Giant second-harmonic generation in rhombohedral transition metal dichalcogenide metasurfaces

The emerging rhombohedral (3R) stacked transition metal dichalcogenides (TMDs), with the broken inversion symmetry in bulk crystal, exhibit significant second-order nonlinear coefficients and high optical index, which play an increasingly important role in nonlinear nanophotonics. Here, we directly design bound states in the continuum (BICs) metasurface in ultrathin 3R-TMDs layers, and further study the interplay between q-BIC mode and nonlinear polarization of 3R-TMDs to form giant enhanced and angle-dependent second-harmonic generation (SHG). With elaborately designed BIC cavity to satisfy maximum alignment between the BIC states and the second-order susceptibility, the nonlinear conversion efficiency can achieve at least a 60,000-fold enhancement compared to that from the bulk film of the same thickness. Our work provides a robust platform for constructing nanophotonic cavities for enhanced second-order nonlinear processes and quantum optics.