Search Results (27)

Electromagnetic vortex waves carrying orbital angular momentum (OAM) provide an additional modal dimension for electromagnetic scattering analysis, but the resulting OAM mode-purity spectra are highly nonlinear and expensive to characterize through repeated full-parameter simulations. To address this issue, this work proposes a dual-path data-driven surrogate framework for the simulation-level prediction of OAM mode-purity spectra in metallic-target vortex-wave scattering. High-frequency datasets were generated within a prescribed workflow that combined an angular-spectrum formulation of Bessel vortex beams with a facet-based physical-optics method. Five representative metallic targets were considered, namely, Plate, Spiral, Spite, Missile, and Dihedral. In the first surrogate path, a numerical-parameter-based regression model was developed to predict the mode-purity spectrum from physical scattering variables for canonical targets. In the second surrogate path, a phase-map-based regression model was introduced to predict the spectrum directly from scattered-field phase maps without explicit geometric parameterization. The results show that the parameter-based surrogate achieves low prediction errors for canonical targets, while the proposed ConvNeXt + GAM model provides strong regression performance across multiple target categories in the phase-map-based setting. Overall, the proposed framework offers an efficient surrogate approximation of the nonlinear mapping between the scattering conditions and OAM mode-purity spectra under simulated conditions. This study is positioned as a simulation-level surrogate modeling investigation, and extension to experimental measurements or real-scene applications remains as future work. Full article

Based on the electron cyclotron maser instability, gyrotrons are capable of generating high-power electromagnetic vortex waves. In conventional axisymmetric configurations, the electron beam typically lifts the azimuthal degeneracy between co-rotating and counter-rotating modes, leading to a state of intense mutual suppression. This study elucidates a fundamental transition from such competitive dynamics to a stable cooperative coexistence, driven by symmetry-breaking perturbations. Using a time-dependent self-consistent interaction theory, we investigate the intermodal dynamics of the counter-rotating ${\mathrm{T}\mathrm{E}}_{6,2}$ mode pair in a terahertz gyrotron. Our results reveal that the azimuthal intermodal phase beating dictates a reciprocal energy exchange that ensures single-mode dominance. However, electron beam misalignment introduces a significant azimuthal non-uniformity in the coupling strength. This non-uniformity effectively neutralizes the competitive disparity between the two modes. At a critical offset, the system undergoes a “territorial division,” where the orthogonal vortex modes spatially segregate by dominating distinct azimuthal segments of the annular beam. This spatial segregation eliminates nonlinear cross-suppression, allowing for the stable coexistence of both rotational states. These findings offer a new perspective on multi-mode interactions in non-ideal systems and establish a robust theoretical framework for the active manipulation of vortex waves in high-performance THz radiation sources. Full article

High frame rate imaging of synthetic aperture radar (SAR), also known as video SAR (ViSAR), has attracted extensive research in recent years. When ViSAR system parameters are fixed, there is a technical trade-off between high frame rates and high resolution. In traditional ViSAR, the frame rate is usually increased by increasing the carrier frequency to increase the azimuth modulation frequency and reducing the synthetic aperture time. This paper attempts to propose a strip non-overlapping mode ViSAR based on Orbital Angular Momentum (OAM) beams, which uses the topological charge of vortex electromagnetic wave (VEW) to improve the azimuth modulation frequency, to improve the frame rate. By introducing the concept of VEW frame splitting, a corresponding time-varying topological charge mode is designed for ViSAR imaging. This design successfully introduces an additional azimuth modulation frequency while maintaining the original imaging resolution, thus significantly improving the frame rate performance of the ViSAR system. However, the Bessel function term in VEW causes amplitude modulation in the echo signal, while the additional frequency modulation causes the traditional matching filter to fail. To address these problems, an improved Range-Doppler algorithm (RDA) is proposed in this paper. By employing the range cell center approximation method, the negative effect of the Bessel function on imaging is reduced effectively. Furthermore, for the introduction of tuning frequency, the azimuth matched filter is specially improved, which effectively prevents the defocusing issues caused by the mismatch of tuning frequency. Finally, the computer simulation results prove that the ViSAR system and imaging algorithm based on VEW can effectively improve the frame rate of ViSAR and maintain the imaging resolution, which provides a research direction for the development of ViSAR technology. Full article

