Harnessing disclination and topological defect edge states for second harmonic generation in photonic crystals

The study of topological phase transitions induced by defects has emerged as a significant research focus. The introduction of topological defects in valley photonic crystals offers a novel degree of freedom for manipulating electromagnetic waves. These defects give rise to robust edge states with bending resistance and flexibility, promising for new waveguide designs. However, their impact on nonlinear effects remains largely unexplored, and disclination states related to nonlinearity have received little attention. Here, we propose a scheme for nonlinear frequency conversion via the coupling of different defect states. By designing harmonic transmission waveguides of various shapes, we achieve a significant enhancement in conversion efficiency. The transmission shape of harmonics can also be customized, offering flexibility. Our work paves new pathways for investigating nonlinear defect states in photonic crystals and holds significant reference value for defect regulation.