Observation of topology transition in Floquet non-Hermitian skin effects in silicon photonics

Non-Hermitian physics has greatly enriched our understanding of nonequilibrium phenomena and uncovered novel effects such as the non-Hermitian skin effect (NHSE) that has profoundly revolutionized the field. NHSE is typically predicted in systems with nonreciprocal couplings which, however, are difficult to realize in experiments. Without nonreciprocal couplings, the NHSE can also emerge in systems with coexisting gauge fields and loss or gain (e.g., in Floquet non-Hermitian systems). However, such Floquet NHSE remains largely unexplored in experiments. Here, we realize the Floquet NHSEs in periodically modulated optical waveguides integrated on a silicon photonics platform. By engineering the artificial gauge fields induced by the periodical modulation, we observe various Floquet NHSEs and unveil their rich topological transitions. Remarkably, we discover the transitions between the normal unipolar NHSEs and an unconventional bipolar NHSE which is accompanied by the directional reversal of the NHSEs. The underlying physics is revealed by the band winding in complex quasienergy space which undergoes a topology change from isolated loops with the same winding to linked loops with opposite windings. Our work unfolds a new route toward Floquet NHSEs originating from the interplay between gauge fields and dissipation effects and offers fundamentally new ways for steering light and other waves

Mid-infrared Polaritonic Coupling between Boron Nitride Nanotubes and Graphene

Boron nitride (BN) is considered to be a promising substrate for graphene-based devices in part because its large band gap can serve to insulate graphene in layered heterostructures. At mid-infrared frequencies, graphene supports surface plasmon polaritons (SPPs), whereas hexagonal-BN (<i>h</i>-BN) is found to support surface phonon polaritons (SPhPs). We report on the observation of infrared polaritonic coupling between graphene SPPs and boron nitride nanotube (BNNT) SPhPs. Infrared scattering type scanning near-field optical microscopy is used to obtain spatial distribution of the two types of polaritons at the nanoscale. The observation suggests that those polaritons interact at the nanoscale in a one-dimensional/two-dimensional (1D/2D) geometry, exchanging energy in a nonplanar configuration at the nanoscale. Control of the polaritonic interaction is achieved by adjustment of the graphene Fermi level through voltage gating. Our observation suggests that boron nitride nanotubes and graphene can interact at mid-infrared frequencies and coherently exchange their energies at the nanoscale through the overlap of mutual electric near field of surface phonon polaritons and surface plasmon polaritons. Such interaction enables the design of nano-optical devices based on BNNT-graphene polaritonics in the mid-infrared range