Method Research Of Optical OAM Generation And Manipulation Based On Subwavelength Structures

Orbital Angular Momentum(OAM) light is a hollow ring beam with the helical phase wavefront. Recently, considering the potential applications in optical communication systems, optically trapped microscopic particles and optical micromanipulation, OAM has attracted much attention. The traditional tools of generating the OAM beams, including spiral light modulators, q-plates, computer induced holograms and annular gratings, are inconvenient to switch the OAM modes easily due to the large scale optical components. Planar optical metasurfaces have been demonstrated to tune the electromagnetic wave in the subwavelength domain. To fully benefit from the ultrathin property, plasmonic metasurfaces constructed by subwavelength structures with space-variant shapes and orientation angles have been exploited to generate OAM beams. In this paper, we mainly focus on the method research of optical OAM generation and manipulation based on subwavelength structures. The research is composed of three parts: studying the resonant property of a subwavelength unit under the circularly polarized(CP) light; designing wavelength-selective OAM generation metasurfaces; expolring the propagation properties of OAM beams in a rotating reference frame. The main achievements are listed below:1. Considering the distinct resonant wavelengths of the subwavelenth units with different dimensions, a design method of a metasurface to generate OAM beams with the different topological charges l in the various regions is proposed. Selective generation and focusing of the OAM beams with l = 1 and l = 2 have been demonstrated both numerically and experimentally and the purities of the OAM light in the focal region are 71.78% and 99.01% when the wavelengths of the left-handed circularly polarized(LCP) incident light are 930 nm and 766 nm, respectively. Moreover, the topological charge of the OAM beam can be confirmed using the linearly polarized(LP) incident light, which simplifies the process of detecting the value of l.2. The relationship between the topological charge of the OAM beam and the velocity of the spinning object is given based on the invariance of the OAM light propagation in a rotating reference frame. Considering that the interference between the OAM light with ±l can propagate both steadily and efficiently in the circular waveguide, a measuring method of detecting the velocity of the spinning object is proposed, which is constructed by the metasurface and the circular waveguide.