Longitudinal Polarization Vortices Generated via Terahertz Ring Resonator

We theoretically and numerically investigate the generation and evolution of different pulsed terahertz (THz) singular beams with an ultrabroad bandwidth (0.1-40 THz) in long gas-plasma filaments induced by a shaped two-color laser field, i.e., a vortex fundamental pulse (ω0) and a Gaussian second harmonic pulse (2ω0). Based on the unidirectional propagation model under group-velocity moving reference frame, the simulating results demonstrate that three different THz singular beams, including the THz necklace beams with a π-stepwise phase profile, the THz angular accelerating vortex beams (AAVBs) with nonlinear phase profile, and the THz vortex beams with linear phase profile, are generated. The THz necklace beams are generated first at millimeter-scale length. Then, with the increase of the filament length, THz AAVBs and THz vortex beams appear in turn almost periodically. Our calculations confirm that all these different THz singular beams result from the coherent superposition of the two collinear THz vortex beams with variable relative amplitudes and conjugated topological charges (TCs), i.e., +2 and -2. These two THz vortex beams could come from the two four-wave mixing (FWM) processes, respectively, i.e., ω0+ω0-2ω0→ωTHz and -(ω0+ω0) + 2ω0→ωTHz. The evolution of the different THz singular beams depends on the combined effect of the pump ω0-2ω0 time delay and the separate, periodical, and helical plasma channels. And the TC sign of the generated THz singular beams can be easily controlled by changing the sign of the ω0-2ω0 time delay. We believe that these results will deepen the understanding of the THz singular beam generation mechanism and orbital angular momentum (OAM) conversion in laser induced gas-filamentation.

We show that continuous and spin-lattice models of chiral ferro- and antiferromagnets provide the existence of an infinite number of stable soliton solutions of any integer topological charge. A detailed description of the morphology of new skyrmions and the corresponding energy dependencies are provided. The considered model is general, and is expected to predict a plethora of particlelike states which may occur in various chiral magnets including ultrathin films, e.g., PdFe/Ir(111), rhombohedral ${\mathrm{GaV}}_{4}{\mathrm{S}}_{8}$ semiconductor, B20-type alloys as ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}\mathrm{Ge}$, ${\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}\mathrm{Si}$, ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$, ${\mathrm{Cu}}_{2}{\mathrm{OSeO}}_{3}$, and acentric tetragonal Heusler compounds.

Cylindrical vector beams (CVBs), which possess polarization distribution of rotational symmetry on the transverse plane, can be developed in many optical technologies. Conventional methods to generate CVBs contain redundant interferometers or need to switch among diverse elements, thus being inconvenient in applications containing multiple CVBs. Here we provide a passive polarization-selective device to substitute interferometers and simplify generation setup. It is accomplished by reversing topological charges of orbital angular momentum based on a polarization-selective Gouy phase. In the process, tunable input light is the only condition to generate a CVB with arbitrary topological charges. To cover both azimuthal and radial parameters of CVBs, we express the mapping between scalar Laguerre–Gaussian light on a basic Poincaré sphere and CVB on a high-order Poincaré sphere. The proposed device simplifies the generation of CVBs enormously and thus has potential in integrated devices for both quantum and classic optical experiments.

Recent studies on orbital angular momentum (OAM) states in the surface plasmon polariton (SPP) field have primarily focused on the generation of single OAM modes and the evolution of OAM states with various topological charges. However, achieving coherent superposition of two OAM states with well-defined phase relations through precise nanostructure design remains challenging. In this work, we propose a plasmonic nanostructure consisting of paired rectangular slits arranged along circular or segmented Archimedes spiral. The Archimedean spiral of various radii in azimuthal angle provides a geometry-dependent helical phase; coupled with a rotated nanoslit pair, it introduces a geometric phase of twice the rotated angle. By combining chiral spiral with nanoslit pair units, the design both generates plasmonic OAM eigenstates with arbitrary topological charges and enables their coherent superposition. The amplitudes of the two constituent OAM states are continuously tunable through the degree of polarization of the incident light, and their relative phase difference is controlled by the polarization angle, enabling arbitrary superposition of the plasmonic OAM states with continuously variable amplitude ratios and phase differences. Theoretical analysis and numerical simulations demonstrate that circularly polarized illumination produces distinct OAM pure states, whereas linearly polarized light leads to equal-amplitude superposition states with structured field distributions. Moreover, rotating the polarization angle continuously adjusts the relative phase between the eigenstates and produces a predictable rotation of the resultant interference pattern. These results provide a new approach for coherent control of plasmonic OAM states and offer design guidelines for multifunctional on-chip optical field manipulation devices.

