Strategy for Optical Transparent Broadband Frequency-Tunable Microwave Absorption: Double Fabry−Pérot Resonant Cavities

The limited modulation range and optical opacity have long restricted the practical application of frequency-tunable microwave absorbers. To address this challenge, we propose a transparent tunable microwave meta-absorber based on double Fabry-Pérot (FP) cavities and a vanadium dioxide (VO2) mesh. By thermally tuning the sheet resistance of the VO2 mesh, the dominant absorption resonance can be switched between FP cavities with different optical thicknesses, while also adjusting the impedance matching with free space, thereby enabling dynamic absorption peak frequency tuning over a 4.2 GHz range and achieving a high amplitude modulation depth of 93.15%. This work also utilizes multibeam interference theory and a reflection coefficient mechanism model to support the theoretical analysis and reveal the physical mechanism underlying the tunable absorption. Experimental results further show that the absorber maintains a broadband optical transmittance exceeding 72.48% across the 380-4000 nm spectral range. Compared with conventional tuning strategies, this work simultaneously achieves structural simplicity, broadband frequency and amplitude modulation capabilities, and high optical transparency, providing a feasible solution for applications in transparent electronics, smart windows, and electromagnetic modulation systems.