“…In order to achieve highly efficient optical frequency conversion, several photonic structures have been developed so far, relying on stronger mode confinement, and thus higher power density (see, e.g., [30,[60][61][62]), material dispersion compensation (see, e.g., [30,63,64]), artificial phase matching in non-birefringent materials (see, e.g., [40,64]), etc. However, these structures also present some drawbacks.…”