Efficient second harmonic generation in the LiNbO₃ metasurface based on quasi-bound states in the continuum under low pump light intensity

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The ultra-compact devices utilized for second harmonic generation (SHG) find extensive applications across multiple domains. However, the decrease in size presents difficulty in achieving efficient SHG. In this study, efficient SHG is successfully realized in the LiNbO 3 metasurface based on quasi-bound states in the continuum (quasi-BIC). The metasurface unit cell is a double-slot cylinder, and quasi-BIC mode is excited by introducing defects through varying the slot sizes, thereby ensuring the localization of electric and magnetic fields within the structure. Furthermore, robust ultra-high quality-factors and field enhancement factors are ultimately achieved by using a lower refractive index substrate. The excited quasi-BIC mode local fields are well coupled to the structure, thus allowing for the full utilization of the largest second-order nonlinear coefficient d 3 3 of the material. A remarkable SHG efficiency of 8.12% is achieved at a fundamental frequency pumping intensity as low as 1 kW/cm 2 .This work provides unique insights for realizing efficient nonlinear processes at nanoscale.