Breaking parity ($\mathcal{P}$) symmetry in $C_6$ crystals is a common routine to implement a valley topological phase. At an interface between two crystals of opposite valley Chern numbers, the so-called valley topological edge states (VTESs) emerge, and they have been proven useful for wave transport with robustness against ${120}^{\circ }$ bending and a certain level of disorder. Here, we first revisit the bending-immune mechanism of VTESs by developing a lattice coupled mode theory to analyze coupling dynamics between VTESs, and further performing a perturbative analysis to estimate the Bragg scattering at the bending corner. Our analysis quantitatively evidences that the bending reflection is suppressed by the unique modal profiles of the VTESs—momentum hot spots localized at high-symmetric $K$ ($K^{\prime }$) points—which only concerns the existence of the valleys in the bulk band diagrams, and imposes no requirement on topology. Second, we reveal the emergence of non-Hermitian supercoupling (SC) effects in (Hermitian) valley systems: Counterflow, momentum-matched incident and transmission ESs could reach $\sim 100\%$ coupling efficiency free of corner reflection and construct an unusual superexponential field distribution. Based on two findings, a coherent understanding of bending immunity for ESs in valley systems is established, and different designs based on rhombic lattices are proposed. Additionally, we demonstrate that the emerged superexponential field profile due to the SC effects provides another way to engineer the photonic Purcell factor.

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• Nanophotonics
• Photonics
• Topological effects in photonic systems