Ultrasmall Optical Vortex Knots Generated by Spin‐Selective Metasurface Holograms

Abstract

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.201900263. Metasurface HologramsOptical vortex, which refers to a beam of photons that propa gates with a singularity in phase, has received an increasing amount of attention. [1][2][3] For example, optical vortex carrying orbital angular momentum (OAM) has been exploited for many important applications, including optical trapping and manipulation, [4][5][6] metrology, [7][8][9] imaging, [10,11] and free space communication. [12][13][14][15] To date, various methods have been pro posed for the generation of optical vortices with the OAM at the macroscopic scale, such as spiral phase plates, [16] computer gen erated holograms, [17] liquid crystal qplates, [18] and spatial light

Creation of acoustic vortex knots

“…In electromagnetic fields, the behavior of a monochromatic light beam propagating along the z -direction should be described by the vector Helmholtz equation. For the transverse paraxial case, the optical vortex knot has been experimentally constructed in the laser beam 16 – 18 , 20 – 23 based on the far-field holographic scheme, where the algebraic knot function at z = 0 plane [ f Hopf ( x , y , z = 0) or f trefoil ( x , y , z = 0)] is embedded into the waist of a Gaussian beam. In another case, it has been proposed theoretically that the longitudinal polarization component of light with the knotted vortex line can be formed by non-paraxially focusing a subwavelength optical beam 19 .…”

“…At present, the knot-type topological structures have been realized in various systems, including plasmas 2 , quantum and classical fluids [3][4][5] , quantum and classical field theory [6][7][8][9] , and liquid crystals 10,11 . In electromagnetic fields, the isolated optical vortex [15][16][17][18][19][20][21][22][23] and polarization knots 24,25 have also been engineered and observed, which are considered to have potential applications to confine cold atoms with complex spatial topologies. However, it is still unknown whether the acoustic wave can also possess the same topological properties.…”

Optical vortex knots and links via holographic metasurfaces

“…Compared with conventional technologies referring to SLMs, holography based on metasurfaces exhibits significant advantages in the control of polarization and large view angle [86], because of their inherent properties of ultra-small pixel, polarization response, and flexible phase gradient. More recently, some subtle metasurfaces have been proposed to the construction of topological optical fields with ultrasmall scale [87][88][89][90]. Typically, a metal type metasurface that consists of rectangular gold nanorods was proposed to realize the holographic reconstruction of vortex knots and links [87].…”

“…More recently, some subtle metasurfaces have been proposed to the construction of topological optical fields with ultrasmall scale [87][88][89][90]. Typically, a metal type metasurface that consists of rectangular gold nanorods was proposed to realize the holographic reconstruction of vortex knots and links [87]. This reflective metal metasurface was designed based on the Pancharatnam-Berry (PB) phase, according to the pure phase-encoded holograms calculated from the complex amplitude of the Hopf linked or trefoil knotted light field at the z = 0 plane.…”

“…A metal metasurface to generate ultrasmall vortex knot and link[87]. (a) Schematic of construction principle.…”