Weyl points of opposite topological charges. By interfacing the metamaterial with a perfect electric conductor (PEC) or a perfect magnetic conductor (PMC), the Fermi arc connecting the two Weyl points can take the form of a half-circle possessing a positive or a negative refractive index. Importantly, due to the topological protection, there is no reflection at the interface between the PEC and PMC covered areas, leading to the observation of all-angle negative refraction without reflection at the boundary. Our work provides a new platform for manipulating the propagation of surface waves, which may find applications in the construction of integrated photonic devices.

Vortex Solitons in Twisted Circular Waveguide Arrays.

We address the formation of topological states in twisted circular waveguide arrays and find that twisting leads to important differences of the fundamental properties of new vortex solitons with opposite topological charges that arise in the nonlinear regime. We find that such system features the rare property that clockwise and counterclockwise vortex states are nonequivalent. Focusing on arrays with C_{6v} discrete rotation symmetry, we find that a longitudinal twist stabilizes the vortex solitons with the lowest topological charges m=±1, which are always unstable in untwisted arrays with the same symmetry. Twisting also leads to the appearance of instability domains for otherwise stable solitons with m=±2 and generates vortex modes with topological charges m=±3 that are forbidden in untwisted arrays. By and large, we establish a rigorous relation between the discrete rotation symmetry of the array, its twist direction, and the possible soliton topological charges.

All-Optical Cross-Connection of Cylindrical Vector Beam Multiplexing Channels

The achievement of cylindrical vector beam (CVB) multiplexing has shown advantages in enlarging communication capacities and all-optical interconnections. While various mode transformation technologies have been developed to process CVB multiplexing signals, previous approaches have failed to cross-connect CVB channels, which could play a pivotal role in all-optical interconnection. Since CVBs can be decomposed into two orthogonal circularly polarized vortex beams with opposite topological charges, it is possible to cross-connect CVB channels via spin-dependent orbital angular momentum mode cross-transformation. Exploiting the spin-orbit interaction of photons in anisotropic liquid crystal q-plates, we performed conjugate helical phase modulations to these two spin components using the local polarization conversion of the birefringent effect, where the conjugate relationship of the two spin components was maintained during the transformation. Constrained by the spin-dependent phase modulation, the spatially separated target CVB mode was independently transformed with other higher-order modes via the partitioned modulation of the liquid crystal q-plate for CVB mode cross-transformation. As a proof of concept, a three-channel CVB multiplexing communication system was constructed. We show that the data signals between two CVB channels were successfully cross-connected, and 150 Gbit/s quadrature phase-shift keying signals were transmitted with a bit-error-rate approaching 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−6</sup> .

Phase-matching quantum key distribution based on orbital angular momentum under atmospheric turbulence

We propose a phase-matching quantum key distribution (PM-QKD) protocol based on orbital angular momentum (OAM) to exceed the linear key-rate bound without quantum repeater in the paper, named OAM-PM-QKD protocol, where the coherent state consisting of OAM with opposite topological charges is used as the information carrier. With the equivalent entanglement distillation model, we analyze its security and then derive its key generation rate. The numerical simulation results show that the proposed OAM-PM-QKD protocol is able to exceed the linear key-rate bound when the transmission distance exceeds 230 km, and the maximum secure transmission distance is also longer than those of original PM-QKD protocol due to no misalignment in the basis. Furthermore, the influence of the atmospheric turbulence (AT) on the key generation rate is also discussed. The stronger the strength of AT is, the shorter the transmission distance is.

Degenerate skyrmionic states in synthetic antiferromagnets

Topological magnetic textures, characterized by integer topological charge S, are potential candidates in future magnetic logic and memory devices, due to their smaller size and expected low threshold current density for their motion. An essential requirement to stabilize them is the Dzyaloshinskii–Moriya interaction (DMI) which promotes a particular chirality, leading to a unique value of S in a given material. However, recently coexistence of skyrmions and antiskyrmions, with opposite topological charge, in frustrated ferromagnets has been predicted using J 1–J 2–J 3 classical Heisenberg model, which opens new perspectives, to use the topological charge as an additional degree of freedom. In this work, we propose another approach of using a synthetic antiferromagnetic system, where one of the ferromagnetic (FM) layer has isotropic and the other FM layer has anisotropic DMI to promote the existence of skyrmions and antiskyrmions, respectively. A frustrated interaction arises due to the coupling between the magnetic textures in the FM layers, which enables the stabilization and coexistence of 6 novel elliptical topological textures.

