Analysis of lossy mode cut-off conditions in planar waveguides with semiconductor guiding layer

It is known that a dielectric waveguide with semiconductor cladding exhibits a maximum attenuation of a given polarisation of light when the cladding thickness corresponds to the cut-off of any guided mode of the same polarisation that the semiconductor layer supports itself. In this paper mode cut-off conditions in the semiconductor layer are analysed and analytic expressions for the layer cut-off thicknesses are developed. The results show good accuracy when compared with more sophisticated numerical analysis techniques consuming a large amount of computer time. Design curves for silicon-clad polarisers on Ti: LiNbO3 and glass waveguides are developed from the presented theory.