Recovery of Tunable Bound States in the Continuum by Odd‐Parity Compensation

Tunable bound states in the continuum (BICs), which occur away from the point in photonic crystal slabs, are highly sensitive to substrate‐induced mirror‐symmetry breaking and typically degrade into finite‐ quasi‐BICs in realistic integrated platforms. Here, it is shown that such degradation can be deterministically reversed. Using temporal coupled‐mode theory and full‐wave simulations, the radiation channel opened by the substrate is canceled by introducing a second, independent odd‐parity perturbation. This strategy restores the singularity of the radiation matrix and thereby recovers tunable BICs in substrate‐supported photonic crystal slabs. The recovered states regain both the polarization vortex and the characteristic scaling. The recovery points further follow a linear relation in the two odd‐parity parameter space, revealing a simple mode‐dependent compensation law. The same mechanism also restores merging‐BIC configurations and can be implemented using graded‐index slabs, tapered holes, extra surface layers, or combined perturbation knobs. These results establish a practical route for preserving tunable BICs and their topological features in substrate‐supported nanophotonic systems.