Inverse-designed non-Hermitian hollow nanowire cavity for generating optical orbital angular momentum

We designed a gallium nitride hexagonal hollow nanowire whispering gallery mode cavity that generates an |m|=6 topological light with orbital angular momentum (OAM). By exploiting the unidirectional propagation that emerges in the vicinity of singularities in non-Hermitian systems, this mechanism enables the generation of OAM. OAM is generated by breaking the cross-sectional mirror symmetry of the nanowire, which creates a non-Hermitian system. This is achieved by replacing the central airhole of the hollow nanowire with a cluster of 6 overlapping circular air holes with rotational offset relative to the hexagonal cross-sectional profile of the nanowire. The design parameters were then further optimized in the finite element method using an inverse design method to maximize the normalized OAM order |l| . We were able to realize a cavity mode with |l| = 5.7, a mode purity of about 97%, and a Q-factor of ∼250. This marks the OAM generating active photonic device design falling within a sub-micron footprint, with additional novelties of being a single component and materially homogeneous.