High efficiency and broad bandwidth terahertz vortex beam generation based on ultra-thin transmission Pancharatnam–Berry metasurfaces*

Abstract

The terahertz (THz) vortex beam generators are designed and theoretically investigated based on single-layer ultra-thin transmission metasurfaces. Noncontinuous phase changes of metasurfaces are obtained by utilizing Pancharatnam–Berry phase elements, which possess different rotation angles and are arranged on two concentric rings centered on the origin. The circularly polarized incident THz beam could be turned into a cross-polarization transmission wave, and the orbital angular momentum (OAM) varies in value by lℏ. The l values change from ± 1 to ± 5, and the maximal cross-polarization conversion efficiency that could be achieved is 23%, which nearly reaches the theoretical limit of a single-layer structure. The frequency range of the designed vortex generator is from 1.2 THz to 1.9 THz, and the generated THz vortex beam could keep a high fidelity in the operating bandwidth. The propagation behavior of the emerged THz vortex beam is analyzed in detail. Our work offers a novel way of designing ultra-thin and single-layer vortex beam generators, which have low process complexity, high conversion efficiency and broad bandwidth.

Ultrathin and Ultra‐Broadband Terahertz Single‐Layer Metasurface Based on Double‐Arrow‐Shaped Resonator Structure for Full‐Space Wavefront Manipulation

“…Vortex wave beam carried with orbital angular momentum (OAM) has a helical phase profile, which can significantly increase the capacity and have potential applications in modern communication and quantum physics. [13,14,26,27,36] To generate a vortex wave beam with a phase distribution of e il𝜑 , where 𝜑 and l are the azimuthal angle and the order number of topological charge, a single-layer MS is constructed by arranging some unit-cells with different orientation angle, similarly to azimuthally varying pattern. With a certain order topological charge of l, the single-layer MS unit-cell should be arranged with a spiral form and the corresponding phase distribution at position (x, y) is written as: [14,43] 𝜑(x, y) = l ⋅ arctan(y∕x) (9) Here, we just consider the vortex wave beams with topological charges of l = +1, −1, +2, and −2 to validate the performance of single-layer MS for both reflection and transmission mode.…”

“…Fortunately, phase gradient MSs from resonance or geometric phase, or both of them opened the scope for achieving various functional devices. [2] In past decade, MSs have been become a ideal platform to achieve various exotic physical phenomena and some extraordinary functionalities successfully, such as anomalous refraction, [6][7][8] holography, [9,10] focusing and imaging, [11,12] vortex beam generation, [13,14] polarization converter, [15,16] perfect absorber, [17,18] beam shaping, [19,20] eta. MS can be used to construct various THz devices with different functions, further promoting the THz application.…”

High efficiency holographic impedance metasurface for dual circular polarized OAM waves

“…4 Phase gradient metasurfaces (MSs) have also been widely used to generate OAM wave due to their unique properties of low profile, easy fabrication, high efficiency, and low cost. [5][6][7][8][9][10][11] However, it is difficult to obtain multifunctional OAM beams with arbitrary direction, polarization and OAM mode by the conventional technique. The THIMS provide a good solution.…”

“…Phased array antennas (usually circular arrays) were utilized for the generation of OAM radiation 4 . Phase gradient metasurfaces (MSs) have also been widely used to generate OAM wave due to their unique properties of low profile, easy fabrication, high efficiency, and low cost 5–11 . However, it is difficult to obtain multifunctional OAM beams with arbitrary direction, polarization and OAM mode by the conventional technique.…”

Ultrafast Modulation of THz Waves Based on MoTe2-Covered Metasurface

“…The appearance of metasurfaces has provided an effective solution to modulate THz waves [ 17 , 18 , 19 ]. By altering the geometry and arrangement pattern of sub-wavelength meta-atoms in metasurfaces, the amplitude, phase and polarization of THz waves can be engineered, presenting numerous unusual applications, such as ultrathin flat lenses [ 20 , 21 ], THz broadband filter [ 22 ], and THz vortex beam generation [ 23 , 24 ]. Generally, once a planar metal structure is fabricated, its electromagnetic (EM) properties are unable to be actively tuned, which blocks the further flexible regulation of EM waves.…”