Plasmon-Induced Optical Anisotropy in Hybrid Graphene–Metal Nanoparticle Systems

Hybrid plasmonic metal–graphene systems are emerging as a class of optical metamaterials that facilitate strong light-matter interactions and are of potential importance for hot carrier graphene-based light harvesting and active plasmonic applications. Here we use femtosecond pump–probe measurements to study the near-field interaction between graphene and plasmonic gold nanodisk resonators. By selectively probing the plasmon-induced hot carrier dynamics in samples with tailored graphene–gold interfaces, we show that plasmon-induced hot carrier generation in the graphene is dominated by direct photoexcitation with minimal contribution from charge transfer from the gold. The strong near-field interaction manifests as an unexpected and long-lived extrinsic optical anisotropy. The observations are explained by the action of highly localized plasmon-induced hot carriers in the graphene on the subresonant polarizability of the disk resonator. Because localized hot carrier generation in graphene can be exploited to drive electrical currents, plasmonic metal–graphene nanostructures present opportunities for novel hot carrier device concepts

Spectral Tuning of Localized Surface Phonon Polariton Resonators for Low-Loss Mid-IR Applications

Low-loss surface phonon polariton (SPhP) modes supported within polar dielectric crystals are a promising alternative to conventional, metal-based plasmonic systems for the realization of nanophotonic components. Here we show that monopolar excitations in 4H-silicon carbide nanopillar arrays exhibit an unprecedented stable efficiency even when the resonator filling fraction is varied by an order of magnitude. This provides a powerful mid-IR platform with excellent spectral tunability and strong field confinement. Combining IR spectroscopy measurements with full electrodynamic calculations, we elucidate the nature of the optical modes in these elongated subwavelength nanostructures by investigating their spectral behavior and local field dependence on the size and periodicity. The present study also gives a clear understanding and practical guidelines for the spectral tuning of localized SPhP and the coupling mechanisms at play. This work is integral with the development of phonon-polariton based applications for surface-enhanced infrared absorption spectroscopy (SEIRA), polychromatic detectors, and thermal imaging

Self-Assembled Spherical Supercluster Metamaterials from Nanoscale Building Blocks

We report on a simple, universal, and large-scale self-assembly method for generation of spherical superclusters from nanoscopic building blocks. The fundamentals of this approach rely on the ultrahigh preconcentration of nanoparticles (NP) followed by using either emulsification strategies or alternatively multiphase microfluidic microdroplets. In both cases drying of the NP droplets yields highly spherical self-assembled superclusters with unique optical properties. We demonstrate that the behavior of these spheres can be controlled by surface functionalization before and after the self-assembly process. These structures show unique plasmonic collective response both on the surface and within the supercluster in the visible and infrared regions. Furthermore, we demonstrate that these strong, tunable optical modes can be used toward ultrasensitive, reproducible, surface-enhanced spectroscopies

Low-Loss, Extreme Subdiffraction Photon Confinement via Silicon Carbide Localized Surface Phonon Polariton Resonators

Plasmonics provides great promise for nanophotonic applications. However, the high optical losses inherent in metal-based plasmonic systems have limited progress. Thus, it is critical to identify alternative low-loss materials. One alternative is polar dielectrics that support surface phonon polariton (SPhP) modes, where the confinement of infrared light is aided by optical phonons. Using fabricated 6H-silicon carbide nanopillar antenna arrays, we report on the observation of subdiffraction, localized SPhP resonances. They exhibit a dipolar resonance transverse to the nanopillar axis and a monopolar resonance associated with the longitudinal axis dependent upon the SiC substrate. Both exhibit exceptionally narrow linewidths (7–24 cm^–1), with quality factors of 40–135, which exceed the theoretical limit of plasmonic systems, with extreme subwavelength confinement of (λ_res³/<i>V</i>_eff)^1/3 = 50–200. Under certain conditions, the modes are Raman-active, enabling their study in the visible spectral range. These observations promise to reinvigorate research in SPhP phenomena and their use for nanophotonic applications