Considerations for structuring the incident and transmitted regions in RCWA

Rigorous coupled-wave analysis (RCWA) is an exact numerical method, which is typically used to investigate the diffraction of coherent light at periodic grating structures. In this application, the grating is situated between the incident and transmitted region with known and constant index of refraction. Yet, there are many situations such as wave guides, fibers, etc., which require also the incident and transmitted regions to be structured. In principle, only slight modifications are already sufficient to achieve this. This extension to RCWA, however, may lead to ambiguities concerning the propagation direction of individual eigenmodes. In these cases the usual separation of the field into forward and backward propagating modes is not anymore possible. In this talk we investigate all the cases, where an unambiguous choice of the sign can be still made, and we also discuss the general case.