Dark mode-in-the-box for enhanced second-harmonic generation in corrugated waveguides

A numerical study of second-harmonic generation (SHG) in cavity resonator integrated grating filters (CRIGFs) shows extraordinary strong conversion rates in planar corrugated waveguides based on lithium niobate layers, not exceeding 625 mu m in length. The key idea is to couple to the dark mode of the structure at the pump frequency that has a Q-factor much stronger than the bright mode commonly used. This is achieved by shifting the grating coupler between the enclosing Bragg mirrors, resulting in an asymmetric structure. Three asymmetric structures are proposed: without any matching, with impedance matching and with indirect phase matching through an additional pair of gratings. They offer respectively increasing conversion rates of eta=4.31 x 10(-3) W-1, 0.149 W-1 and 2.38 W-1, which compared to a symmetric CRIGF (eta=7.12x10(-6) W-1) offer respectively more than 2, 4 and 5 orders of magnitude improvement of the conversion rate. (C) 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Automated summary

The numerical study explores strong conversion rates of second-harmonic generation in cavity resonator integrated grating filters (CRIGFs) based on lithium niobate layers. By coupling to the dark mode and creating asymmetric structures, significant improvements in conversion rates are achieved compared to symmetric CRIGFs.