Fig. 1. (a) Schematic diagram of an all-silicon terahertz metasurface consisting of periodic imperforated air-hole tetramers when normally illuminated by a terahertz wave polarized along the x-direction. (b) Unit cell of the metasurface. The periods of the unit cell are P_x = P_y = P = 300 µm. The depth of air-holes is denoted as t₁, and the thickness of the substrate is t₂. (c) SEM picture of one fabricated metasurface when r₁ = 25 µm, r₂ = 55 µm, and t₁ = 150 µm.

Fig. 2. (a) Simulated transmittances of the metasurface with different r₁, when r₂ = 55 µm. (b)–(e) Near-field electric and magnetic field distributions of resonances f₁ and f₂ at 0.489 THz and 0.521 THz, when r₁ = 25 µm. (f) Resonance frequencies of f₁ and f₂ with respect to r₁. (g) Q-factors of f₁ and f₂ versus r₁.

Fig. 3. (a) Dispersion curves of the four related TE and TM eigenmodes, as a function of k P/2π, when r₁ = r₂ = 55 µm, where k is a propagating constant. The inset shows the first Brillouin zone of the square lattice. (b) Electric (E_z) and magnetic (H_z) near-fields in the xy plane of the four modes at Γ = 0.

Fig. 4. (a), (c) Influence of air-hole depth t₁ on the frequency and (b), (d) Q-factor of the resonances f₁ and f₂, when thickness of metasurface t = 200 µm.

Fig. 5. (a) Calculated and (b) measured transmittances of four metasurfaces. (c) and (d) Enlarged views of measured resonance f₂ for the sample r₁ = 40 µm (blue) and f₁ for the sample r₁ = 35 µm (yellow) and their Fano fitted curves (pink).