Topological quadratic-node semimetals enabled by cross-pseudospin coupling in photonic microring arrays

We experimentally realize a topological quadratic-node semimetal in a photonic microring lattice, where a second-order nodal point at the Brillouin zone center coexists with two Dirac points. This system is enabled by the unique cross-pseudospin coupling Hamiltonian in a triangular resonator lattice. The quadratic-node semimetal goes beyond graphene by supporting both massive and massless chiral states within a single lattice. In a photonic p–n junction configuration, we observe simultaneous Klein and anti-Klein tunneling, highlighting the unique coexistence of two distinct chiral transport behaviors. The Dirac valley transmits fully, while the quadratic valley reflects, demonstrating a remarkable hybrid of massless and massive tunneling modes. This work introduces a previously unexplored topological nodal material, potentially guiding the development of novel photonic and electronic device architectures.