Extremely large-angle beam deflection based on low-index sparse dielectric metagratings

Metasurfaces for beam engineering are usually implemented by using dense-plasmonic or high-contrast dielectric building blocks with deep-subwavelength feature size and strong field confinement. Here, we theoretically and experimentally demonstrate extremely large angle deflection with a very steep phase gradient based on low-contrast ($\delta n=0.57$) metagratings composed of very sparse building blocks. Only two ridges are included in a 4π variation supercell, which diffracts the normal excitation to desired angles as large as 80° with a theoretical efficiency of more than 80%. Due to the limited beam radius relative to the metasurfaces, the angular distribution of the intensity is broadened with reduced peak intensity. Such metagratings are 3D-printed for 0.14 THz operation. The measured peak intensities are 57% of the theoretical ones due to material loss and fabrication errors. Repeatability, bandwidth and angular sensitivity are systematically studied, and the results pave the way towards using practical THz elements for large-gradient wavefront shaping. (paper)