Bifunctional metasurface for cross-polarization conversion and ultra-broadband absorption in terahertz range

A bifunctional terahertz (THz) metasurface based on vanadium dioxide (VO2) was proposed, which can achieve near-perfect cross-polarization conversion (CPC) and ultra-broadband absorption. When VO2 is in insulator state, the metasurface acts as a cross-polarization converter, converting linearly polarized (LP) and circularly polarized (CP) waves into the corresponding cross-polarized waves. The polarization conversion ratio (PCR) exceeds 99.5% in the frequency range of 4.1∼6.7 THz at normal incidence, and can be maintained more than 98.3% even at an incident angle of 20°. When VO2 is in metal state, the metasurface becomes a THz broadband absorber with absorption efficiency above 90% in a wide frequency range of 2.9 THz∼8.4 THz. The absorption efficiency is almost unchanged when the incident angle increases from 0° to 15° for TE wave. In addition, the absorption efficiency can maintain above 90% at 4.0∼7.1 THz when the incident angle is up to 60° for TM wave. Moreover, the interference theory model was utilized for verifying theoretically the validity of the numerical simulation results of our proposed metasurface, and the theoretical calculation results are in good agreement with the simulation results. We believe that this proposed metasurface has potential applications in many fields, such as sensing, imaging, non-destructive detectors, super lenses, and telecommunications.