Tractor beams have received increasing attention. The generation of tractor beams is an important issue in practical applications. In this paper, Pancharatnam-Berry (PB) metasurfaces were designed to generate non-paraxial Bessel tractor beams. The optical pulling forces (OPFs) exerted on dielectric particles with specific radii on the axis of the non-paraxial Bessel beam were obtained. The presence of OPFs depended on the size of the particles, indicating the potential of the non-paraxial Bessel tractor beam for separating particles. Such a feature illustrated the possibility of selective optical manipulation and sorting. This setup of metasurface has good potential in such aspects as lab-on-a-chip.
Optical chains have received widespread attention due to their unique characteristics of high intensity and multiple potential wells. Many methods have been used to generate optical chains, such as using diffractive optical elements or 4Pi systems to modulate vector beams. But these methods require additional phase elements or more complex optical systems. In this work, a single-layer polarization-insensitive metalens with phase distribution of the binary optical element was used to focus the radially polarized beam to generate optical chains. The Richards-Wolf vector diffraction theory was employed to calculate the focal field distribution of the radially polarized beam. Optical chains generated by both the simulated and theoretical calculation are composed of alternating solid points and bubbles which indicates that the theoretical results are in agreement with the simulation results. The introduction of metalens reduces the volume of the optical system, which is conducive to the miniaturization and integration of the optical system. This work may contribute to particle trapping and manipulation, optical micro-nano processing, etc.
Controlling the polarization state of light has important applications in optical communications, imaging, and detection. The latest developments in metamaterials can miniaturize optical components to the sub-wavelength range. The previous work cannot simultaneously take into account the two aspects of simple structure and high efficiency. Here, we propose a simple and effective circular polarization converter, which is composed of three layers of rotated gold split-rings and SiO2 substrate. It can convert LCP and RCP into orthogonally polarized light in two adjacent bands, with conversion efficiencies of 70% and 52%, respectively. Meanwhile, an asymmetric transmission of about 0.5 is achieved efficiently. The error caused by allowable changes of structural parameters will not affect the efficiency of asymmetric polarization conversion, indicating the high stability of our converter. As an ultra-thin planar optical element, the proposed metasurface can be used in integrated photonics, optical sensing and other fields. The asymmetric transmission in adjacent frequency bands may contribute to information encoding and decoding in applications of optical communication.
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