Fig. 1. （a）Conventional thin-film structure design：reflection process of a quarter-wave film with low losses on a perfectly reflecting substrate，the reflected partial waves are represented on the complex plane，（b）subwavelength thin-film stack metamaterials design：reflection process from a highly absorbing，ultra-thin film on a perfectly reflecting substrate and the corresponding partial waves phasor diagram^［18］

Fig. 2. （a）Schematic diagram of periodic subwavelength thin-film stack metamaterials^［49］，（b）the corresponding calculation process of transfer matrix method^［63］

Fig. 3. （a）Schematic of periodic multilayered thin-film metamaterials composed of alternating layers with permittivities of ε₁ and ε₂，and thicknesses of d₁ and d₂，respectively，and surrounded by homogeneous media with permittivities of ε_in and ε_out，schematic diagram of the rigorous transfer matrix method（TMM）and the corresponding effective-medium theory（EMT），（b）transmission versus number of periods（N）for TMM calculations and local effective-medium theory（LEMT）perspectives，and（c）transmission versus N for TMM calculations and nonlocal effective-medium theory（NEMT）perspectives under the same conditions ^［63］

Fig. 4. Structural colors based on subwavelength thin-film stack metamaterials（a）reflectance of Ge/Au samples and the corresponding structural color patterns generated by the designed structures^［5］，（b）reflectance of Cu/Au and CuO/Au samples，photographs of the angular responses from the color effects and the CIE 1931 chromaticity coordinates of the colors ^［115］，（c）schematic configurations of planar thin-film Ag-SiO₂-Ag（MIM）metamaterials，photograph of five different large-area color filters and the corresponding transmittance spectra for MIM metamaterials^［116］

Fig. 5. Subwavelength thin-film stack metamaterials for photoluminescence（PL）enhancement（a）schematic of the hyperbolic metamaterial structure and time-resolved PL data from QDs^［129］，（b）sketches of the single ENZ（MIM）and double ENZ（MIMIM）structures and normalized emission spectra of QDs^［122］，（c）schematic of sample structure and measured PL spectra for QDs on different thickness Ag films^［130］

Fig. 6. Subwavelength thin-film stack metamaterials for narrow-band thermal emitter（a）cross-sectional view of thin-film stacks enhanced resonant thermal emitter，and the corresponding normal emittance of the theoretical proposed structure^［144］，（b）schematic view of thin-film stacks thermal emitter and experimentally measured emission spectra for different temperatures operate in mid-wave infrared region^［84］，（c）schematic view of the thin-film stacks thermal emitter and experimentally measured emission spectra for different temperatures operate in long-wave infrared region^［148］，（d）Bayesian optimized narrow-band thermal emitter structure and the corresponding normal emittance ^［149］

Fig. 7. Subwavelength thin-film stack metamaterials for infrared stealth（a）schematic of the infrared stealth selective emitter，calculated and measured emissivity of the selective emitter，schematics of the experimental apparatus，and infrared image of different emitters in the 3~5 μm and 8~14 μm ranges^［166］，（b）schematic of ultrathin titanium carbide（MXene）films for high-Temperature thermal camouflage and the corresponding infrared thermal image^［167］

Fig. 8. Other interesting applications for subwavelength thin-film stack metamaterials（a）optical gas sensing applications^［75］，（b）radiation cooling applications^［89］ and（c）energy-saving window applications^［177］