A photonic waveguide for conducting light having first and second wavelengths, the waveguide comprising superposed first and second strips of light conducting semiconductor materials having first and second refractive indexes, wherein the second wavelength is shorter than the first wavelength and the second refractive index is higher than the first refractive index, wherein the width and height of the first strip of light conducting semiconductor material are such that the first strip of light conducting semiconductor material is adapted to confine an optical mode of the first wavelength and the width and height of the second strip of light conducting semiconductor material are such that the second strip of light conducting semiconductor material is adapted to confine an optical mode of the second wavelength but is too narrow to confine an optical mode of the first wavelength.