Theoretical investigation of a tunable vanadium dioxide-based metastructure for ultra-wideband absorption and linear-to-circular polarization conversion across triple-octave frequencies.

An ultra-wideband absorption and tripling octave frequency linear-to-circular polarization conversion tunable metastructure (MS) is proposed, utilizing the phase transition property of vanadium dioxide (VO). In its metallic state, the MS is demonstrated to function as a polarization-insensitive ultra-wideband MS absorber, achieving an absorption value exceeding 90% within a frequency range of 2.37 THz to 4.56 THz. The absorption mechanism is elucidated through an equivalent circuit model, impedance matching theory, and electric field distribution analysis. In the insulating state of VO, the MS is shown to operate as a converter tripling the octave frequency in linear-to-circular polarization, with an axial ratio below 3 dB across the frequency ranges of 0.95-1.68 THz and 2.25-4.49 THz, corresponding to relative bandwidths of 55.51% and 66.27%. Remarkably, the polarization conversion in the insulating state exhibits tripling octave frequency characteristics, with a fundamental bandwidth spanning 0.95-1.49 THz and a tripling octave frequency bandwidth spanning 2.85-4.48 THz. The MS can switch between distinct functionalities within the terahertz regime, offering significant potential for applications such as spectral analysis, signal encryption, stealth material preparation, and other advanced terahertz technologies.