Objective Metasurface effective and flexible manipulation of electromagnetic waves in sub-wavelength size has attracted widespread attention.Many metasurface-based devices have been reported in recent years,such as anomalous reflectors/refractometers,vortex beam generators,and polarization converters.However,most reported metasurfaces can only manipulate transmitted or reflected electromagnetic waves.Due to the limitations of half-space manipulation of terahertz waves,it is necessary to design a full-space metasurface,which can realize the manipulation of terahertz waves with reflected and transmitted modes.However,the reported full-space metasurfaces can only manipulate one of the polarized waves for circularly polarized and linearly polarized waves.Therefore,it is very urgent to design a metasurface that can simultaneously reflect and transmit terahertz waves of both circularly polarized and linearly polarized waves.Methods In this paper,we propose an omnidirectional bifunctional terahertz metasurface to generate the modulation of radiometric polarization and reflect circularly polarized waves.The unit cell has nine layers,which are elliptical metal pattern,polyimide,metal grating,polyimide,rectangular metal strip structure,polyimide,metal grating,polyimide,and elliptical metal pattern.When a circularly polarized terahertz wave is incident in the metasurface,it can produce reflected vortex beam splitting,deflection vortices,and superimposed vortices.When the y(x)linearly polarized wave is incident along the±z direction,the designed metasurface produces the x(y)linearly polarized wave transmission and transformation.This terahertz metasurface device offers great flexibility in terahertz wave regulation.Results and Discussions When the circularly polarized terahertz wave is incident,the metasurface structure generates vortex beams with topological charges of l=±1 and±2 at frequencies of 1.4 THz and 1.5 THz,four offset vortex beams with l=-1 and+2 at 1.3 THz,and superposition vortex beams with l=-1 and+2 at 1.5 THz.When the linearly polarized terahertz wave is incident along the±z direction,the designed metasurface realizes the polarization conversion function of the transmitted wave at the frequency of 0.72 THz,and the polarization conversion rate is greater than 95％.This metasurface provides an innovative idea for the design of bidirectional multifunctional terahertz wave control devices.Conclusions In this paper,an omnidirectional bifunctional terahertz metasurface device is proposed,and the metasurface element structure is composed of the top elliptical metal pattern,polyimide,metal grating,polyimide,rectangular metal strip,polyimide,metal grating,polyimide,and elliptical metal pattern from top to bottom.When circularly polarized terahertz waves are incident to the metasurface,vortex beams with topological charges of l=±1 and±2 are generated at frequencies 1.4 THz and 1.5 THz.Four vortex beams with l=-1 and one deflected vortex beam with l=+2 are generated at 1.3 THz,and superposition vortex beams with l=-1 and+2 are generated at 1.5 THz.When the y(x)polarized wave is incident to the metasurface from the-z(+z)direction,a transmission mode x(y)polarization wave is generated at 0.72 THz.The omnidirectional bifunctional terahertz metasurface proposed in this paper provides a new idea for manipulating terahertz waves with multi-polarized and bidirectional terahertz waves.
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