**2.3 Choice of material**

To reach phase matching and a high conversion efficiency, we simulated various waveguide geometries before settling on high index contrasts and a pump of order 2 in the direction of growth. To maximize index contrast in the vertical direction, the waveguide is surrounded by Al0.8Ga0.2As in the bottom cladding and by air above. However, in the region of transfer, the top cladding will be provided by whatever material separates laser and underlying waveguide core. We set this material to be Al0.3Ga0.7As, since it is already used as cladding in GaAs lasers. This layer structure is summarized in **Table 2**. It is identical to a standard laser data sheet, apart from the modified separation layer and added nonlinear waveguide and cladding. In the region of frequency conversion, all layers are etched, except for the last three: nonlinear waveguide, bottom cladding, and substrate.


## **Table 2.**

*Layer structure proposed for the coupled-cavity design. In the region of conversion, all layers are etched except the last three.*
