On-chip waveguide digital metalenses via inverse design

Given the recent success of metasurfaces in free-space applications, these concepts can be leveraged to an even larger extent in on-chip waveguide systems. The in-plane diffractive metasurfaces enable the manipulation of guiding waves in the multimode regime with greater parallelism than conventional single-mode or few-mode waveguides, leading to exciting opportunities in signal processing and optical computing systems. Beam focusing is one of the basic functionalities of wavefront shaping, which can be implemented using phase gradient metalenses consisting of arrays of meta-atoms. The meta-atoms are mainly realized by etched trenches with varying lengths, which are assembled into a one-dimensional transmit array with a specific phase response. However, this kind of periodic arrayed structure has significantly limited design freedom compared to its free-space counterparts. Here, we propose a digital metalens that consists of a seamless array of pixelated unit cells, which are engineered via inverse design. In contrast to conventional focusing metalenses based on transmit arrays, highly functional digital metalenses have been demonstrated: (1) achromatic focusing lens; (2) extended depth of focus (EDOF) lens; (3) Airy beam lens. These devices were fabricated on a silicon photonic platform and characterized in near-infrared. The intersection of digital structures and algorithm-driven optimizations offers greater versatility for on-chip wavefront shaping.