  Topological photonic systems represent a new class of optical materials
supporting boundary modes with unique properties, not found in conventional
photonics. While the early research on topological photonics has focused on
edge and surface modes in 2D and 3D systems, respectively, recently
higher-order topological insulators (HOTIs) supporting lower-dimensional
boundary states have been introduced. In this work we design and experimentally
realize a photonic kagome metasurface exhibiting a Wannier-type higher-order
topological phase. We demonstrate and visualize the emergence of a topological
transition and opening of a Dirac cone by directly exciting the bulk modes of
the HOTI metasurface via solid-state immersion spectroscopy. The open nature of
the metasurface is then utilized to directly image topological boundary states.
We show that, while the domain walls host 1D edge states, their bending induces
0D higher-order topological modes confined to the corners. The demonstrated
metasurface hosting topological boundary modes of different dimensionality
paves the way to a new generation of universal and resilient optical devices
which can controllably scatter, trap and guide optical fields in a robust way.
