Multifunctional VO2 metasurface thermal emitter with switchable radiation bandwidth and direction

The controllability of the spectral width and intensity of long-wave infrared (LWIR) emission is essential for various applications, including optical stealth, infrared radiation sources, and infrared lasers. Here, we proposed a multifunctional LWIR metasurface emitter with a switchable radiation state, which consists of germanium (Ge) rectangular pair resonators placed on a vanadium oxide (VO2) film. By manipulating the reversible metal-to-insulator phase transition of phase change material VO2, the spatial and spectral emission characteristics could be dynamically controlled. The high loss of metallic VO2 enables broadband emission in the LWIR band. The average emissivity in the 8–12 μm range reaches 91.3% and exhibits wide-angle emission properties. When the VO2 is transformed to the insulating phase, the coupling effect of the symmetry-breaking rectangle with two quasi-bound states in the continuum modes enhances the emission intensity of VO2 at a selective wavelength. This results in narrowband emission with a high-quality factor (Q-factor) and high directional selectivity. Notably, the emission peak's linewidth is only 28 nm, with a Q-factor of 293, and the average emissivity in the 8–12 μm range drops to 9.4%. Overall, the proposed metasurface device achieves an LWIR emissivity switching ratio of 9.7 between its two operating modes, showcasing significant differences in spectral and spatial radiation characteristics.