The measurement setup was designed to switch the optical beam path between two modes: one for the reflection mode with a broadband thermal emission source turned on (hot plate turned off) and the beam splitter on top of the tunable metasurface, and the other for the thermal emission mode without the beam splitter and the hot plate turned on. The intensity pattern originating from the interference between the reflected light from a fixed mirror and one from the moving mirror was recorded in a HgCdTe (MCT) IR detector. The Fourier transformed signal reveals the spectral information from the tunable metasurface.

For the thermal emission measurement, we prepared two regions: One was covered by Al pad near the Al grating metasurfaces, which was used as the normalization, and the other was painted with carbon black, heat-safe paint. For a given sample having certain width and period of the Al gratings, electrical bias, and hot plate temperature, the four FT-IR spectra were recorded: Isample (signal from a sample), IAl (signal from the mirror-like Al pad), Icarbon (signal from the carbon paint region), and Ibackground (signal from out-focused from the sample plane, measuring the background signal). The emission intensity was IsampleIAl. The block body radiation was IcarbonIbackground. The emissivity was then given by (IsampleIAl)/(IcarbonIbackground).

Regarding the electrical properties of the Al2O3 layer, the DC permittivity (εDC) was measured to be 7.74 via a capacitance-voltage (CV) measurement (fig. S6). We performed current-voltage (IV) measurements to characterize a dielectric strength (EBD, breakdown electric field) of 7.3 MV cm−1 (fig. S7).

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