Polarization converter is firstly proposed with the functions of simultaneously converting a linear polarization (LP) and a circular polarization (CP) to its corresponding cross polarizations, and polarization conversion ratio (PCR) of the converter with graphene strips is also controllable in this paper. A reflection-type converter is designed based on a metasurface in the terahertz region. It consists of periodic unit cells with strip-loaded half elliptical rings, a dielectric spacer layer, and a gold layer. The converter with PCR regulating also includes graphene strips on the gold layer. Both the LP and CP incidences can be converted into their cross-polarized ones with good PCR performances in a 2-THz band (2.1–2.9 THz, 32%). The graphene strips on the gold layer that disturbs the gold layer currents can regulate the PCR, typically from 2% to 98% for LP illuminations and from 60% to 98% for CP illuminations by adjusting the chemical potential ${\mu} _{{\text{c}}}$ of the graphene. Also, the physical mechanisms of the polarization conversions were investigated.

An ultra broadband all-dielectric transmitarray antenna (TA) is designed based on genetic algorithm optimization (GAO) and 3-D print fabrication technology. The array element is composed of all-dielectric pillars with different sizes distributed on the two sides of a substrate. The element is operating in the out-of-band resonance (OBR) zone, whose transmission amplitude is nearly one and the phase can be easily controlled. Afterward, the geometric parameters of the elements on different positions of the TA surface were optimized using generalized Snell's Law-based GAO, which achieved broadband phase compensation and good transmission performance. As the structure is all- dielectric, it can be easily manufactured using 3-D printing technology. The TA was fed by a ridged horn, and the antenna gains are enhanced by 7-12 dB in the measuring band (9-20 GHz). Moreover, the 1 dB steady gain ranges from 14.5 to 20 GHz (up to 29%).

Decoupling of a wideband circularly polarized (CP) conformal array is studied by a single-negative (SNG) metamaterial (MTM) isolator. The conformal array consists of two wideband CP antennas, which are assembled on an arched carrier, and a wideband SNG structure is placed between the antenna elements for coupling reduction. The SNG structure consists of four wheel-shaped units concentrically. The SNG structure is bianisotropic, and its constitutive parameters are tailored to form a wide isolation band. By using the proposed wideband SNG structure, the mutual coupling between the conformal antenna elements is reduced up to 25 dB at 4.5 GHz, and the mutual coupling is improved in the 3.9-5.05 GHz band. In addition, the loading of the SNG structure has little effect on the axial-ratio bandwidth. Besides, the gain and radiation patterns are improved. This research provides some basis in the decoupling of the conformal array.

Terahertz electromagnetic (EM) wave components usually have a single function, such as they can only convert the polarization state of an incident wave or absorb the incident energy, which would be a limitation for their applications. To make a breakthrough, a multi-functional device (MFD) is proposed in this paper, and it is capable of switching between absorption mode and polarization conversion mode. The device has a low-profile and simple structure, and it is constructed by graphene-based absorbing metasurface (AM) and gold-based polarization conversion metasurface (PCM). By controlling the chemical potential (μc) of the graphene, the leading role is transferred between the AM and the PCM, which leads to steerable absorption and polarization conversion (PC) modes. For the PC mode, the simulated polarization conversion ratio (PCR) is larger than 0.9 in the 2.11–3.63-THz band (53.0% at 2.87 THz). For the absorption mode, the simulated absorptivity is larger than 80% in the 1.59–4.54-THz band (96.4% at 3.06 THz). The physical mechanisms and operating characteristics of the MFD are discussed. This research has potential applications in terahertz imaging, sensors, photodetectors, and modulators.

A wideband vortex beam reflectarray is designed and fabricated to generate orbital angular momentum (OAM) waves for l=2 mode. A quarter wavelength element is proposed to achieve a reflection phase of 360° at the designed frequency of 6 GHz. The element consists of a loop patch, a cross dipole patch, and four square patches. The symmetric structure of the element makes it suitable for dual polarization. It is revealed that the OAM mode purity and operational bandwidth of the reflectarray are increased by using the quarter-wavelength element. The array size is 50×50 cm $^2$ that achieves measured gain of 18.98 dBi at 6 GHz and operational OAM bandwidth of 42%.

In this paper, high-order orbital angular momentum (OAM) vortex beams (OAMVBs) are generated using reflective metasurface. Generating high-order OAMVBs using traditional reflective metasurface methods deteriorates the radiation performance that affects the mode purity of OAMVBs. A reflective metasurface is proposed to generate OAMVBs with..=4 mode at the designed frequency of 6 GHz. It is revealed that radiation performance has been improved and mode purity for..=4 is increased 12.6% by using quarter wavelength unit cell and circular aperture metasurface as compared to conventional metasurface at designed frequency. Moreover, the reflective metasurface can generate OAMVBs with..=4 mode from 5 GHz to 7.5 GHz. The measured gain is 17.93 dBi which agrees well with the simulated gain.