Zero crosstalk in asymmetric multimode waveguides for higher-order mode crosstalk suppression via subwavelength gratings.

We present a generalized zero-crosstalk design method for asymmetric multimode waveguides using anisotropic subwavelength grating (SWG) claddings. Based on non-orthogonal coupled-mode theory, we show that optical power transfer in asymmetric systems is governed by the effective coupling coefficient κeff, and the zero-crosstalk condition is achieved when κeff = 0. By engineering the anisotropic dielectric perturbation of the SWG cladding, the longitudinal field component contributing to κeff can be counterbalanced, enabling complete suppression of coupling between dissimilar waveguides and different orders of transverse-electric (TE) modes. Geometric parameter spaces achieving zero-crosstalk trajectories are identified, and the concept is validated with a manufacturable discrete SWG structure with strong tolerance to parameter deviations. This approach provides a scalable solution for higher-order mode crosstalk suppression in dense multimode photonic integrated circuits.