Second-harmonic generation of trapezoidal grating lithium niobate films based on quasi-bound states in the continuous

Optical second harmonics (SH) has important applications in many fields, such as micro-imaging techniques and extending the wavelength band of laser light sources. Lithium niobate (LiNbO₃) can achieve a high SH conversion efficiency by its large second-order nonlinear coefficient. Conventional LiNbO₃ devices require long interaction lengths and meet phase matching requirements. Thin-film lithium niobium oxide enables better optical mode confinement and improves SH conversion efficiency. A distributed Bragg reflection trapezoidal grating SH- conversion device based on quasi-bound states of continuous LiNbO₃ thin films is designed. The device can generate narrow-band resonances with five high Q-factors over a broadband width of 500–800 nm. Based on the multistage expansion algorithm and the magnetic field map obtained by simulation, a suitable resonant mode is selected to obtain a higher SH generation efficiency. By rationally designing the structural parameters of the device as well as the incident light intensity, a high SH generation efficiency of up to 3.35 × 10⁻³ can be obtained. As a result, our devices are designed for high efficiency, low loss and multi-band SH generation devices.