Tunable reflecting terahertz bandstop filter based on chirped metamaterial structure is demonstrated by numerical simulation. In the metamaterial, the metal bars are concatenated to silicon bars with different lengths. By varying the conductivity of the silicon bars, the reflectivity, central frequency and bandwidth of the metamaterial could be tuned. Light illumination could be introduced to change the conductivity of the silicon bars. Numerical simulations also show that the chirped metamaterial structure is insensitive to the incident angle and polarization-dependent. The proposed chirped metamaterial structure can be operated as a tunable bandstop filter whose modulation depth, bandwidth, shape factor and center frequency can be controlled by light pumping.

The paper reports an absorption–transmission multifunctional tunable metamaterial based on graphene. Its pattern graphene layer can achieve broadband absorption, while the frequency selective layer can achieve the transmission of specific band. Furthermore, the absorption and transmission can be controlled by applying voltage to regulate the chemical potential of graphene. The analysis results show that the absorption of the metamaterial is adjustable from 22% to 99% in the 0.72 THz~1.26 THz band and the transmittance is adjustable from 80% to 95% in 2.35 THz. The metamaterial uses UV glue as the dielectric layer and PET (polyethylene terephthalate) as the flexible substrate, which has good flexibility. Moreover, the metamaterial is insensitive to incident angle and polarization angle, which is beneficial to achieve excellent conformal properties.

A bendable Ag-coated hollow waveguide (HWG) based on the acrylonitrile-butadiene-styrene (ABS) tube is fabricated for low-loss transmission and imaging of low-frequency terahertz (THz) waves. ABS plastic tubing is selected since it allows growing a smooth silver (Ag) reflective film with good adhesion to the inner wall and contributes to constructing a bendable THz HWG structure. The low-loss transmission window of the ABS/Ag-coated HWG is at 0.2-1.0 THz. The attenuation characteristics of the as-prepared waveguide samples with different bore sizes (2.3, 3.2, and 4.2 mm) at 100 and 300 GHz are investigated. The 3.2-mm-bore THz waveguide exhibits good bendability and transmission performances. Its measured straight transmission loss is 1.02 dB/m at 300 GHz, and the bending loss reaches 1.56 dB/m at 150° (the curvature radius is 10 cm). Moreover, a far-field confocal waveguide-based THz imaging system is established based on the ABS/Ag-coated HWG. High-quality imaging with a super-resolution of 1.12 mm was achieved using the 3.2-mm-bore waveguide samples at 100 GHz.

A terahertz metamaterial waveguide (meta-waveguide) and a meta-waveguide-based lens-free imaging system are presented. The meta-waveguide not only inherits the low-loss transmission performance of a waveguide but also breaks through the diffraction limit under the action of the metamaterial, achieving subwavelength focusing. The focusing distance is far greater than the Rayleigh length, thus enabling far-field scanning imaging. For verification, a metal ring-based meta-waveguide was fabricated by 3D printing and metal cladding technology. Then, a transmission scanning imaging system working at 0.1 THz was built. High quality terahertz images with a resolution of 1/3 of the wavelength were obtained by placing the imaging targets at the focus and performing two-dimensional scanning. The focusing and transmission of terahertz wave in the meta-waveguide were simulated and analyzed.