Light carrying orbital angular momentum (OAM) has emerged as a promising tool for manipulating light–matter interactions, providing an additional degree of freedom to explore chiral-optical phenomena at the nanoscale. When such vortex beams interact with chiral metamaterials, a unique phenomenon of optical asymmetry known as vortical dichroism (VD) arises. Nevertheless, most existing chiral metamaterials exhibit limited VD responses, and the underlying physical mechanisms are yet to be fully clarified. In this work, we propose three-dimensional spiral metamaterials that achieve gigantic VD effect. This pronounced VD effect originates from the intrinsic coupling between the spiral structure and the chirality inherent to optical vortices, which leads to strongly asymmetric scattering intensities for left- and right-handed OAM beams of opposite topological charges. Numerical simulations confirm a remarkable VD value of 0.69. Further analysis of electric field distributions reveals that the asymmetric VD response stems from a handedness-dependent excitation of distinct electromagnetic modes. For opposite handedness, spatial mode mismatch results in enhanced scattering. In contrast, matching handedness enables efficient energy coupling into a guided spiral mode, which suppresses scattering. These findings not only deepen the physical understanding of VD mechanisms but also establish a versatile platform for developing advanced chiral photonic devices and enhancing OAM-based light–matter interactions. Full article

Exploding or concentrating beams, vortex beams, and cylindrical vector beams have a precisely shaped transversal amplitude profile such that they produce a continuously concentrating and intensifying focal spot upon focusing as the lens aperture is opened. This effect is the physical manifestation of the mathematical fact that Fresnel diffraction integral predicts an infinite intensity at the focus when the aperture effects are ignored. Here, using a full electromagnetic, nonparaxial focusing model, we show that the singularity in exploding cylindrical vector beams is an artifact of the paraxial approximation. Nevertheless, the exploding or concentrating effect, alien to any other light beam with finite power, keeps going up to unit numerical aperture, equivalent to infinite aperture radius. This unique feature enables a dynamic control of the focal intensity and spot size down to the sub-wavelength scale using a single light beam, imitating similar control when focusing an ideal plane wave, but requiring a finite amount of power. Full article

Multifunctional metasurfaces, capable of flexible electromagnetic wave manipulation, have become a focus of research for their high integration and utility. In particular, those operating simultaneously in transmission and reflection modes have attracted growing interest, as they integrate multiple functions within a single aperture, save physical space, and further expand wave control capabilities across full space. In this work, an inspiring strategy of transmission-reflection-integrated bifunctional metasurface by hybridizing geometric phase and propagation phase is proposed. The transmission and reflection modes can be independently and flexibly controlled in full space: the co-polarized reflection under left-handed circular polarization (LCP) incidence is governed by rotation-induced geometric phase modulation, while the co-polarized transmission under right-handed circular polarization (RCP) incidence is modulated through scaling-induced propagation phase modulation. Moreover, arbitrary amplitude modulation of the co-polarized transmission under RCP incidence can be realized by incorporating lumped resistors. As a proof of concept, a bifunctional meta-device is constructed, which can generate vortex beam carrying arbitrary topological charge for LCP reflected wave and achieve high-quality holographic imaging for RCP transmitted wave. Both the simulated and experimental results validate the feasibility of the proposed strategy, which significantly enhances the integration density of multifunctional metasurfaces while reducing inter-functional crosstalk, expanding its potential applications in electronic engineering. Moreover, it can also serve as a fundamental machine learning platform, facilitating multimodal fusion and cross-modal learning in radar signals and visual imaging. Full article

The photon spin Hall effect (PSHE) arises from the spin–orbit interaction of light. Metasurfaces enable precise control over the PSHE through their influence. Using electromagnetic simulations as its foundation, this work engineers a metal–insulator–metal (MIM) metasurface for generating vortex beams in the near-infrared band, targeting enhanced modulation of the PSHE. Electromagnetic simulations embed vanadium dioxide (VO₂)—a thermally responsive phase-change material—within the MIM metasurface architecture. Numerical evidence confirms that harnessing VO₂’s insulator–metal-transition-mediated optical switching dynamically tailors spin-dependent splitting in the illuminated MIM-VO₂ hybrid, thereby achieving a significant amplification of the PSHE displacement. Electromagnetic simulations determine the reflection coefficients for both VO₂ phase states in the MIM-VO₂ structure. Computed spin displacements under vortex beam incidence reveal that VO₂’s phase transition couples to the MIM’s top metal and dielectric layers, modifying reflection coefficients and producing phase-dependent PSHE displacements. The simulation results show that the displacement change of the PSHE before and after the phase transition of VO₂ reaches 954.7 µm, achieving a significant improvement compared with the traditional layered structure. The dynamic modulation mechanism of the PSHE based on the thermal–optical effect has been successfully verified. Full article

