  Band topology and related spin (or pseudo-spin) physics of photons provide us
with a new dimension for manipulating light, which is potentially useful for
information communication and data storage. Especially, the quantum spin Hall
effect of light, where electromagnetic waves propagate along surfaces of
samples with strong spin-momentum locking, paves the way for achieving
topologically protected photonic spin transport. Recently, the conventional
bulk-edge correspondence of the band topology has been extended to higher-order
cases that enables the explorations of topological states with codimensions
larger than 1 such as hinge and corner states. Here, for the first time, we
demonstrate a higherorder quantum spin Hall effect of light by utilizing an
all-dielectric C6v-symmetric photonic crystal. We observe corner states with
opposite pseudospin polarizations at different corners owing to nontrivial
higher-order topology and finite spin-spin coupling. By applying the
spin-polarized excitation sources, we can selectively excite the corner states
at different spatial positions through spin-momentum-locked decaying edge
states, resembling the quantum spin Hall effect in a higher-order manner. Our
work which breaks the barriers between the spin photonics and higher-order
topology opens the frontiers for studying lower-dimensional spinful classical
surface waves and supports explorations in robust communications.

Materials Science;Optics