Fig. 1. Mid-infrared metasurface structures based on different electromagnetic wave control mechanism. (a) Metasurface quarter-wave plate based on the V-shaped antenna^[44]; (b) Dielectric Huygens metasurface operating near the mid-IR wavelength of 5.2 µm， schematic tilted view of a rectangular meta-atom structure, the phase shift and transmittance corresponding to the eight meta-atom elements used to construct the meta-optical device, and the scanning electron microscope image of the fabricated metasurface structure, respectively^[51]; (c) Long-wavelength infrared metalens composed of silicon nanopillars arranged on a square lattice. The building block of all-silicon metalens (left) and simulated amplitude and phase for eight selected nanopillars (right)^[31]

Fig. 2. Mid-infrared polarization devices. (a) Schematic illustration of the BAFOV generation with polarization-dependent functions (left). The birefringent meta-atoms are made of monocrystalline Si (right)^[69]; (b) Schematic of the GIAM-based polarimeter^[71]; (c) Schematic of the Mid-IR full-Stokes polarization detection device design with seven cells for direct Stokes parameter measurement^[73]

Fig. 3. GST phase-change material metasurfaces. (a) Sketch of the switchable perfect absorber device (left) and measured reflection spectra in amorphous and crystalline conditions for different antenna sizes (right)^[36]; (b) Active plasmonic metasurface for beam switching (left) and experimental results for the cylindrical bifocal lens (right)^[84]; (c) Artistic rendering of a reconfigurable varifocal metalens (left) and well-resolved lines of USAF-1951 resolution charts (right)^[90]

Fig. 4. Graphene electrically tunable metasurfaces. (a) Mid-infrared optical modulator based on an electrically tunable metasurface absorber^[35]. Schematic of the ultrathin optical modulator based on a tunable metasurface absorber and a scanning electron microscope (SEM) image of the metasurface on graphene (left). Measured reflection spectra from the metasurface absorber for different gate voltages (right); (b) The gate-tunable graphene-gold reconfigurable mid-infrared metasurface^[92]. Schematic of a gate-tunable device for control of reflected phase and SEM image of gold resonators on graphene (left). The scale bar indicates 1 μm. Phase modulation at wavelengths of 8.2 µm, 8.5 µm, and 8.7 µm (circles-experiment, line-simulation) (right); (c) Hybrid graphene metasurface allows for electrically tunable resonant absorption^[95]. Schematic of the hybrid graphene metasurface (left) and measured reflection spectra when applying different gate voltages (right)

Fig. 5. A schematic of the substrate prior to stretching with Au split ring resonators attached and a schematic of a stretched array (Top); The measured reflectance spectra and representative environmental scanning electron microscope (ESEM) images for the double SRR array for various degrees of strain (bottom)^[77]

Fig. 6. Metal-based metasurfaces for surface enhanced infrared absorption. (a) Chemically specific, label-free nanophotonic biosensor in the mid-infrared^[16]; (b) MOF-SEIRA platform for simultaneous sensing of CO₂ and CH₄ gases^[21]

Fig. 7. Graphene metasurfaces for surface-enhanced infrared absorption. (a) Schematic of graphene plasmon enhanced molecular fingerprint sensor^[117]; (b) Experimental scheme of the graphene plasmon device for gas identification^[20]

Fig. 8. Molecular fingerprint retrieval and spatial absorption mapping of a mid-IR nanophotonic sensor based on all-dielectric high-Q metasurface elements^[120]