A broadband Terahertz metamaterial absorber enabled by a water-assisted square-pyramid architecture with optical transmittance

Terahertz (THz) absorbers with broad bandwidth and high absorption are significant interest for applications in electromagnetic shielding and photonic devices. In this work, we propose a broadband THz metamaterial absorber consists of square-pyramid structure with optical transmittance, composed of liquid water, polyethylene terephthalate, and indium tin oxide. The continuously varying square-pyramid configuration promotes multiple reflections and electromagnetic field superposition, in combination with the strong intrinsic THz absorption of water, which leads to efficient THz absorption. As a result, the proposed absorber achieves absorption exceeding 90% from 0.5 to 10 THz, over 95% from 0.65 THz to 10 THz, and over 99% from 1.2 THz to 10 THz. In addition, the absorber exhibits polarization-insensitive behavior and maintains a broadband absorption bandwidth of 7 THz with absorptance above 90% for incident angles up to 70°. These features highlight the proposed design as a promising platform for broadband THz absorption in practical applications where visible-band optical transmittance is desirable.