Terahertz tight focusing based on low-index metagratings with free phase modulation

Metagrating-based high-NA metalenses are studied, which can be made of any transparent dielectric due to the nonlocal phase modulation mechanism. Besides of spatial modulation of the diffraction angle, the diffraction phase of metagratings are tailored for the first time to achieve coherent superposition of the diffracted fields. Metalenses made of low-index polymer metagratings with NA of 0.94 at 0.14 THz are demonstrated in theory and experiment. The proposed lightweight and planar high-NA metalenses with straightforward design principles may find wide applications in terahertz imaging and communications.