YN is grateful for partial support by Japan Society for the Promotion of Science (JSPS), Grants-in-Aid for Scientific Research (20H02545), JSPS Bilateral Joint Research Projects between Japan and Lithuania(JPJSBP1). This work was partially supported by Japan Science and Technology Agency (JST) A-STEP(19216760). SJ is grateful for support via the ARC LP190100505 grant. fraction of SiO 2 is high, the space-averaged refractive index of the dielectric spacer in MIM becomes high too and the absorption loss increases significantly. I n t he c ase o f o f S iO 2 :CaF 2 fractions 2.5:7.5, the (1 -R) is almost 100% and the average refractive index is about 2.5. It was found that modulation of the refractive index by too large absorption is a factor that inhibits the coupling between MIM metasurface and its I-spacer. Experimental results (Fig. ) were also confirmed by FDTD c alculations (Fig. ). The results were calculated for Palik's dielectric constant from 1.5 µm to 10 µm wavelengths (Lumerical database) and range of nanodisk diameters (Fig. ). The results of FDTD calculations are in good agreement with the experimental results. In particular, the convergence of the absorption wavelength when the strong coupling occurs is consistent with the two absorption peaks obtained from the transmission spectrum. At the same time, the peak intensity can be maximized by thinning the SiO 2 layer, which is in good agreement with the experimental observations. The value of (1 -R) is closely related to the emissivity of the black-body radiation source, which is essential to obtain a higher emissivity. At the same time, the longer wavelengths (at smaller wavenumbers 1000 cm -1 ) enter the forbidden band of wavelengths longer than λ > 10 µm.