Calibration of the Soleil–Babinet Compensator Based on the Vectorial Optical Field

The Soleil–Babinet compensator (SBC) is a variable retarder and has been used in a variety of application fields. A scheme based on the vectorial optical field is proposed to calibrate the SBC by transforming the change of the phase retardation into the visible rotation of the petal-like pattern. The relationship between the rotation angle of the petal-like pattern and the phase retardation of the SBC is established theoretically. In the experiment, the vector beam is generated by using the spiral phase plate (SPP) and the modified Mach–Zehnder interferometer based on the superposition principle of two orthogonal circularly polarized vortex beams with opposite topological charges. Taking advantage of the image processing method, the rotation angles of the acquired petal patterns are calculated, and the relationship between the phase retardation of the SBC and the displacements of its micrometer screw is determined. The measured phase retardation of the SBC ranges from −277.00° to 516.57°. By linearly fitting the experimental data, the phase sensitivity is 33.076 ± 0.147 °/mm, and the coefficient of determination value that shows the linearity of the experimental data is 0.9995. The experimental results agree well with the theoretical data.

Pancharatnam–Berry phase reversal via opposite-chirality-coexisted superstructures

Recently discovered reflective Pancharatnam–Berry phase (PB phase) from chiral anisotropic media (e.g., cholesteric liquid crystal, CLC) has aroused great interest in the emerging frontier of planar optics. However, the single chirality of common CLCs results in the intrinsic limitation of the same spin-selective PB phase manipulation, which means the reversal of the input spin cannot realize the conjugated PB phase. In this work, an innovative scheme based on opposite-chirality-coexisted superstructures is proposed to simultaneously modulate orthogonal circular polarization and get PB phase reversal. Through refilling CLC into a washed-out polymer network with opposite chirality and delicate photo-patterned structures, reflective optical vortex (OV) with opposite topological charges and vector beams with conjugated spiral PB phases are efficiently generated depending on the incident polarization. Furthermore, OV holograms are encoded to reconstruct polarization-selective OV arrays, indicating the strong capability of such opposite-chirality-coexisted anisotropic media. This work provides a new compact platform for planar optics, and sheds light on the architectures and functionalities of chiral superstructures.

Nonlinear rotation of spin-orbit coupled states in hollow ring-core fibers.

We experimentally demonstrate that when two spin-orbit coupled orbital angular momentum (OAM) modes of opposite topological charge co-propagate in the Kerr nonlinear regime in a hollow ring-core optical fiber, the vectorial mode superposition exhibits a unique power-dependent rotation effect. This effect is analogous to nonlinear polarization rotation in single-mode fibers, however, the added spatial dimension produces a visually observable rotation of the spatial pattern emerging from the fiber when imaged through a linear polarizer. A dielectric metasurface q-plate was designed and fabricated to excite the desired mode combination in a hollow ring-core fiber that supports stable propagation of OAM modes. The observed spatial patterns show strong agreement with numerical simulations of the vector coupled nonlinear Schrödinger equations. These results constitute the first measurements of what can be described as the spin-orbit coupled generalization of the nonlinear polarization rotation effect.

Controlling Topology and Polarization State of Lasing Photonic Bound States in Continuum

AbstractRecently, optical bound states in continuum (BICs) incorporated with optical gain have been reported to exhibit lasing. Here, it is shown that each of the four BICs supported by C symmetric photonic crystal slab can be made to lase, allowing the control over the topological charge of the resulting laser beam. The type of each BIC and their topological charges are identified by imaging the far‐field polarization vortices of the lasing signal. Results are compared with experimentally obtained dispersions, finite element method simulations, and multipole decomposition method based on the microscopic polarization currents in the photonic crystal plane. A demonstration of multimode lasing of two non‐degenerate BICs with opposite topological charges is presented. The momentum space overlap of the BICs results in a unique polarization pattern. The study provides a generalizable example for engineering the topological properties of coherent light.

Observation of Weyl exceptional rings in thermal diffusion

SignificanceThermal diffusion is dissipative and strongly related to non-Hermitian physics. At the same time, non-Hermitian Weyl systems have spurred tremendous interest across photonics and acoustics. This correlation has been long ignored and hence shed little light upon the question of whether the Weyl exceptional ring (WER) in thermal diffusion could exist. Intuitively, thermal diffusion provides no real parameter dimensions, thus prohibiting a topological nature and WER. This work breaks this perception by imitating synthetic dimensions via two spatiotemporal advection pairs. The WER is achieved in thermal diffusive systems. Both surface-like and bulk states are demonstrated by coupling two WERs with opposite topological charges. These findings extend topological notions to diffusions and motivate investigation of non-Hermitian diffusive and dissipative control.

Coupled Focused Acoustic Vortices Generated by Degenerated Artificial Plates for Acoustic Coded Communication

AbstractAcoustic vortices (AVs) show great significance in acoustic communication. However, the lack of confidentiality and the cross‐talks in communications based on traditional AVs remain problems. Here, a degenerated centrosymmetric circular hole plate (DCHP) is designed to generate coupled focused acoustic vortices (CFAVs) with pure orbital angular momentums (OAMs) in both air and water. When an acoustic plane wave transmits through DCHP, two focused acoustic vortices (FAVs) with opposite topological charges are produced simultaneously, and the coupling between them results in a focused acoustic petal beam (FAPB). The OAM of the CFAV can be simply modulated by the angular interval of the adjacent holes on DCHP and the CFAV can be focused at any distances by adjusting the positions of holes. The DCHPs are demonstrated to be able to generate FAPBs numerically and experimentally. Furthermore, 8 FAPBs with different OAMs are set as 8 bases to represent 8 bits of ASCII binary code, and the information encoding and decoding for acoustic communication are realized in water using them. The proposed DCHP provides a convenient method to achieve CFAVs, which may be helpful to enhance the security, reduce the false information, and increase the information capacity in acoustic communication.