Quantum oscillations of response functions in high magnetic fields tend to reveal some of the most interesting properties of metals. In particular, the oscillation phase shift is sensitive to topological band features, thereby helping to identify the presence of Weyl fermions. In this work we predict characteristic parameter dependence of the phase shift for Weyl fermions with tilted and overtilted dispersion (type I and type II Weyl fermions) and an arbitrary topological charge (multi-Weyl fermions). For type-II Weyl fermions our calculations capture the case of magnetic breakthrough between the electron and the hole part of the dispersion. Here the phase shift turns out to depend only on the quantized topological charge due to cancellation of non-universal contributions from the electron and the hole part.

Topological charge plays a significant role in a range of physical systems. In particular, observations of real-space topological objects in magnetic materials have been largely limited to skyrmions - states with a unitary topological charge. Recently, more exotic states with varying topology, such as antiskyrmions, merons, or bimerons and 3D states such as skyrmion strings, chiral bobbers, and hopfions, have been experimentally reported. Along these lines, the realization of states with higher-order topology has the potential to open new avenues of research in topological magnetism and its spintronic applications. Here, real-space imaging of such spin textures, including skyrmion, skyrmionium, skyrmion bag, and skyrmion sack states, observed in exfoliated flakes of the van der Waals magnet Fe3-x GeTe2 (FGT) is reported. These composite skyrmions may emerge from seeded, loop-like states condensed into the stripe domain structure, demonstrating the possibility to realize spin textures with arbitrary integer topological charge within exfoliated flakes of 2D magnets. The general nature of the formation mechanism motivates the search for composite skyrmion states in both well-known and new magnetic materials, which may yet reveal an even richer spectrum of higher-order topologicalobjects.

Carrying orbital angular momentum per photon, the optical vortex has elicited widespread interest. Here, we demonstrate that dual coaxial longitudinal polarization vortices can appear upon a nonparaxial propagation of a tightly focused Pancharatnam-Berry tailored Laguerre-Gaussian beam. Most importantly, it is capable of accessing arbitrary independent topological charges for both vortices, as well as predesigned tunable spacing distances between them.

An advanced isolation control technique is proposed for a terahertz (THz) multi-input multi-output (MIMO) antenna, significantly enhancing port-to-port isolation. This approach utilizes a silicon-based dielectric resonator antenna for high-performance tunable THz MIMO system. The design follows a structured evolutionary process, beginning with a dual-feed (DF) configuration without a dielectric resonator as the baseline model. Enhancements were introduced by incorporating a graphene metal coating, improving tunability and isolation. Further refinements involved integrating a split square ring resonator without an RDRA, enhancing isolation and polarization characteristics. The design was then optimized by introducing a metal-coated RDRA, improving gain, radiation efficiency, and circular polarization purity. Additionally, a complementary split ring resonator-loaded circularly polarized DF structure was incorporated for further performance enhancement. The proposed MIMO antenna achieves 25.5 dB isolation at 7.2 THz without a metal coating. By applying a graphene coating, isolation increases to 49 dB at 7.42 THz, using graphene’s tunable conductivity under an external DC bias for dynamic reconfiguration. The antenna exhibits excellent pattern diversity, with an envelope correlation coefficient below 0.003 and a diversity gain of 9.94 dB. Further isolation improvement is achieved by integrating a split-ring resonator on the dielectric material, enhancing isolation up to 59 dB. These advancements make the proposed antenna highly suitable for THz applications.

Nested skyrmion bags, as magnetic solitons with arbitrary integer topological charges (Q), hold potential for applications in data encoding. A crucial issue is the local manipulation of skyrmions within nested skyrmion bags to control the total Q. In this study, we explore different possible ground states and resonant excitation spectra of nested skyrmion bags through micromagnetic simulations. More importantly, we demonstrate that the manipulation of the Q of nested skyrmion bags can be achieved by using microwave magnetic fields, i.e., the inner, middle, and outer skyrmions within the nested skyrmion bags are selectively excited by using the diverse out-of-plane excitation modes. By calculating the energy of skyrmions, we further analyze the relationship between the annihilation of skyrmions at different positions and the out-of-plane microwave magnetic fields. Our findings present a promising approach for manipulating the Q of nested skyrmion bags, potentially advancing their application in storage and logic devices.

The generation of surface plasmon vortices with arbitrary higher order vortex topological charges with novel plasmonic vortex lens is experimentally demonstrated. It is shown that the polarization sensitivity of the plasmonic vortex lens can be utilized for the dynamic switching of the surface plasmon vortices with different topological charges. A simple algebraic rule related to the vortex topological charge change in the dynamic switching is formulated, and its proof is provided with theory and experiment. The synthesis and dynamic switching of higher order surface plasmon vortices have profound potential in optical trapping, optical data storage, and other related fields.

