Dynamically tunable terahertz ultra-wideband absorber based on a photosensitive silicon ring-coupled structure

Aiming to address the complexities of structural design and the limited dynamic tuning capabilities of traditional phase-change material absorbers, this study proposes a dynamically tunable terahertz ultra-wideband absorber based on a photosensitive silicon ring-coupled structure. The device features a periodic array design with a square-ring-circular coupling structure, forming a three-dimensional resonant system through a polycyclic olefin copolymer dielectric layer and a continuous gold film reflection layer at the bottom. The results indicate that when the photosensitive silicon conductivity (<i>σ</i>_{<i>S</i><i>i</i>}) is modulated to 1.5×10⁵<i>S</i>/<i>m</i>, the device achieves over 90% absorption in the spectral range of 1.83-4.07 THz. Conversely, when <i>σ</i>_{<i>S</i><i>i</i>} is reduced to 1.0×10³<i>S</i>/<i>m</i>, the device exhibits greater than 95% total reflection characteristics, resulting in a remarkable bistable response. The symmetry of the structure imparts polarization insensitivity, and the photosensitive silicon conductivity can be modulated by adjusting the power of the pump light source, allowing for flexible adjustment of the absorption bandwidth, with a maximum modulation depth reaching 94.61%. The principles underlying broadband high absorption are elucidated by integrating impedance matching theory, electric field current distribution, and electric dipole resonance. Additionally, an equivalent circuit model is introduced for quantitative analysis, while the Fresnel equation further elucidates the mechanism behind the blue-shift phenomenon of the absorption band under large-angle incidence. The designed absorber possesses a simple structure and dynamic tunability, indicating potential applications in stealth, high-resolution imaging, and 6G communication.