Arbitrary large-gradient wavefront shaping: from local phase modulation to nonlocal diffraction engineering

The powerful wavefront manipulation capability of metasurfaces originates from their subwavelength or deep subwavelength elements with designable optical responses, especially phase responses. However, they usually suffer from performance degradation as the spatial phase gradient is large. To solve this issue, we propose an accurate and efficient nonlocal diffraction engineering mechanism to tailor an arbitrary large-gradient wavefront utilizing superwavelength-scale elements. The fast-varying phase profile is cut into segments according to 2 pi zones rather than subwavelength discretization. Each phase segment is accurately implemented by precisely tailoring the diffraction pattern of the element, where diffraction angles, efficiencies, and phases are controlled simultaneously. As proof of the concept, high numerical aperture cylindrical metalenses are designed using this method and experimentally validated at the terahertz band. The cylindrical metalens is further extended to a full-space metalens, which enables high-quality subwavelength imaging with resolved details of 0.65 lambda. The proposed mechanism offers an efficient way to capture the fast-varying wavefront using relatively coarse geometries with new physical insights. (C) 2022 Chinese Laser Press

Automated summary

This article introduces an accurate and efficient nonlocal diffraction engineering mechanism that utilizes superwavelength-scale elements to manipulate large-gradient wavefronts, solving the problem of performance degradation for metasurfaces with large spatial phase gradients. Experimental validation demonstrates the applicability of this mechanism in designing high numerical aperture metalenses for high-quality subwavelength imaging.