Gallium phosphide (GaP) offers unique opportunities for nonlinear and quantum nanophotonics due to its wide optical transparency range, high second-order nonlinear susceptibility, and the possibility to tailor the nonlinear response by a suitable choice of crystal orientation. However, the availability of single crystalline thin films of GaP on low index substrates, as typically required for nonlinear dielectric metasurfaces, is limited . Here we design resonant monolithic GaP metasurfaces optimized for efficient second harmonic (SH) generation. We experimentally realized the metasurfaces from bulk (110) GaP wafers using electron-beam lithography and an optimized inductively coupled plasma etching process. SH generation measurements show good agreement with numerical simulations and a high NIR-to-visible conversion efficiency reaching up to 10^(-5) for SH emission along the optical axis. Furthermore, we investigated the potential of the suggested monolithic GaP metasurface for SH wavefront shaping applications.
Efficient manipulation of the valley degree of freedom in Transition Metal Dichalcogenide (TMD) monolayers at the nanoscale becomes very desirable for future developments in valleytronics. Resonant optical nanostructures are considered as potential tools in this endeavor; however, it is still unclear how they affect polarization properties of valley-specific monolayer emission. Here, we present a systematic experimental and numerical study that is aimed to bridge this gap. As a simple model, we consider a hybrid system where valley-polarized photoluminescence [1] or second harmonic from MoS2 - monolayer is coupled with a plasmonic nanosphere. Through this study, we are not only aimed to refine the exciting simulation approaches for valleytronic devices, but also contribute to the deeper understanding of the rich physics of light-matter interactions at the nanoscale.
Topological edge states draw their unique robustness against perturbations from a topological invariant of the bulk of the system. As long as the topological properties persist, the edge transport is not perturbed by static defects, which is referred to as the bulk-edge correspondence.
In our work we demonstrate that local periodic perturbations of the interface can destroy the topological protection even if the bulk of the system stays unperturbed. As model system we consider the Su-Schrieffer-Heeger (SSH) model realized in coupled plasmonic waveguide arrays with alternating short and long separations. Interfacing two SSH chains with different dimerizations we induce the topological edge mode. The temporal perturbations are realized by periodically bending the waveguide at the interface. The spatial evolution of surface plasmon polaritons (SPPs) in the array is monitored by real- and Fourier space leakage radiation microscopy. In Fourier space we observe that time-periodic perturbations of the interface create Floquet replicas of the topological edge mode. If the driving frequency is in the range for which the first Floquet replicas cross the static bands, the topological edge state couples to bulk states and the topological protection is destroyed resulting in delocalization of SPPs in real space. Otherwise the topological protection is conserved and SPPs stay localized at the interface. Our experimental findings are in full agreement with the theoretical analysis based on Floquet theory and illuminates the generalization of the bulk-edge correspondence for Floquet systems for the special case of a static bulk.
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