Fig. 1. Schematic diagram of silicon nitride sandwich waveguide structure.

Fig. 2. Mode field cross-section of silicon nitride sandwich waveguide at different wavelengths

Fig. 3. The dispersion coefficient diagram of the silicon nitride sandwich waveguide. The gray area represents the abnormal dispersion region，and the white area represents the normal dispersion region.

Fig. 4. Phase matching （PM） diagram of silicon nitride sandwich waveguide at 1550 nm input wavelength.

Fig. 5. Effect of different W on phase matching and SCG under 1 550 nm pulse，Hl=500 nm，Hs=50 nm，Hu=200 nm conditions：（a） Phase matching plots of silicon nitride sandwich waveguides under different W conditions； （b） SCG spectra of silicon nitride sandwich waveguides under different W conditions

Fig. 6. Effect of different Hl on phase matching and SCG under 1 550 nm pulse，W=1 800 nm，Hs=50 nm，Hu=200 nm conditions：（a） Phase matching plots of silicon nitride sandwich waveguides under different Hl conditions； （b） SCG spectra of silicon nitride sandwich waveguides under different Hl conditions

Fig. 7. Effect of different Hu on phase matching and SCG under 1 550 nm pulse，W=1 800 nm，Hs=50 nm，Hl=500 nm conditions： （a） Phase matching plots of silicon nitride sandwich waveguides under different Hu conditions； （b） SCG spectra of silicon nitride sandwich waveguides under different Hu conditions

Fig. 8. Effect of different Hs on phase matching and SCG under 1 550 nm pulse，W=1 800 nm，Hu=200 nm，Hl=500 nm conditions： （a） Phase matching plots of silicon nitride sandwich waveguides under different Hs conditions； （b） SCG spectra of silicon nitride sandwich waveguides under different Hs conditions

Fig. 9. SEM image of silicon nitride waveguide covered with ALD sapphire film： （a） Side view of waveguide； （b） Cross-section of waveguide