Phase-changing chalcogenide-based metasurface as dual-band modulator for terahertz photonic applications

In this study, we numerically investigate the evolution of dual-band Fano resonance into dual-band electromagnetically induced transparency (EIT) due to increased structural asymmetry. Utilising face-to-face oriented twisted E-type metasurface (MS) structures, we observe bright mode-dark mode and dark mode-dark mode coupling at distinct resonance frequencies, facilitating the dual-band EIT phenomenon. The emergence of narrow transparency bands is predominantly attributed to the excitation of longitudinal magnetic dipole and scattering suppression from all dominant moments due to destructive interference, respectively. These conclusions are confirmed by studying the excited surface current under plane wave illumination and further validated in computations using multipole scattering formalism. The optimised terahertz metasurface thus designed exhibits interesting dual-band modulation characteristics by combining the unique optical properties of various phases of chalcogenide phase change materials. Proper placement of these phase change materials in the form of patches within the capacitive gaps of the metasurface allows superior modulation control of resonances compared to the traditional modulation techniques. The independent control of individual resonances was leveraged to realise NOR and NAND logic gates, demonstrating the capability for selective resonance modulation. This approach enhances the modulation of terahertz resonances and provides a promising framework for developing advanced terahertz modulators.