We demonstrate the design, characterization, and interference-theory interpretation of a terahertz triple-band metamaterial absorber (MA). The experiments show that the fabricated MA has three distinctive absorption peaks at 0.5, 1.03, and 1.71 THz with absorption rates of 96.4%, 96.3%, and 96.7%, respectively. We use the multi-reflection interference theory to investigate the physical insight of the proposed triple-band terahertz MA, which provides a design guideline for MA of such type. The theoretical predictions of the interference model have excellent agreements with experimental results. The designed multiband absorber is easy to manufacture and insensitive to incident polarizations with high absorption, which is favorable for various applications.

This paper introduces a dual-band terahertz imaging system as a potential product for nondestructive testing using heterodyne detectors and continuous-wave sources. The operating frequencies of the system are 110.4 and 220.8 GHz. Multiband fusion technology combines the advantages of the greater spatial resolution of the high-frequency band and the enhanced sensitivity of the low-frequency band to improve the detection ability of the system. Additionally, the interference cancellation technology is used to obtain a superior image quality. The spatial resolution of this system was approximately 3 mm. The results show that the system can be used for bonding quality and embedded defect detection in radomes and foam materials adhered to metal plates in aircrafts.

This paper introduces a method for optimizing the terahertz (THz) imaging resolution in a specified field of view (FoV) for a scanning dual reflector system (Gregory or Cassegrain system) based on the theory of field curvature. The authors built a Gregory scanning system to verify the ability of this method. The operating frequency of the verification system is 220 GHz, the FoV at a specified distance of 8 m is approximately 50 cm * 100 cm, and the imaging resolution throughout the entire FoV is better than 3 cm. Experimental results show that the imaging resolution of the classical dual reflector system can be optimized, and the proposed verification system can realize active THz imaging of the human body.

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In this study, we illustrate the effective medium theories in the designs of three-dimensional composite metamaterials of both negative permittivity and negative permeability. The proposed metamaterial consists of random coated spheres with sizes smaller compared to the wavelength embedded in a dielectric host. Simple design rules and formulas following the effective medium models are numerically and analytically presented. We demonstrate that the revised Maxwell-Garnett effective medium theory enables us to design three-dimensional composite metamaterials through the assembly of coated spheres which are random and much smaller than the wavelength of the light. The proposed approach allows for the precise control of the permittivity and the permeability and guides a facile, flexible, and versatile way for the fabrication of composite metamaterials.