Bifunctional Metal-Free VO₂ Metasurface for Ultra-Broadband THz Absorption and Transmission

Abstract

This study presents the design and numerical investigation of a metasurface absorber/transmission for terahertz (THz) wave applications. A resonance-engineering technique is employed to generate multiple closely spaced resonances and merge them to achieve ultra-wideband absorption. The metasurface unit cell is realized using thermally switchable vanadium-dioxide (VO₂) as both the resonator and ground plane, while silicon-dioxide (SiO₂) is utilized as the dielectric cavity and impedance-matching layer. In the conducting phase of VO₂, the proposed structure exhibits ultra-wideband absorption more than 90% ranging from 4.04 THz to 19.47 THz, corresponding to an absolute bandwidth of 15.43 THz and a fractional bandwidth of 131.2% among the highest reported for VO₂-based THz absorbers to the best of current knowledge. The temperature-controlled phase transition of VO₂ enables a reconfigurable transmission response, where more than 80% transmission is achieved over the 2–13.31 THz band, yielding a fractional bandwidth of approximately 147.7% in the insulating phase. The absorber performance is further analysed under oblique incidence to assess its practical applicability, demonstrating stable absorption for incident angles up to approximately 60° for both transverse electric and transverse magnetic polarizations. A transmission line based equivalent circuit model is used to further validates its simulated results, demonstrating good agreement between the theoretical and simulated responses. Detailed fabrication steps, working mechanism using electric, magnetic and power-flow graph of the proposed structure are presented, and a comprehensive parametric analysis is carried out to evaluate fabrication tolerance. These features make the metasurface highly promising for emerging THz applications, spectroscopic imaging, non-invasive biosensing, stealth technology, electromagnetic shielding, and frequency-selective filtering.