Fig. 1. (a) Original inversely-designed structure, where p_x and p_y are the periods in the x and y direction, respectively. (b) The transmission spectra of the target (black) and inverse design (red). (c) Adjusted inverse design metasurface structure. (d) Processed ultimate inverse design metasurface with 200-nm-thick amorphous Ge film covering the face. The scale bar in the pictures is 50 µm.

Fig. 2. (a) Experimental (black) and simulated (red) transmission spectra of the ultimate inverse design metasurface. (b) Time-domain THz spectra when the THz pulse passes the quartz substrate (black) and metasurface (red). Experimental modulation of THz double EIT (c) transmission and (d) group delay for various pump fluences ranging from 0 to 2.25 mJ/cm².

Fig. 3. Experimentally extracted (a) differential transmittance and (b) transient conductivity of 200 nm pure Ge film at pump fluence of 2.25 mJ/cm². (c), (e) Map of experimental THz transmission and group delay as a function of relative time delay between the femtosecond laser and THz pulse. (d), (f) Map of simulated THz transmission and group delay as a function of relative time delay between the femtosecond laser and THz pulse.

Fig. 4. Static near-field simulation results. (a), (b) Near-field distributions of the inversely-designed metasurface in the first transparency window (peak1) with various conductivities of Ge film, 10 and 1000 S/m, respectively. (c), (d) Near-field distributions of the inversely-designed metasurface in the second transparency window (peak2) with various conductivities of Ge film, 10 and 1000 S/m, respectively.