Ruggero Verre

Optical magnetism and plasmonic Fano resonances in metal-insulator-metal oligomers.

The possibility of achieving optical magnetism at visible frequencies using plasmonic nanostructures has recently been a subject of great interest. The concept is based on designing structures that support plasmon modes with electron oscillation patterns that imitate current loops, that is, magnetic dipoles. However, the magnetic resonances are typically spectrally narrow, thereby limiting their applicability in, for example, metamaterial designs. We show that a significantly broader magnetic response can be realized in plasmonic pentamers constructed from metal-insulator-metal (MIM) sandwich particles. Each MIM unit acts as a magnetic meta-atom and the optical magnetism is rendered quasi-broadband through hybridization of the in-plane modes. We demonstrate that scattering spectra of individual MIM pentamers exhibit multiple Fano resonances and a broad subradiant spectral window that signals the magnetic interaction and a hierarchy of coupling effects in these intricate three-dimensional nanoparticle oligomers.

Dimer-on-mirror SERS substrates with attogram sensitivity fabricated by colloidal lithography

Nanoplasmonic substrates with optimized field-enhancement properties are a key component in the continued development of surface-enhanced Raman scattering (SERS) molecular analysis but are challenging to produce inexpensively in large scale. We used a facile and cost-effective bottom-up technique, colloidal hole-mask lithography, to produce macroscopic dimer-on-mirror gold nanostructures. The optimized structures exhibit excellent SERS performance, as exemplified by detection of 2.5 and 50 attograms of BPE, a common SERS probe, using Raman microscopy and a simple handheld device, respectively. The corresponding Raman enhancement factor is of the order 10(11), which compares favourably to previously reported record performance values.

Controlled in situ growth of tunable plasmonic self-assembled nanoparticle arrays

Self-assembled silver nanoparticle (NP) arrays were produced by deposition at glancing angles on transparent stepped Al2O3 templates. The evolution of the plasmonic resonances has been monitored using reflection anisotropy spectroscopy (RAS) during growth. It is demonstrated that the morphology of the array can be tailored by changing the template structure, resulting in a large tunability of the optical resonances. In order to extract detailed information on the origin of the measured dichroic response of the system, a model based on dipolar interactions has been developed and the effect of tarnishing and morphological dispersion addressed.

Self-assembled broadband plasmonic nanoparticle arrays for sensing applications

Highly ordered noble metal nanoparticle (NP) arrays are produced using a glancing angle deposition on stepped substrates. The versatility of the technique is demonstrated by depositing different metals, resulting in shifts of the resonance positions. The behaviour of the NP arrays grown is predicted by a dipolar model, and it is measured using reflectance anisotropy spectroscopy (RAS). Fine tuning of the resonances can be finally realised by selecting the deposition parameters. The combined application of both RAS and deposition at glancing angles provides a unique tool to grow NP arrays with the tunable plasmonic resonances in the entire visible range.

Hot Electron Generation and Cathodoluminescence Nanoscopy of Chiral Split Ring Resonators

Three-dimensional chiral plasmonic nanostructures have been shown to be able to dramatically boost photon-spin selective light-matter interactions, potentially leading to novel photonics, molecular spectroscopy, and light-harvesting applications based on circularly polarized light. Here, we show that chiral split-ring gold nanoresonators interfaced to a wide band gap semiconductor exhibit a contrast in hot-electron transfer rate between left-handed and right-handed visible light that essentially mimics the far-field circular dichroism of the structures. We trace down the origin of this effect to the differential excitation of the thinnest part of the split-ring structures using dichroic-sensitive cathodoluminescence imaging with nanometer spatial resolution. The results highlight the intricate interplay between the near-field and far-field chiral response of a nanostructure and establishes a clear link to the emerging field of hot carrier plasmonics with numerous potential applications in photocatalysis and solar light harvesting.

