(a) Schematic structure of the Ref-sample (without Bi₂Se₃ thin film) and TI-sample (with 11-nm-thick Bi₂Se₃ thin film) with dimensions of 400 μm by 400 μm. The width of the Au gratings is W = 400 nm, 600 nm, 800 nm, 1000 nm, 1200 nm, and 1500 nm, with period 2W. (b) The scanning electron microscopy (SEM) image of the grating structures with notation of incident light parallel or perpendicular to the ribbon direction. (c) The reflectivity of bare sapphire substrate at T = 10 K and 296 K in the spectral range from 500 cm⁻¹ to 7100 cm⁻¹.