Multi-beam multi-mode vortex beams generation based on metasurface in terahertz band

Abstract

The generating of vortex beams in the terahertz (THz) band attracts significant attention due to their applications in high-speed communication and high-resolution imaging. In this article, a novel reflective metasurface working in the THz band is designed to generate four vortex beams with different topological charges in different directions. The unit cell is designed based on the geometric phase, and it consists of two metallic (gold) layers and one dielectric layer in between. The top layer of the unit cell includes an elliptic patch and a circular ring, and the bottom layer of the unit cell is a metallic ground. The reflection efficiency of the unit cell is very high due to the presence of metallic ground. To break through the limitations of traditional methods, the metasurface is a good choice to generate beams that carry orbital angular momentum (OAM). Using the concept of geometric phase, the reflection phase of reflective circular polarization (CP) electromagnetic waves can be controlled in an ingenious way. Owing to the property of the geometric phase, inverse phase shift can be achieved for left-handed circular polarization and right-handed circular polarization waves. By utilizing this trait of geometric phase, one can decompose a linear polarization wave into two orthogonal circular polarization waves and control their properties respectively. By rotating the top layer of the unit cell, 360-degree phase shift and the phase distribution satisfying the requirement for generating the multi-beam multi-mode vortex beam can be achieved. In order to control the direction and the topological charge of each beam, based on the geometric phase, the theory of reflectarray and the phase composition principle, the phase distribution of the reflective metasurface is calculated to provide the phase compensation to make the vortex beams point to certain directions. It is worthwhile to point out that the method presented in this paper provides a way to generate complex multi-mode vortex beams in the THz band. The simulations and measurements show that the metasurface can generate four vortex beams with topological charges <i>l</i> = ± 1 and ± 2 in different directions in the THz band. These results also indicate that our design has great potential applications in wireless communication and high-resolution imaging.

Six‐Mode Orbital Angular Momentum Generator Enabled by Helicity‐Assisted Full‐Space Metasurface with Flexible Manipulation of Phase, Polarization, and Spatial Information

“…[ 35,36 ] The efficiencies are mainly limited by the transmission amplitudes and the mutual interference of the meta‐atoms. And the purity of generated OAM modes are also assessed according to Equation (): [ 35,45 ] Here, A l is the amplitude of the OAM mode with topological charges of l , and E (ω) represents the electric field distribution at different azimuth ω in selected analysis area. The normalized amplitudes A l of generated multiple OAM modes are provided in Figure a, which indicates that the OAM modes of = −1 and = −3 occupy the extremely high proportion in the generated two vortex beams and further proves the effectiveness of our design.…”

“…[35,36] The efficiencies are mainly limited by the transmission amplitudes and the mutual interference of the meta-atoms. And the purity of generated OAM modes are also assessed according to Equation ( 18): [35,45] A E e d l il…”

A broadband tunable asymmetric transmission structure design

“…The underlying theory is based on the Generalized Snell's Law (GSL) proposed by Yu [1] et al in 2011, which states that when a beam passes through the interface of two media, the beam will produce phase mutation at the interface. If the phase is reasonably designed and arranged at the interface, many strange physical phenomena can be realized, such as abnormal refraction [2][3][4], beam focusing [5,6], self-acceleration [7], vortex [8][9][10], asymmetric transmission [11][12][13], etc. Once proposed in the field of optics, metasurfaces were rapidly applied to the field of acoustic/elastic files.…”

Arbitrary polarization angle and wavefront manipulation of linearly polarized waves using PB phase

“…As for EM wavefront manipulations, PB phases and resonant phases have been applied to realize desired phase distributions on MS apertures to generate certain shaped beams [17][18][19][20][21][22][23][24][25]. MSs designed based on resonant phases generally have a limited frequency bandwidth.…”

“…PB phase has attracted more attentions due to its characteristics of wideband and convenient phase control by tuning the azimuth angle of unit cells [21]. MSs designed based on PB phase have been used to achieve radar cross section (RCS) reduction, focusing, anomalous reflection, and imaging [21][22][23][24][25]. In [21], an ultra-wideband MS designed based on PB phases was proposed to achieve RCS reductions.…”