Metamaterials are artificial composite materials or composite structures composed of unit structures with a specific spatial arrangement and have extraordinary macroscopic physical properties. They are widely used in energy-harvesting, subwavelength imaging, perfect absorbers, and photovoltaic devices. Once the structure is established, the common metamaterial electromagnetic absorber can only achieve specific absorption in a specific wavelength range, without tunability. Tunable materials are integrated into the metamaterial absorber to realize a tunable metamaterial absorber. Because the tunable material VO₂ has the characteristics of fast response and high regulation intensity, most of the reported VO₂-based metasurface absorbers are concentrated in the visible and terahertz bands, and relatively few studies have been conducted in the infrared band. Therefore, metamaterial absorbers operating in the infrared band have attracted much attention in research hotspots. Thus far, it has been difficult for existing infrared absorbers to achieve both ultra-broadband absorption and tunability owing to limitations such as material loss and fabrication precision. In this study, an ultra-broadband infrared absorber based on VO₂ is designed with a simple structure, wide absorption bandwidth, and wide tunable range.

In this study, a truncated four-layer infrared tunable broadband absorber based on the phase change materials VO₂, titanium dioxide (TiO₂), lithium fluoride (LiF), and SiO₂ was designed. The finite element method was used for the simulation using the COMSOL Multiphysics simulation software. According to the different material properties, the corresponding mesh division was performed on the domains, boundaries, and edges of the different materials. Simultaneously, the infrared rays are incident along the wave vector direction, the incident angle is α, and the azimuth angle is φ. Periodic boundary conditions were set in the X and Y directions. Periodic port boundary conditions were used in the positive and negative directions of the Z-axis. The two ports were set with one open and one closed. Subsequently, according to the requirements of the absorption curve, the parameters of the research band were scanned, the structural parameters, incident angle, and polarization angle were adjusted, and the parameters were repeatedly optimized. At the end of the simulation, the magnetic field and electric field distribution maps at the corresponding structural positions in the corresponding bands were derived.

When the conductivity of VO₂ is 2×10⁵ S/m, after optimization, a set of optimal parameters are obtained, namely: the period of the cell structure, p=6.3 μm; the thickness of the bottom layer of Au, h₁=0.5 μm; the thickness of the SiO₂ layer, h₂=1.2 μm; the thickness of the VO₂ layer, h₃=3.0 μm; the thickness values of the TiO₂ layer and the nested LiF cross ring, h₄=4.5 μm; the width of the bottom composite layer, W₁=4.9 μm; the width of the top composite layer, W₂=3.5 μm; the length and width of the two LiF rectangular rings in the top composite layer are a=3.2 μm and b=1.6 μm, respectively, and the ring width of the cross ring, c=0.6 μm. From Fig. 3(a), it is observed that when the transverse magnetic (TM) and transverse electric (TE) waves are incident at 0°, the absorptivity of the absorber exceeds 90% in the wavelength range of 16?60 μm, including the wavelengths of 21?26 μm, 28?36 μm, and 42?53 μm. The absorptivity in this range could exceed 98%. The absorption curve consists of four absorption peaks: p₁ located near 11 μm, p₂ located near 24 μm, p₃ located near 30 μm, and p₄ located near 50 μm. As shown in Fig. 3(b), when the azimuth angle of the incident wave increases from 0 to 90°, the absorptivity remains stable and basically unaffected. Owing to the symmetry of the structure, the absorber is polarization-insensitive. Figure 4 shows the contour plot of the absorptivity of the absorber as a function of the wavelength and incident angle when the TE and TM waves are incident obliquely. When the incident angle is varied from 0 to 60°, the absorptivity is maintained at approximately 80%. Figure 5 shows that the absorption mechanism of the ultra-broadband absorber is plasmon resonance. Figure 6 shows the graph of the relative impedance of the structure as a function of wavelength, indicating that the relative impedance value of the absorber in the corresponding band matches the relative impedance value in the free space. Figure 7 shows the absorptivity curves for different structural parameters and the corresponding magnetic field distribution diagrams used to determine the optimal parameters. Figure 8 plots the change in the absorptivity of the absorber with wavelength when other structural parameters keep the optimal parameter values constant, and the VO₂ conductivity changes. Finally, Fig. 9 shows that the absorber with the LiF cross-ring pattern has a significantly higher absorptivity when the structural parameters are the same and the TE wave is incident vertically.

In this study, a dynamically tunable ultra-broadband angle-insensitive infrared ideal metamaterial absorber based on VO₂, LiF, and TiO₂ was designed. When the TM and TE waves are vertically incident, the absorptivity is greater than 90% in the 16?60 μm band, and the absorption bandwidth can reach 54 μm. The absorptivity rate can reach more than 98% in the wavelength ranges of 21?26 μm, 28?36 μm, and 42?53 μm. The TM wave and TE wave polarizations have incident and polarization insensitivity, and the absorptivity of the absorber is tuned by adjusting the conductivity of VO₂ from 20 S/m to 2×10⁵ S/m. The metamaterial is expected to have a wide range of applications, such as polarization detectors, thermal radiators, and infrared sensors.