Efficient Wide-Band Large-Angle Refraction and Splitting of a Terahertz Beam by Low-Index 3D-Printed Bilayer Metagratings
Xipu Dong, Jierong Cheng, Fei Fan, Xianghui Wang, Shengjiang Chang
Abstract
Near-perfect anomalous reflection and refraction have been demonstrated using passive lossless metasurfaces and metagratings operating at microwave, infrared, and visible frequencies, while related studies at terahertz frequencies are lacking. Here we propose low-index (with a refractive index of 1.57) 3D-printed dielectric metagratings for efficient wide-band diffraction engineering at low terahertz frequencies. A simplified analytical model reveals that the number of propagating waveguide modes inside the grating is a key factor in diffraction engineering, and is insufficient in a low-index design regardless of the detailed dimensions in a period. Additional waveguide modes are introduced in asymmetric bilayer and trilayer metagratings, providing sufficient degrees of freedom for efficient large-angle anomalous refraction and beam splitting. These metagratings are inherently wide-band, benefiting from low dispersion of the waveguide modes. Three metagratings are designed, 3D printed, and tested experimentally at 0.14 THz for ${70}^{\ensuremath{\circ}}$ refraction, ${80}^{\ensuremath{\circ}}$ refraction, and $\ifmmode\pm\else\textpm\fi{}{70}^{\ensuremath{\circ}}$ beam splitting. The measured efficiency shows good agreement with the design. The proposed metagratings, with simple structures and large feature sizes, can be easily scaled to applications at higher terahertz frequencies.