An underfill adhesive may be used to mechanically stabilize a photonic integrated circuit chip (PIC) onto an electrical substrate; however, when the PIC is optically coupled to an external optical fiber at or near an edge of the chip, e.g. using an edge coupler, the underfill may flow into the optical interface impacting optical coupling quality. A photonic integrated circuit apparatus according to the disclosure comprises an electrical substrate, which includes a cavity underneath the edge coupler for preventing underfill material from entering the optical interface by impeding capillary action thereof.