  A vectorial optical mode converter that can transform an orthogonal set of
multiple input vector beams into another orthogonal set of vector beams is
attractive for a wide range of applications in optics and photonics. While
multi-plane light conversion (MPLC) and metasurface technologies have been
explored to individually address multiple spatial mode conversion and
polarization mode manipulation, there is no universal methodology to
simultaneously convert a set of multiple vectorial modes, having non-uniform
spatial distributions in both their complex amplitude and polarization, to
another set of multiple vectorial modes. In this paper, we introduce a novel
device based on the MPLC concept, incorporating multi-layer locally
birefringent metasurfaces, and present a general design formalism to realize
complete vectorial mode conversion for arbitrary cases. The efficacy of the
proposed method is confirmed through the demonstration of two distinctive
devices: a mode-division-multiplexed dual-polarization coherent receiver and a
spatial-mode-multiplexed vectorial holography system. Due to its versatility,
the demonstrated protocol can be applied to design a myriad of
multi-input-multi-output devices, offering a powerful tool to realize universal
optical mode converters for diverse applications.
