The Realization of Super-Capacity Perfect Vector Vortex Beams

Research Background and Problem Statement

Optical vortices, with their unique orbital angular momentum (OAM) characteristics, have demonstrated great potential in applications such as optical multiplexing, particle manipulation, imaging, holographic display, optical communication, and optical encryption. However, traditional vortex beams are usually generated using a global phase modulation method, which results in a single topological charge (TC) and a uniform intensity distribution, limiting further exploration of spatial information. Although previous studies have attempted to enhance information capacity by introducing degrees of freedom such as polarization, local spatial intensity information remains underexplored.

To overcome this limitation, a research team from Tsinghua Shenzhen International Graduate School, the Hong Kong Polytechnic University, Jinan University, and other institutions proposed an entirely new concept of “spatial-frequency patching metasurface” for generating a novel type of super-capacity perfect vector vortex beam (SC-PVVB). This beam achieves localized control in three dimensions: morphology, polarization azimuth, and ellipticity angle, supporting at least 13 channels of information encoding, greatly enhancing the information capacity and application potential of the beam.

Paper Source and Author Information

This research paper, titled “A Spatial-Frequency Patching Metasurface Enabling Super-Capacity Perfect Vector Vortex Beams,” was co-authored by Yu Zhipeng, Gao Xinyue, Yao Jing, and others, with Yu Zhipeng and Gao Xinyue as co-first authors. The corresponding authors are Lai Puxiang, Li Xiangping, and Song Qinghua. The research team comes from several renowned institutions, including Tsinghua Shenzhen International Graduate School, the Department of Biomedical Engineering at the Hong Kong Polytechnic University, and the Institute of Photonics Technology at Jinan University. The paper was published in the open-access journal eLight in 2024, with the DOI: 10.1186/s43593-024-00077-3.

Research Methods and Experimental Procedures

a) Research Workflow and Experimental Design

The core of this study is the realization of spatial frequency segmentation control of vortex beams through a “spatial-frequency patching metasurface.” The entire research is divided into the following main steps:

1. Theory and Design of Spatial-Frequency Patching

The research team first proposed a new mathematical method to decompose irregular seamless curves in the far field into the superposition of several elliptic arcs and apply the desired spatial-frequency distributions to each partial region in the near field. This approach allows for local control of the morphology and TC of vortex beams. Specifically, for a complete elliptic perfect vortex beam (EPVB), its phase distribution can be described by the following formula: [ \phi_{oam}(l, a, b) = l \cdot \arctan\left(\frac{a}{b} \tan(\theta)\right) ] where (a) and (b) are the horizontal and vertical normalization factors, (l) is the TC, and (\theta) is the radial angle. By breaking the complete ellipse into four quarter elliptic arcs (Segments I-IV), the research team calculated the phase distribution of each part and stitched them together into a complete beam.

2. Design and Fabrication of Metasurface Structure

The research team designed a geometric metasurface based on a titanium dioxide (TiO₂) nanopillar array. These nanopillars have a height of 600 nm and are arranged on a square lattice with a period of 300 nm. By adjusting the rotation angle of the nanopillars, precise control of the geometric phase (Pancharatnam-Berry Phase, PB Phase) of the vortex beam can be achieved. The team used electron beam lithography (EBL) and reactive ion etching (RIE) techniques to fabricate the metasurface samples.

3. Experimental Verification and Data Analysis

The experimental setup includes a supercontinuum laser, acousto-optic tunable filter (AOTF), polarizer, lens group, and scientific CMOS camera. The research team illuminated the metasurface with linearly polarized light, measured the optical field distribution at different wavelengths, and analyzed the polarization state using Stokes parameters. The experimental results show that the generated SC-PVVB achieves independent control in three dimensions: morphology, polarization azimuth, and ellipticity angle.

b) Key Research Results

1. Local Control of Morphology and Topological Charge

The experimental results demonstrate that the research team successfully generated SC-PVVBs with locally controllable morphology and TC using the spatial-frequency patching method. For example, in a beam composed of four elliptic arcs, the TCs of the respective parts are 2, 6, 4, and 8, with an equivalent TC of 5. This localized control capability significantly surpasses traditional methods.

2. Multi-Dimensional Control of Polarization State

The research team further achieved independent control of polarization azimuth and ellipticity angle by superimposing two orthogonally circularly polarized SC-PVVBs. Experimental data shows that the polarization state (such as linear, left-handed elliptical, or right-handed elliptical polarization) of each elliptic arc can be precisely controlled, thereby supporting at least 13 channels of information encoding.

3. Broadband Response and Robustness

The experiments verified the broadband response characteristics of SC-PVVBs within the visible spectrum (460–650 nm). Additionally, the research team enhanced information capacity further by designing optimized Dammann gratings for multiplexed transmission.

c) Research Conclusions and Significance

Scientific Value

This study is the first to propose the concept of “spatial-frequency patching metasurfaces,” breaking through the global limitations of traditional vortex beams in terms of morphology and TC, achieving localized control capabilities. This achievement provides new insights into the fundamental research of optical vortices.

Application Value

The multi-dimensional control capabilities and ultra-high information capacity of SC-PVVBs make them highly promising for applications in optical encryption, high-density data communication, and particle manipulation. For instance, by encoding 13 channels into binary values, a single SC-PVVB can generate (2^{13}) possible combinations, greatly enhancing the security and efficiency of information transmission.

d) Highlights of the Study

1. Innovative Method: The proposal of the spatial-frequency patching method offers a new perspective for the design of vortex beams.
2. Multi-Dimensional Control: Independent control in three dimensions—morphology, polarization azimuth, and ellipticity angle—is achieved.
3. Ultra-High Capacity: A single beam supports at least 13 channels, significantly surpassing existing technologies.
4. Broadband Response: Demonstrates excellent performance stability across the visible spectrum.

e) Other Valuable Information

The research team also developed an optimization method based on a genetic algorithm to design the phase distribution of Dammann gratings, further improving the efficiency of multiplexed transmission.

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Summary

This paper successfully achieved the generation of super-capacity perfect vector vortex beams through the innovative method of “spatial-frequency patching metasurfaces.” Its research outcomes not only theoretically expand the control dimensions of optical vortices but also demonstrate immense practical application potential. This study marks a new phase in optical information processing technology, laying a solid foundation for the future development of high-density, high-security optical communication systems.