Second harmonic generation enhancement based on quasi-BICs in centrosymmetric materials

In centrosymmetric optical materials, the second-order nonlinear polarization of the bulk electric dipolar contribution is zero. More effective utilization of the contribution of the surface term is one of the key methods to efficiently obtain second-order nonlinear responses on these materials. Herein, a design of densely packed slotted nanopillar arrays based on quasi-bound states in the continuum (quasi-BICs) is proposed. The quasi-BICs are analyzed by using the finite element method as an example of silicon and the second harmonic generation (SHG) process is simulated. In the structure, normal-incidence linearly polarized light excites magnetic dipole-like quasi-BICs with a high quality factor which effectively promotes light-matter interactions. Increasing the nanopillar radius or decreasing the lattice constant within a certain range can cause the distribution of quality factors in k-space of the ky direction to contract toward the Gamma point, which leads to a quasi-BIC with higher quality factors at the Gamma point. By conjunctively adjusting the nanopillar radius and lattice constant or changing the slot azimuth, the resonance wavelength can be adjusted over a wide range (about several hundred nanometers) or finely (within about one nanometer) while maintaining high quality factors. When the symmetry perturbation introduced by the slot is small, it is calculated that the SHG conversion efficiency is about 10-6 similar to 10-5 at an incident light power density of 1 MW/m2, and the SHG power is about 107 similar to 108 times enhancement compared with the structure without slots. As the slot width decreases, higher SHG conversion efficiency with more significant SHG enhancement can be achieved at a specific slot length. The results provide new insights into the modulation of the resonant wavelength and quality factor of quasi-BICs, as well as the control of second-order nonlinear effects in centrosymmetric materials.