Observation of tunable charged exciton polaritons in hybrid monolayer WS2 – plasmonic nanoantenna system

Formation of dressed light-matter states in optical structures, manifested as Rabi splitting of the eigen energies of a coupled system, is one of the key effects in quantum optics. In pursuing this regime with semiconductors, light is usually made to interact with excitons, electrically neutral quasiparticles of semiconductors; meanwhile interactions with charged three-particle states, trions, have received little attention. Here, we report on strong interaction between localized surface plasmons in silver nanoprisms and excitons and trions in monolayer tungsten disulfide (WS2). We show that the plasmon-exciton interactions in this system can be efficiently tuned by controlling the charged versus neutral exciton contribution to the coupling process. In particular, we show that a stable trion state emerges and couples efficiently to the plasmon resonance at low temperature by forming three bright intermixed plasmon-exciton-trion polariton states. Our findings open up a possibility to exploit electrically charged polaritons at the single nanoparticle level.

Manipulating and probing the growth of plasmonic nanoparticle arrays using light

Highly ordered self-assembled silver nanoparticle (NP) arrays have been produced by glancing angle deposition on faceted c-plane Al2O3 templates. The NP shape can be tuned by changing the substrate temperature during deposition. Reflectance anisotropy spectroscopy has been used to monitor the plasmonic evolution of the sample during the growth. The structures showed a strong dichroic response related to NP anisotropy and dipolar coupling. Furthermore, multipolar resonances due to sharp edge effects between NP and substrate were observed. Analytical and numerical methods have been used to explain the results and extract semi-quantitative information on the morphology of the NPs. The results provide insights on the growth mechanisms by the glancing angle deposition. Finally, it has been shown that the NP morphology can be manipulated by a simple illumination of the surface with an intense light source, inducing changes in the optical response. This opens up new possibilities for engineering plasmonic structure over large active areas.

Polarization conversion-based molecular sensing using anisotropic plasmonic metasurfaces

Anisotropic media induce changes in the polarization state of transmitted and reflected light. Here we combine this effect with the refractive index sensitivity typical of plasmonic nanoparticles to experimentally demonstrate self-referenced single wavelength refractometric sensing based on polarization conversion. We fabricated anisotropic plasmonic metasurfaces composed of gold dimers and, as a proof of principle, measured the changes in the rotation of light polarization induced by biomolecular adsorption with a surface sensitivity of 0.2 ng cm(-2). We demonstrate the possibility of miniaturized sensing and we show that experimental results can be reproduced by analytical theory. Various ways to increase the sensitivity and applicability of the sensing scheme are discussed.

Directional Light Extinction and Emission in a Metasurface of Tilted Plasmonic Nanopillars

Plasmonic optical antennas and metamaterials with an ability to boost light-matter interactions for particular incidence or emission angles could find widespread use in solar harvesting, biophotonics, and in improving photon source performance at optical frequencies. However, directional plasmonic structures have generally large footprints or require complicated geometries and costly nanofabrication technologies. Here, we present a directional metasurface realized by breaking the out-of-plane symmetry of its individual elements: tilted subwavelength plasmonic gold nanopillars. Directionality is caused by the complex charge oscillation induced in each individual nanopillar, which essentially acts as a tilted dipole above a dielectric interface. The metasurface is homogeneous over a macroscopic area and it is fabricated by a combination of facile colloidal lithography and off-normal metal deposition. Fluorescence excitation and emission from dye molecules deposited on the metasurface is enhanced in specific directions determined by the tilt angle of the nanopillars. We envisage that these directional metasurfaces can be used as cost-effective substrates for surface-enhanced spectroscopies and a variety of nanophotonic applications.

Quasi-isotropic surface plasmon polariton generation through near-field coupling to a penrose pattern of silver nanoparticles.

Quasicrystals are structures that possess long-range order without being periodic. We investigate the unique characteristics of a photonic quasicrystal that consists of plasmonic Ag nanodisks arranged in a Penrose pattern. The quasicrystal scatters light in a complex but spectacular diffraction pattern that can be directly imaged in the back focal plane of an optical microscope, allowing us to assess the excitation efficiency of the various diffraction modes. Furthermore, surface plasmon polaritons can be launched almost isotropically through near-field grating coupling when the quasicrystal is positioned close to a homogeneous silver surface. We characterize the dispersion relation of the different excited plasmon modes by reflection measurements and simulations. It is demonstrated that the quasicrystal in-coupling efficiency is strongly enhanced compared to a nanoparticle array with the same particle density but only short-range lateral order. We envision that the system can be useful for a number of advanced light harvesting and optoelectronic applications.