A semiconductor structure according to the present disclosure includes a buried oxide layer, a first dielectric layer disposed over the buried oxide layer, a first waveguide feature disposed in the first dielectric layer, a second dielectric layer disposed over the first dielectric layer and the first waveguide feature, a third dielectric layer disposed over the second dielectric layer, and a second waveguide feature disposed in the second dielectric layer and the third dielectric layer. The second waveguide feature is disposed over the first waveguide feature and a portion of the second waveguide feature vertically overlaps a portion of the first waveguide feature.