Fig. 1. Schematic diagram of an angle-multiplexed inverted all-dielectric metagrating on a metal substrate for enhanced THz absorption of thin films. The symbols w₁, w₂, w₃, and w₄ denote the widths of the four grating elements, while p, h₁, h₂, and d represent the unit cell period, grating layer height, WG layer height, and analyte thickness, respectively

Fig. 2. Reflection spectrum of the metagrating structure when the incidence angle increased from 3° to 20°. (a) Reflection curves for uncoated α-lactose; (b) reflection curves for coated 0.5 μm α-lactose

Fig. 3. (a) Dispersion relation of the fundamental TE mode in the metagrating; (b) absorbance (A) of the structure covered with 1 μm α-lactose as a function of the frequency and incident angle (θ)

Fig. 4. Terahertz absorption spectra of lactose thin films enhanced by the metagrating structure in this paper. (a) Absorption spectrum of 0.2 μm pure α-lactose film (blue solid line), absorption spectrum coated with 0.2 μm α-lactose (gray solid line), and the envelope of the absorption spectrum coated with 0.2 μm α-lactose (red dashed line); (b) the real and imaginary parts of the α-lactose dielectric constants in the band of 0.48 THz~0.58 THz; (c) the x-z plane normalized electric field distributions of the thin-film coating of α-lactose (d=0.2 μm) and uncoated α-lactose under different incidence angles θ at corresponding resonant frequencies

Fig. 5. (a) Changes in electric field intensity with and without α-lactose coating under different incident angles (θ) and frequencies. The numbers above the red dots represent the percentage reduction in the maximum electric field intensity of the structure with α-lactose coating compared to that without the coating; (b) reflectance spectra of the metasurface at different γ values with a terahertz wave incident angle of θ = 5°

Fig. 6. Absorption enhancement spectra versus film thickness and metagrating parameters (h₁=h₂=140 μm, w₁=w₃=150 μm, w₂=60 μm, w₄=20 μm). (a) Absorbance envelope curves for different thicknesses of α-lactose on the metasurface; (b) absorbance envelope curves at different h₁; (c) absorbance envelope curves at different h₂; (d) absorbance envelope curves for different γ