We have developed the paraxial approximation for electromagnetic fields in arbitrary isotropy-broken media in terms of the ray–wave tilt and the curvature of materials’ Fresnel wave surfaces. We have obtained solutions of the paraxial equation in the form of biaxial Gaussian beams, which is a novel class of electromagnetic field distributions in generic isotropy-broken materials. Such beams have been previously observed experimentally and numerically in hyperbolic metamaterials but have evaded theoretical analysis in the literature up to now. Biaxial Gaussian beams have two axes: one in the direction of the Abraham momentum, corresponding to the ray propagation, and another in the direction of the Minkowski momentum, corresponding to the wave propagation, in agreement with the recent theory of refraction, ray–wave tilt, and hidden momentum [Durach, 2024]. We show that the curvature of the wavefronts in the biaxial Gaussian beams correspond to the curvature of the Fresnel wave surface at the central wave vector of the beam. We obtain the higher-order modes of the biaxial beams, including the biaxial Hermite–Gaussian and Laguerre–Gaussian vortex beams, which opens avenues toward studies of the optical angular momentum (OAM) in isotropy-broken media, including generic anisotropic and bianisotropic materials. Full article

We derive the analytical expressions for root-mean-square (rms) beam wander (BW) and relative BW of a twisted electromagnetic elliptical vortex (TEEV) beam propagating through non-Kolmogorov atmospheric turbulence with the help of the extended Huygens–Fresnel principle and the second-order moments of the Wigner distribution function (WDF). Our numerical findings demonstrate that the BW of a TEEV beam with a small ellipticity, a large topological charge as well as a small waist width and initial coherent length is less affected by the turbulence. It can be also found that the effect of turbulence with a larger outer scale of turbulence, a generalized exponent parameter, and a generalized structure parameter on BW is more obvious. It is interesting to find that the effect of atmospheric turbulence on BW for a TEEV beam can be effectively reduced by regulating jointly the symbols and sizes of the twisted factor and topological charge. Therefore, modulation of the structure parameters of a TEEV beam provides a new way to mitigate turbulence-induced beam wander. Our work will be useful for free-space optical communications, remote sensing, and lidar distance measurement. Full article

In this study, we obtained the intensity and orbital angular momentum (OAM) spectral distribution of the scattering fields of vortex electromagnetic beams illuminating electrically large targets composed of different materials. We used the angular spectral decomposition method to decompose a vortex beam into plane waves in the spectral domain at different elevations and azimuths. We combined this method with the physical optics algorithm to calculate the scattering field distribution. The OAM spectra of the scattering field along different observation radii were analyzed using the spiral spectrum expansion method. The numerical results indicate that for beams with different parameters (such as polarization, topological charge, half-cone angle, and frequency) and targets with different characteristics (such as composition), the scattering field intensity distribution and OAM spectral characteristics varied considerably. When the beam parameters change, the results of scattering from different materials show similar changing trends. Compared with beams scattered by uncoated metal and dielectric targets, the scattering field of the coating target can better maintain the shape and OAM mode of beams from the incident field. The scattering characteristics of metal targets were the most sensitive to beam-parameter changes. The relationship between the beam parameters, target parameters, the scattering field intensity, and the OAM spectra of the scattering field was constructed, confirming that the spiral spectrum of the scattering field carries the target information. These findings can be used in remote sensing engineering to supplement existing radar imaging, laying the foundation for further identification of beam or target parameters. Full article

