  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, we have demonstrated a free-space optical
multi-port beam splitter (MPBS) based on a polarization-independent
all-dielectric metasurface. By applying an optimized phase-pattern paradigm via
a gradient-descent-based iterative algorithm to amorphous silicon (a-Si)
metasurfaces, we have prepared a variety of MPBS samples with arbitrarily
predetermined output port number (2~7), power ratio and spatial distribution of
output beams. The experimental results reveal that the fabricated MPBSs could
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). We envision that such MPBS could provide fabulous flexibility for optical
integrated system and diverse applications.
