An optical waveguide structure of a semiconductor photonic device includes a first semiconductor waveguide, a second semiconductor waveguide, and an air seam between the first and second semiconductor waveguides. The semiconductor waveguides extend in a first direction, and a plurality of air seams extend in a second direction. Each of the air seams is disposed between two adjacent semiconductor waveguides. A distance between the two adjacent semiconductor waveguides is less than a width of each semiconductor waveguide.