Smooth waveguide structures and manufacturing methods

A method of manufacturing a photonic integrated circuit (PIC) structure, the method comprising:

• creating, in a semiconductor device layer of a semiconductor-on-insulator substrate, a wire waveguide having a top surface and straight vertical sidewalls and comprising a tapered region, the semiconductor device layer made from a first semiconductor material;
• crystallographically wet-etching the straight vertical sidewalls of the wire waveguide to create, along a length of the wire waveguide in the tapered region, undercuts defining sidewall portions coinciding with crystallographic planes of the wire waveguide;
• depositing, above the device layer, a layer of a second semiconductor material that is different from the first semiconductor material;
• and
• patterning and etching the layer of the second semiconductor material to create a second waveguide of the second semiconductor material above the wire waveguide in the tapered region, the wire waveguide and the second waveguide together forming an optical mode converter.