  Metasurfaces have achieved fruitful results in tailoring complexing light
fields in free space. However, a systematic investigation on applying the
concept of meta-optics to completely control waveguide modes is still elusive.
Here we present a comprehensive catalog capable of selectively and exclusively
excite almost arbitrary high-order waveguide modes of interest, leveraging
silicon metasurface-patterned silicon nitride waveguides. By simultaneously
engineering the phase-matched gradient of the metasurface and the vectorial
spatial modal overlap between the nanoantenna near-field and target waveguide
mode for excitation, either single or multiple high-order modes are
successfully launched with high purity reaching 98% and broad bandwidth.
Moreover, on-chip twisted light generators are also theoretically demonstrated
with configurable OAM topological charge \ell from -3 to +2, serving as a
comprehensive framework for metasurface-enabled guided mode optics and
motivating further applications such as versatile integrated couplers,
demultiplexers, and mode-division multiplexing-based communication systems.
