We propose a vortex beam generator based on a nanometer spiral slit and explore the propagation rule of the topological charge. Compared to the common methods of generation of a vortex beam with a fixed topological charge, the optical vortex generated by the proposed vortex beam generator has the topological charge varying with the propagation distance. The value of topological charge can be modulated by the geometric charge of the spiral slit and the propagation distance. Theoretical analysis predicts the variation rule of the topological charge of vortex beam in the near field, and numerical simulations and experimental measurement verify the proposed scheme. Discussion on the shape and structure of the spiral slit is also presented. This work provides the theoretical foundation for the generation of a vortex field with variable topological charge. The simple geometry of the vortex beam generator and the flexible modulation of the topological charge must inspire applications of the vortex beam.

Due to its unbounded and orthogonal modes, the orbital angular momentum (OAM) is regarded as a key optical degree of freedom (DoF) for future information processing with ultra-high capacity and speed. Although the manipulation of OAM based on metasurfaces has brought about great achievements in various fields, such manipulation currently remains at single-DoF level, which means the multiplexed manipulation of OAM with other optical DoFs is still lacking, greatly hampering the application of OAM beams and advancement of metasurfaces. In order to overcome this challenge, we propose the idea of multiplexed coherent pixel (MCP) for metasurfaces. This approach enables the manipulation of arbitrary complex-amplitude under incident lights of both plane and OAM waves, on the basis of which we have realized the multiplexed DoF control of OAM and wavelength. As a result, the MCP method expands the types of incident lights which can be simultaneously responded by metasurfaces, enriches the information processing capability of metasurfaces, and creates applications of information encryption and OAM demultiplexer. Our findings not only provide means for the design of high-security and high-capacity metasurfaces, but also raise the control and application level of OAM, offering great potential for multifunctional nanophotonic devices in the future.

A kind of metasurface cylindrical vector light (CVL) generator in the visible region is proposed. This kind of CVL generator consists of nanometer holes etched on silver film, and it can change any linearly or circularly polarized light into the CVL in nanoscale. The order of the generated CVL is controlled by the rotation of the holes and its polarization state changes with the incident polarization condition. The base transformation theory guides is used to design the metasurface. The numerical simulations for the transmission of the proposed CVL generators confirm the validity of the theoretical predictions, and they also provide the available parameters for practical metasurface devices. The experimental results verify the performance of the proposed metasurface CVL generators. This kind of vector light generator has the advantages of thin and compact structure, polarization multiplexing and convenient manufacture. This work paves a new path for designing the miniature devices to generate the vector light field and it will promote the applications of polarization devices in optical integration and micro-manipulation.

In view of wide applications of structured light fields and plasmonic vortices, we propose the concept of compound plasmonic vortex and design several structured plasmonic vortex generators. This kind of structured plasmonic vortex generators consists of multiple spiral nanoslits and they can generate two or more concentric plasmonic vortices. Different from Laguerre-Gaussian beam, the topological charge of the plasmonic vortex in different region is different. Theoretical analysis lays the basis for the design of radially structured plasmonic vortex generators and numerical simulations for several examples confirm the effectiveness of the design principle. The discussions about the interference of vortex fields definite the generation condition for the structured vortex. This work provides a design methodology for generating new vortices using spiral nanoslits and the advanced radially structured plasmonic vortices is helpful for broadening the applications of vortex fields.

In view of wide applications of vector beams and powerful light manipulation ability of metasurfaces, this paper studies the generation of two kinds of vector beams with different polarization singularities based on metasurfaces. One kind of vector beams are the linearly polarized vector beam with uncertain polarization orientation and the other kind of vector beams are the elliptically polarized vector beam with hybrid polarization states with uncertain polarization orientation, ellipticity and handedness. These vector beams can be decomposed into two or more uniform polarization states carrying the spiral phases. The metasurfaces consisting of rotated cross nanoholes are designed to generate vector beams on basis of the decomposition of vector beams and phase modulation of nanoholes. The simulation results verify the availability of the designed metasurfaces and the experiment results validate the generation of two kinds of vector beams. The generation of complex vector beams based on compact metasurfaces can bring more application possibilities of vector beams in classical physics and quantum sciences.

Highly customized and miniaturized structured light is expected in many application fields. A kind of structured vortex generators is proposed based on a metasurface consisting of rectangular nanoholes etched in a silver film, and the generated vortices with the same or different topological charges are distributed along the radial direction. The geometric metasurface is completed with the help of optical holography technology, and the structured vortex generator possesses high working efficiency and large information capacity. The proposed vortex generators work under circularly polarized light illumination, and the reproduced vortices of multiplexing vortex generator depend on the handedness of the circularly polarized light. This work paves a way to generate new structured light fields. The radially distributed vortices may be utilized to simultaneously screen or separate microparticles. The compact design of the structured vortex generator and the convenient switch of different structured vortices will be a benefit to expand the applications of structured vortex fields.