This paper presents an ultra-thin and compact metamaterial absorber (MMA) capable of achieving near-perfect absorption peaks across the C, X, Ku, and K frequency bands. The MMA structure features a modified metallic plus-shaped resonator surrounded by symmetric L-shaped resonators within a compact size of 13 × 13 mm². The absorber exhibits seven absorption peaks at different resonant frequencies including 4.23, 6.48, 10.62, 12.92, 14.03, 17.39, and 18.11 GHz. With a thickness of 0.0225λ and a compact size of 0.1833λ at the lowest frequency, the absorber offers remarkable thinness and compactness. Different characteristics of the absorber, such as normalized impedance, surface current distribution, and electric field distribution, are also examined. The polarization-insensitive behavior of the MMA is assessed through absorption and reflection responses under different polarization and incident angles. The Equivalent Circuit Model (ECM) of the metamaterial absorber is also designed to accurately represent the MMA unit cell across all resonant frequencies. Experimental validation of the proposed MMA confirms its performance consistency with simulation results. The proposed MMA design holds potential for applications in defense, detection, and sensing. The sensing ability of the MMA is analyzed using simulations at different refractive index values.

Metal-free organic polymer photocatalysts have attracted dramatic attention in the field of visible light-induced hydrogen evolution reaction (HER). Herein, we showed a polymeric O-linked heptazine polymer (OLHP) decorated with S, N co-doped graphene quantum dots (S,N-GQDs) as a photosensitizer to generate hydrogen upon quantum dot sensitization. Both of these heptazine-based systems show effective photosensitization with strong π–π interactions and enhanced photocatalytic H₂ generation (24 times) as metal-free systems. Electrochemical impedance and optical measurements show effective charge transfer kinetics with decreased charge recombination, which is responsible for the enhanced photocatalytic activity. As a result, a significant high apparent quantum yield (AQY) with highest value of 10.2% was obtained for our photocatalyst OLHP/S,N-GQD10.