Metasurface-Based Free-Space Multi-Port Beam Splitter with Arbitrary Power Ratio

A beam splitter (BS) is one of the most critical building blocks in optical systems. Despite various attempts of flat-type BSs to miniaturize the conventional cube BS reported, it remains a challenge to realize an ultrathin optical BS with multi-port output, non-uniform splitting ratio, and steerable outgoing directions. Herein, a free-space optical multi-port beam splitter (MPBS) based on a polarization-independent all-dielectric metasurface is demonstrated. By applying an optimized phase-pattern paradigm via a gradient-descent-based iterative algorithm to amorphous silicon (alpha-Si) metasurfaces, a variety of MPBS samples with arbitrarily predetermined output port number (2-7), power ratio, and spatial distribution of output beams are prepared. The experimental results reveal that the fabricated MPBSs can achieve high total splitting efficiency (TSE, above 74.7%) and beam-splitting fidelity (similarity, above 78.4%) within the bandwidth of 100 nm (1500-1600 nm). Such proposed MPBS can provide fabulous flexibility for optical integrated system and diverse applications.