A Three-Step Procedure for the Design of Broadband Terahertz Antireflection Structures Based on a Subwavelength Pyramidal-Frustum Grating

Materials in the terahertz (THz) region have high refractive indices, thus leading to significant Fresnel reflection loss. We present a step-by-step 2-D THz antireflection grating design procedure using thin film theory and coupled-coefficient-matrix method. A thin film with desirable properties is first optimized by genetic algorithm, and a `translation' from thin film to multilevel grating is then implemented with the coupled-coefficient-matrix method. A final smoothing step leads to a square-pyramidal-frustum grating. Using rigorous coupled-wave analysis, we demonstrate a subwavelength square-pyramidal-frustum grating based on silicon producing broadband (0.5 to 5 THz) and efficient (transmittance>95%) polarization-independent antireflection effects. The grating parameters are evaluated, showing that our design is resistant to parameter deviations such as grating depth and incidence angle.