Broadband solar-thermal absorption in a refractory TiN-based metamaterial

We propose a refractory metamaterial absorber consisting of a TiN square-ring resonator on a VO ₂ spacer and a TiN back-reflector, in which the phase-change spacer crosses the metal–insulator transition near 68 °C without disrupting broadband solar absorption. An FDTD sweep over the four structural parameters identifies the ring outer length and ring thickness as the only parameters that must be retuned when the elemental metal layers of a W/SiO ₂ /Ti reference design are replaced by TiN. The optimized structure sustains an AM1.5G solar-weighted absorption of 96.58–97.56% across the full 20–80 °C window, with absorption exceeding 92% at every wavelength from 280 to 2500 nm. Near-field maps trace the broadband response to a lateral gap-plasmon mode supported by the ring-to-ring spacing in both phases, supplemented by additional plasmonic channels that emerge in the NIR when VO ₂ enters the metallic phase. Replacing the VO ₂ spacer with alternative inorganic dielectrics or flexible polymers preserves the broadband envelope, which extends the design framework toward flexible and substrate-diverse refractory absorber platforms.