Yanzhong Yu

Circularly polarized light, owing to its intrinsic spin angular momentum, plays a pivotal role in emerging fields such as ultrafast optomagnetic manipulation, chiral matter discrimination, and high-density information storage. In circularly polarized focal fields, the polarization purity and its spatial distribution critically determine the efficiency of light-matter interactions and the transfer of angular momentum. Despite their significant application potential, the realization of spatially structured circularly polarized fields with both high polarization purity and dynamic tunability at the diffraction limit remains a key challenge, largely constrained by conventional optical modulation approaches.To address this issue, we propose a method based on an array of orthogonal electric dipoles, enabling the generation of a dynamically tunable circularly polarized ring-shaped focal field in a 4Pi focusing system. The resulting ring focus exhibits a circular polarization state along the radial cross-section at any point on the ring, thereby forming a closed-loop photonic spin angular momentum distribution. This unique spatial polarization configuration is expected to induce a well-matched ring-shaped effective magnetic field via the inverse Faraday effect, offering a promising route toward efficient and low-energy ultrafast magnetization control.

Multi-band circular dichroism (CD) with spectral tunability is highly desirable for chiral metasurface-based sensing and polarization control. In this work, we propose a mid-infrared (MIR) metal–insulator–metal (MIM) chiral metasurface absorber composed of gold-alumina-gold (Au-Al₂O₃-Au) layers, where chirality is introduced by symmetry breaking between two gold elliptical bars in the top layer. Finite element calculations show that the structure operates over 3.5–6.5 µm and produces dual-band CD responses with values of 0.83 and −0.81. The CD magnitude in each band can be independently tuned by adjusting the semi-major or semi-minor axis of the elliptical bars. In addition, rectangular slots inserted into the bars enable continuous redshift of the resonance wavelengths, and the tunability can be further enhanced by optimizing the slot dimensions. These results provide a practical strategy for designing tunable dual-band chiral absorbers and may be useful for future chiral sensing and polarization imaging applications.

In a 4Pi focusing system, this paper proposes a method for constructing flat-top optical focus fields based on the radiation pattern of a spatially uniform line source antenna array. The approach employs a genetic algorithm to optimize key parameters of the antenna array, thereby enabling adjustable length and width as well as controllable spatial orientation of the resulting flat-top focus field. Compared with traditional optical field modulation techniques, this method avoids the complexity of optical system design while offering greater degrees of freedom in field shaping. Numerical results demonstrate that the optimized flat-top focus field exhibits high intensity uniformity exceeding 90% within the defined boundary, indicating a significant advantage in maintaining uniform energy distribution. The proposed focus field provides theoretical support and technical potential for applications such as precision laser fabrication, optical trapping, and high-resolution microscopy.

We present a method for generating three-dimensional super-resolution light-induced magnetization spot arrays with multiple degrees of freedom in the focal region of a 4π-focusing system. By employing the radiation of an orthogonal dipole pair (ODP) with a phase difference of π/2, the required illumination field of the focusing system is derived through a reversing calculation. Based on the vectorial Debye diffraction theory and the inverse Faraday effect (IFE), the tightly focused field and its induced magnetization field are quantitatively evaluated. The results demonstrate that the light-induced magnetization field exhibits super-resolved dimensions, and its orientation can be arbitrarily controlled by adjusting the ODP direction. Furthermore, ODPs are utilized as elements to form antenna arrays with diverse configurations. By optimizing the amplitude and phase modulation of individual array elements, we obtain the illumination at the pupil plane for the desired magnetization fields. The results reveal the generation of a 3D super-resolved magnetization spot array with uniform intensity, where the orientation of each magnetization spot can be independently tailored. This all-optical control of super-resolved magnetization spot array provides a theoretical and technical framework for developing high-density, large-capacity, low-energy-consumption, and high-speed all-optical magnetic storage technologies, demonstrating broad application potential in magneto-optical data storage.

Optical skyrmions, as quasiparticles with non-trivial topological structures, have garnered significant attention in recent years. This paper proposes a method for customized spin angular momentum (SAM) distribution in highly localized focal fields, thereby enabling the generation of SAM skyrmion and bimeron topologies. The skyrmionic SAM textures can be flexibly controlled, such as polarity, vorticity, and helicity. In addition, the two-dimensional projection plane can be arbitrarily oriented within three-dimensional space. By utilizing time-reversal techniques, we obtain the required illumination fields of the 4π-focusing system and subsequently evaluate the tightly focused field using vector Debye integral theory. Our results show that the SAM orientation within the focal field is controlled by the orientation of orthogonal dipole pairs. Using the radiation field of a multi-concentric array of orthogonal dipole pairs, the distribution of SAM orientation in the target plane can be tailored to generate SAM topological structures such as skyrmions and bimerons. Highly localized and tunable SAM engineering holds great potential for applications in optical manipulation, light–matter interactions, optical information processing, transmission, and storage.

Spatially controllable focal fields play a pivotal role in light manipulation and provide significant opportunities for precisely manipulating light–matter interactions in a wide range of applications. In particular, the double-helix focal field—characterized by a distinctive helical structure—exhibits exceptional optical properties, thus differentiating it apart from conventional focal fields. However, the rapid construction of a double-helix focal field with controllable characteristics and a uniform intensity remains a challenging task. Based on the theory of pattern synthesis of an antenna array, we propose and realize the generation of three-dimensional doughnut-spot arrays and double-helix focal fields with specified characteristics in a 4π system by reverse-solving the radiation field of the virtual antenna. Numerical examples indicate that the desired novel focal fields, including features such as shape, orientation, length, and period, could be rapidly, conveniently, and flexibly customized by selecting appropriate parameters for the magnetic dipole array antennas. This method could reveal an avenue for enhanced light manipulation for applications in materials processing, optical lithography, and optical communications.

Optical cages have recently received extensive research and attention due to their potential applications in optical cloaking, microscopic imaging, and trapping particles or atoms. However, the creation of a controllable spatial multi-optical cage array with tunable dark regions comes with a great challenge, which limits the effectiveness of the above applications. To this end, we analyze the radiation pattern of a reverse-focused magneto-electric dipole antenna in a 4Pi optical focusing system. It has been found that three-dimensional (3D) optical cages with different dark regions can be produced by using magneto-electric dipoles with different combination coefficients K. Numerical results indicate that the value of K can be employed to adjust the shapes and sizes of the dark regions, and specifically, when K equals 0.8209, it allows the realization of a perfectly spherical dark region with null central intensity and minimal volume. Furthermore, it is possible to flexibly create spatial multi-optical cage arrays with specified orientations, adjustable numbers, and controllable positions by constructing spatial antenna arrays according to practical requirements. This work may enhance its ability to meet the distinct requirements for optical cages across diverse fields and catalyze the advancement of multi-particle optical trapping and manipulation.

Spatially controllable focal fields play a pivotal role in modern optical engineering, offering opportunities for precise manipulation of light-matter interactions in various applications. Double-helix focal fields, in particular, have gained attention due to their unique helix structure that exhibit distinct optical properties. In this paper, we propose a method for generating a controllable spatial double-helix focal field without optimization. This is accomplished by utilizing the radiation pattern of a virtual helix antenna consisting of magnetic units, which is reverse-focused in a 4π-system. Numerical results indicate that the double-helix focal field can be flexibly customized by selecting the shape and parameters of the helix antenna. The novel and customizable double-helix focal field expands the applications of optical engineering in complex opto-fluidics and optical lithography.