06 November 2017 Graphene Superlattices for Plasmonic Topological Orders

Illustration of a honeycomb graphene superlattice with 100% right-coupling efficiency at each junction

An ICFO study, in collaboration with Wuhan University, proposes a new way to realize topologically protected graphene plasmons. Topologically protected photonic states exhibit remarkable robustness against disorder and backscattering, which make them extremely useful for the realization of defect-immune photonic devices. These states have been explored in conventional metamaterials and photonic crystals involving sophisticated structural designs, which greatly complicate their fabrication and limit their miniaturization for nanoscale optical integration.

A powerful class of such states is based on time-reversal symmetry (T-symmetry) breaking of the optical response. While this approach has been demonstrated in the microwave regime, the weak magneto-optical response of most materials at visible and infrared frequencies renders it difficult to achieve substantial T-symmetry breaking in these technologically important spectral ranges.

Recent studies have shown that Dirac fermion (DF) systems possess a giant magneto-optical response in the infrared regime and exhibit substantial T-symmetry breaking. Exploiting this property in a recent study published in Nature Communications, ICFO researchers Deng Pan and ICREA Prof. at ICFO Javier García de Abajo, in collaboration with researchers Rui Yu and Hongxing Xu from Wuhan University, have theoretically demonstrated that topologically protected plasmonic states can be robustly realized in Dirac Fermion superlattices, constructed from single-layer graphene under exposure to a static magnetic field of only 2 tesla.

In their study, they have used graphene nanoribbons to construct the honeycomb network superlattice. They showed that the application of a magnetic field of a few tesla (as provided by commertially available permanent magnets) already induces significant asymmetry in the guided ribbon plasmon modes, thus resulting in directional coupling at the junctions of the structure. With this they have shown that, as a direct consequence of this directional coupling, localized modes are formed at defects inside the superlattice, and edge states at the boundary.

The results of their study provide a simple, yet robust platform for fast speed, ultra-compact, nonreciprocal optical computing networks, thus paving the way toward realistic applications of topological photonics.