The present invention provides optical integrated circuits. The optical integrated circuit includes a substrate including a single crystalline semiconductor material. The optical integrated circuit includes an insulation region in a trench in the substrate. The optical integrated circuit includes a first core on the insulation region. The first core includes the single crystalline semiconductor material. Moreover, the optical integrated circuit includes a second core that is spaced apart from the first core. The second core includes a material having a refractive index that is lower than thatof the first core.