Currently, terahertz metamaterials are studied in many fields, but it is a major challenge for a metamaterial structure to perform multiple functions. This paper proposes and studies a switchable multifunctional multilayer terahertz metamaterial. Using the phase-transition properties of vanadium dioxide (VO₂), metamaterials can be controlled to switch transmission and reflection. Transmissive metamaterials can produce an electromagnetically induced transparency-like (EIT-like) effect that can be turned on or off according to different polarization angles. The reflective metamaterial is divided into I-side and II-side by the middle continuous VO₂ layer. The I-side metamaterials can realize linear-to-circular polarization conversion from 0.444 to 0.751 THz when the incident angle of the y-polarized wave is less than 30°. The II-side metamaterials can realize linear-to-linear polarization conversion from 0.668 to 0.942 THz when the incident angle of the y-polarized wave is less than 25°. Various functions can be switched freely by changing the conductivity of VO₂ and the incident surface. This enables metamaterials to be used as highly sensitive sensors, optical switches, and polarization converters, which provides a new strategy for the design of composite functional metamaterials.

This letter presents an extremely low-profile dual-band antenna based on a single-layer square microstrip patch for 5G mobile application. The antenna has a height of only 0.8 mm (about 0.009λ _L at 3.45 GHz) and a footprint of 30×30 mm ² (0.345×0.345λ _L ² ). The TM ₁₀ mode at around 3.56 GHz and the TM ₁₁ mode at around 4.91 GHz for the antenna are obtained by etching a pair of symmetrical circular notches and a narrow rectangular slot on the square patch. The TM _1/2,0 mode at around 3.45 GHz and the TM _1/2,1 mode at around 4.83 GHz of the antenna are generated by further placing two 2-order matrix-style shorting pins. The combination of the four modes can overcome the effect of insufficient bandwidth due to the very low profile, resulting in 3.4–3.6 GHz (partial 5G N78 band) and 4.79–4.94 GHz (partial 5G N79 band) dual-band operation. The measured results are also presented.

We propose and design a metamaterial broadband stop-band filter with a steep cut-off in the terahertz region. The filter is based on the flexible structure of metal-dielectric-metal-dielectric-metal (MDMDM). Simulation results show that the filter has a center frequency of 1.08 THz, the square ratio reaches 0.95, and the −20 dB bandwidth reaches 1.07 THz. In addition, it has excellent flat-top characteristics with an average transmission rate in the resistive band of no more than 5%. The relative bandwidth has been up to 99%, and stopband absorption rate has reached more than 98%. The effects of the main structural parameters on the transmission characteristics are discussed. The role of each layer of metal in the filter is explored by studying the effect of the variation of the number of metal layers on the filter. The symmetry of the structure ensures the polarization insensitivity of the filter at normal incidence. The correctness of the simulation results was verified by analyzing the effective permittivity and magnetic permeability. To investigate the transmission characteristics of the metamaterial filter in-depth, we analyzed the electric field strength and surface current distribution at the center frequency of the filter. The designed terahertz filter may have potential applications in terahertz communications, sensors, and emerging terahertz technologies.