Terahertz absorption characteristics of a VO ₂ /SiO ₂ /Au plasmonic metamaterial with rotational symmetry

In this research, a plasmonic metamaterial absorber (MMA) composed of radially symmetric alternating triangular wedges, trapezoids, and partial ring segments made of vanadium dioxide (VO 2 ) phase-change material (PCM) on a SiO 2 layer backed by a gold ground plane in the terahertz (THz) frequency range is proposed. Two geometrically scaled versions of the proposed structure were investigated. The larger unit cell (56 × 56 µm 2 ) supported multiple overlapping resonant modes throughout the 1-8 THz range, resulting in a broad, multimodal absorption spectrum, whereas the miniaturized unit cell (40×40 µm 2 ) exhibited fewer, spectrally narrower resonances confined to a tighter frequency interval. The sensing performance of the miniaturized unit cell was then investigated as a refractive index sensor. As the refractive index increased, the resonance frequency displayed a clear redshift, decreasing from 4.724 THz at n = 1.32 to 4.703 THz at n = 1.35, which corresponds to a sensitivity of 0.7 THz/refractive index unit (RIU). Furthermore, the effect of changing VO 2 thickness was examined, and it was observed that at a thickness of 200 nm, the absorption reached nearly 100 percent, accompanied by a narrow spectral width and a high quality factor (Q). Additionally, the miniaturized unit cell exhibited polarization-insensitive behavior due to the rotational symmetry of the geometry. Moreover, continuous conductivity-dependent tunability of both the broadband and miniaturized absorber configurations was demonstrated through multiple intermediate conductivity states of VO 2 during the phase transition process. Finally, for the smaller unit cell, results showed that the SiO 2 layer with a thickness of 9 µm resulted in a sharp, spectrally narrow resonance with peak absorption close to 100 percent, making this configuration well suited for high-Q sensing applications.