The present disclosure provides an integrated circuit device comprising a substrate having a back surface and a sensing region disposed in the substrate and being operable to sense radiation projected towards the back surface of the substrate. The device further includes a waveguide disposed over the back surface of the substrate. The waveguide is aligned with the sensing region such that the waveguide is operable to transmit the radiation towards the aligned sensing region. The waveguide includes a waveguide wall, and an inner region disposed adjacent to the waveguide wall. A diffractive index of the waveguide wall is less than a diffractive index of the inner region.