Degeneracy-protected quasi-bound state in the continuum with polarization-robust nonlinear enhancement

We demonstrate a degeneracy-protected quasi-bound state in the continuum (quasi-BIC) in a germanium metasurface for polarization-robust nonlinear enhancement in the mid-infrared. By combining Brillouin zone folding with C 4 symmetry, two orthogonal modes become degenerate at the Γ point, enabling polarization-independent excitation through coherent superposition. Introducing controlled symmetry breaking converts the ideal BIC into a quasi-BIC with finite radiation leakage, allowing efficient coupling while maintaining a high quality factor. Compared with the ideal BIC, the quasi-BIC achieves stronger field enhancement due to improved coupling near the critical condition. In addition, the structure confines the electromagnetic field within the germanium region, enhancing light–matter interaction. Enhanced third-harmonic generation (THG) is observed, although the improvement is limited by strong absorption and the absence of resonant modes at the harmonic frequency.