In a vector beam (VB), the polarization rotates spatially within the electromagnetic-field distribution. In this paper, we propose a simple method for using a circularly polarized pump pulse that is tightly focused onto a (100)-cut ZnTe crystal to generate a VB with anti-vortex polarization (i.e., with topological charge ℓ = -1) in the terahertz (THz) regime. Relative to normal incidence, we expect the THz output to be enhanced for the oblique incidence of a focused pump pulse. We formed a ring-shaped pump beam using a pair of axicon lenses and focused it onto a (100)-cut ZnTe crystal to achieve oblique-dominant excitation. We then performed two-dimensional polarization imaging based on THz time-domain spectroscopy, and we evaluated the results using cylindrically polarized Laguerre-Gaussian (CPLG) mode decomposition optimized by using the information entropy. We confirmed the successful generation of THz VBs with ℓ = -1, with the output efficiency increasing with focal angle from 6.4° to 17.3°. These results indicate feasibility of using THz VBs for various applications.

Terahertz (THz) vortex waves carrying orbital angular momentum (OAM) hold great potential in dealing with the capacity crunch in wireless high-speed communication systems. Nevertheless, it is quite a challenge for the widespread applications of OAM in the THz regime due to the beam divergence and stringent alignment requirement. To address this issue, an all-dielectric lens (ADL) is proposed for the arbitrary manipulation of quasi-nondiffractive THz OAM waves (QTOWs). On the basis of the concept of the optical conical lens and the multivorticity metasurface, the beam number, the topological charge (TC), and the deflection angle as well as the nondiffractive depth of the generated THz OAM waves are controllable. For proof-of-concept, two ADLs are 3D printed to create single and dual deflected QTOWs, respectively. Remarkably, measured by a THz imaging camera, the desired QTOWs with high mode purity are observed in predesigned directions with a nondiffractive depth predefined theoretically. The proposed designs and experiments, for the first time, verified that the QTOWs could be achieved with a nondiffractive range of 55.58λg (λg = wavelength at 140 GHz) and large deflection angles of 30° and 45°.

Optical resonators are widely utilized to enhance light–matter interaction by focusing electromagnetic waves into deep sub-wavelength regions. Here, we first present a metallic bowtie split ring (BSR) optical resonator as an asymmetric light coupler for a terahertz (THz) graphene photothermoelectric (PTE) detector. The giant THz field enhancement in the slit region of BSR is mediated by two types of resonances: the inductor–capacitor (LC) and the dipole resonances, which greatly increase the THz absorption, resulting in the sensitivity improvement of the THz PTE detector. In detail, the LC and dipole resonant behaviors of BSR are systematically investigated in both theoretical and experimental aspects. Compared with the dipole resonance, the LC resonance leads to stronger electric field localization and enhancement. An optimized BSR is designed and integrated with a graphene THz PTE detector, and an ultrasensitive THz PTE response is demonstrated. At room temperature and in zero-bias mode, the key detection parameters—responsivity, sensitivity (noise-equivalent power), and speed—are 138 V/W, 25 pW/Hz1/2, and 3.7 µs, respectively. Our results indicate that the LC resonance supported by BSR can introduce strong local field enhancement, which is helpful for realizing high sensitivity THz detectors.

Metamaterials exhibiting exotic optical properties have played a significant role over the years in guiding the concept of invisibility cloaking from the realm of being fiction to reality. However, due to the difficulties in fabricating the 3D cloaking devices and lack of exotic plasmonic materials at terahertz (THz) frequencies, the experimental realization of cloaking phenomenon in the THz spectrum is challenging. In this work, a new mechanism for invisibility cloaking based on the resonant scattering cancellation technique in a 2D nonconcentric composite metamaterial device, consisting of a split ring resonator (SRR) and a microelectromechanical system (MEMS) reconfigurable closed ring resonator (CRR) at THz frequencies is reported. A strong magnetic interaction between the SRR and CRR eliminates the scattering effects from the SRR at its fundamental eigen mode frequency, thereby making it invisible to the incident THz wave. Further, by voltage actuation of MEMS-reconfigurable CRR, an active switching between the visible and cloaked states of SRR structure is demonstrated. The proposed technique provides a simple design and technique for realizing invisibility cloaks by utilizing the resonant near-field interactions in the subwavelength structures across microwave to optical frequencies, thereby circumventing the need for materials with complex geometry and exotic properties

A study is performed of features of surface-emitting quantum-cascade lasers with a ring resonator and second-order diffraction grating. The coefficient of coupling can be raised to 12 cm−1 by selecting the modes for direct ion lithography etching. Lasing is done near 7.6 μm with a threshold current density of only 0.3 kA/cm2.