As a promising platform for versatile electromagnetic (EM) manipulations, metasurfaces have drawn wide interest in recent years due to their unique EM properties and small footprints. However, although great efforts have been made to achieve multifunctionalities, the design of tunable metasurfaces with high compactness is still challenging. Here, a simple yet powerful design methodology for single-layered reconfigurable metasurfaces composed of nonvolatile phase-change material Ge₂Sb₂Se₄Te₁ (GSST) is proposed with average working amplitudes of 72.6% and 53% at different crystallization levels. The proposed metasurfaces could not only enable independent phase control at different crystallization levels but also introduced another polarization degree of freedom. As a proof of concept, we numerically demonstrate three kinds of metadevices in the infrared region achieving a multi-focus metalens with tunable foci, multistate vortex beam generator with adjustable topological charges and multi-channel meta-hologram with three independent information channels. It is believed that these multifunctional metasurfaces with both tunability and compactness are promising for various applications including information encryption, chiroptical spectroscopy, chiral imaging and wireless communication. Full article

The phase and polarization of electromagnetic waves can be conveniently manipulated by the dynamic phase and geometric phase elements. Here, we propose a compact optical integration of dynamic phase and geometric phase to generate arbitrary vector vortex beams on a hybrid-order Poincaré sphere. Two different technologies have been applied to integrate dynamic and geometric phase elements into a single glass plate to modulate the phase and polarization of light simultaneously. A spiral phase structure is made on one side of a glass substrate with optical lithography and a geometric phase metasurface structure is designed on the other side by femtosecond laser writing. The vector polarization is realized by the metasurface structure, while the vortex phase is generated by the spiral phase plate. Therefore, any desirable vector vortex beams on the hybrid-order Poincaré sphere can be generated. We believe that our scheme may have potential applications in future integrated optical devices for the generation of vector vortex beams due to its the high transmission efficiency and conversion efficiency. Full article

Metasurface regulates the polarization, phase, amplitude, frequency, and other characteristics of electromagnetic waves through the subwavelength microstructure. By using its polarization characteristics, it can realize the functions of optical rotation and vector beam generation. It is the most widely used method of regulation. However, parallel optical manipulation, imaging, and communication usually require polarization-insensitive focused (or vortex) arrays of beams, so polarization-independent wavelength multiplexing optical systems need to be considered. In this paper, the genetic algorithm combined with the computer-generated hologram (CGH) is used to control the transmission phase of the structure itself, and on the basis of wavelength multiplexing, the corresponding array of focused or vortex beams without the polarization selection property is realized. The simulation software results show that the method has a huge application prospect in optical communication and optical manipulation. Full article

In this paper, using a Richards–Wolf method, which describes the behavior of electromagnetic waves at the sharp focus, we show that high-order spin and orbital Hall effects take place at the focal plane of tightly focused laser beams. We reveal that four local subwavelength regions are formed at the focus of a linearly polarized optical vortex with unit topological charge, where the spin angular momentum behaves in a special way. Longitudinal projections of the spin angular momentum are oppositely directed in the adjacent regions. We conclude that this is because photons falling into the neighboring regions at the focus have the opposite spin. This newly observed phenomenon may be called a spin Hall effect of the 4-th order. We also show that tightly focusing the superposition of cylindrical vector beams of the m-th and zero-order produces 2m subwavelength regions in the focal plane, such that longitudinal projections of the orbital angular momentum are oppositely directed in the neighboring regions. This occurs because photons falling into the neighboring regions at the focus have the opposite signs of the on-axis projections of the orbital angular momentum. This phenomenon may be termed an orbital Hall effect of the 2m-th order. Full article

We propose a scheme to exchange optical vortices beyond electromagnetically induced transparency (EIT) but based on four-wave mixing (FWM) in a five-level atomic system consisting of two $\Lambda$ subsystems linked via a weak driving field. When the laser fields are strong enough, the quantum interference responsible for the EIT in each $\Lambda$ subsystem is washed out, giving rise to the Autler-Townes splitting (ATS). When only one of the control fields carries an optical vortex, it is shown that the generated FWM field obtains the vorticity of the vortex control. We distinguish between three different regimes, i.e., a pure EIT, a joint EIT-ATS, and a dual-ATS, where the optical angular momentum (OAM) translation can take place. Elaborating on the distinction between three regimes through numerical analysis, we find that the maximum energy conversion efficiency is obtained in the joint EIT-ATS and dual-ATS regimes. The latter is more favorable as the absorption losses vanish as the beam propagates into the atomic cloud. The results may find applications in the implementation of high-efficient frequency and OAM conversion devices for quantum information processing. Full article