Quantum Monte Carlo Study of Weyl Superconductivity

Using auxiliary-field quantum Monte Carlo we study superconductivity in a simple extension of the two-dimensional negative-U Hubbard model which hosts Weyl nodes near the Fermi surface. We discuss the model and our method as applied to the study of superconductivity in this system. We discuss the nature of pairing between Weyl quasiparticles which carry opposite spin and opposite topological charge leading to a spin-singlet pairing amplitude and to a mixture of pseudo-spin singlet and pseudo-spin triplet pairing.

Doping Control of Magnetic Anisotropy for Stable Antiskyrmion Formation in Schreibersite (Fe,Ni)3 P with S4 symmetry.

Magnetic skyrmions, vortex-like topological spin textures, have attracted much interest in a wide range of research fields from fundamental physics to spintronics applications. Recently, growing attention is also paid to antiskyrmions emerging with opposite topological charge in non-centrosymmetric magnets with D2d or S4 symmetry. In these magnets, complex interplay among anisotropic Dzyaloshinskii-Moriya interaction, uniaxial magnetic anisotropy, and magnetic dipolar interactions generates various magnetic textures. However, the precise role of these magnetic interactions in stabilizing antiskyrmions remains to be elucidated. In this work, the uniaxial magnetic anisotropy of schreibersite (Fe,Ni)3 P with S4 symmetry is controlled by doping and its impact on the stability of antiskyrmions is investigated. The authors' magnetometry study, supported by ferromagnetic resonance spectroscopy, shows that the variation of the Ni content and slight doping with 4d transition metals considerably change the magnetic anisotropy. In particular, doping with Pd induces easy-axis anisotropy, giving rise to formation of antiskyrmions, while a temperature-induced spin reorientation is observed in an Rh-doped compound. In combination with Lorentz transmission electron microscopy and micromagnetic simulations, the stability of antiskyrmion as functions of uniaxial anisotropy and demagnetization energy is quantitatively analyzed, and demonstrated that subtle balance between them is necessary to stabilize the antiskyrmions.

The tailoring effect of two symmetrically distributed vortices on circular Airy beam

We investigate the tailoring effect of the point vortices on the foci of the circular Airy vortex beam (CAVB). Two optical vortices (OVs) with same or opposite topological charges (TCs) are symmetrically loaded on the beam. It is found that the loading distance between these two OVs (denoted as dv) can significantly tailor the auto-focusing behaviour of the beam. For two OVs with same TCs, as dv increases from 0 to infinite, the intensity pattern at the focal plane changes from a ring to a bright spot. For opposite OVs, when dv is near 0 or very large, there is a bright foci. But within a certain range, two foci are generated at the focal plane. Good agreements have been achieved between simulations and experiments. We qualitatively explain the tailoring phenomena from the perspective of the inward energy flow of the main ring of CAVB and the interaction between two OVs. The tailoring effect of dv of the two OVs on the focal pattern may have potential applications in micromanipulation, communication and other fields.

Orbital Angular Momentum Modes (De)multiplexer without Mode Conversion based on Strongly Guiding Coupled Ring-core Fibers

The mode-matching&#x002F;finite-element (MM&#x002F;FE) method based on complex power matching is applied for the design of strongly guiding coupled ring-core fibers (RCFs) to modulate orbital angular momentum (OAM) beams. The evolutions of polarization and OAM states of light in the coupler are investigated. Due to the effects of linear birefringence and spin-orbit interaction (SOI), the OAM beam with opposite polarization and topological charge (TC) is observed at a certain distance to the end of the coupler after the coupling region in the form of complex supermodes. The impacts of separation distance and wavelength on the maximum coupling efficiency and inter-modal crosstalk are investigated to achieve the optimum trade-off between the crosstalk and the coupling length. Two types of OAM couplers with separated OAM modes of TC <inline-formula><tex-math notation="LaTeX">$l = \pm1$</tex-math></inline-formula> are proposed, both of which can achieve high coupling efficiency (<inline-formula><tex-math notation="LaTeX">$\gt 99\%$</tex-math></inline-formula>) and low crosstalk (<inline-formula><tex-math notation="LaTeX">$\rm{\lt - 21\,dB}$</tex-math></inline-formula>) at 1550 nm wavelength without mode conversion. The proposed design approach has great potential in the (de)multiplexing of OAMs since it leads to a novel mechanism for sorting OAM modes in waveguides without destroying the integrity of OAM modes.