Merging mirror-coupled bound states in the continuum at THz frequency.

We achieve merging mirror-coupled bound states in the continuum (BICs) at THz frequency in the metasurface consisting of Si block arrays on an Au mirror with SiO as a spacer. Compared with the conventional mirror-coupled BIC, the merging BIC exhibits a higher Q-factor and local field enhancement at the same in-plane wavevector . Further, merging mirror-coupled BICs can combine the advantage of mirror-coupled BICs without breaking the symmetry of the nanostructure and the superiority of merging BICs, which is robust to imperfections of the nanostructure. Efficient third-harmonic generation and low threshold optical bistability in the metasurface are obtained assisted by merging BICs. The results indicate the potential applications of merging mirror-coupled BICs in advanced all-optical devices.