  The emerging field of topology has brought device effects to a new level.
Higher-order topological insulators (HOTIs) go beyond traditional descriptions
of bulk-edge correspondence, broadening the understanding of topologically
insulating phases. In this paper, a second-order split-ring photonic crystal
(SSPC) with zero-dimensional (0D) corner states and one-dimensional (1D) edge
states is proposed. Based on the coupling strength determined by the opening
direction between the split-rings, the electronic transition strength of the
electronic system is imitated, and the topological trivial and non-trivial
transformation of the topological two-dimensional (2D) SSH model are realized
by using the rotating split-ring lattice. Theory and simulation find that SSPC
has non-trivial topological edge states that can be quantified by bulk
polarization. As the opening direction of the split-rings gradually changes
within one period, there will be transitions between four different topological
polarizations of the lowest energy bands, which can be conveniently used to
achieve transitions between different topological phases. Our research can be
extended to higher dimensions and broaden research paths for higher-order
photonic topological insulators and semimetals.
