Yan Francescato

Fano Resonances in Nanoscale Plasmonic Systems: A Parameter-Free Modeling Approach

The interaction between plasmonic resonances, sharp modes, and light in nanoscale plasmonic systems often leads to Fano interference effects. This occurs because the plasmonic excitations are usually spectrally broad and the characteristic narrow asymmetric Fano line-shape results upon interaction with spectrally sharper modes. By considering the plasmonic resonance in the Fano model, as opposed to previous flat continuum approaches, here we show that a simple and exact expression for the line-shape can be found. This allows the role of the width and energy of the plasmonic resonance to be properly understood. As examples, we show how Fano resonances measured on an array of gold nanoantennas covered with PMMA, as well as the hybridization of dark with bright plasmons in nanocavities, are well reproduced with a simple exact formula and without any fitting parameters.

Plasmonic Systems Unveiled by Fano Resonances

We show in detail how a derivation of Fano theory can serve as a new paradigm to study, understand, and control the interaction of nano-objects with light. Examples include a plasmonic crystal, a dolmen-type structure sustaining dark and bright plasmon modes, and a nanoshell heptamer. On the basis of only three coupling factors, a straightforward analytical formula is obtained, only assuming a plasmonic resonance for the continuum, and retaining the nonclassical character of the original formalism. It allows one to predict, reproduce, or decompose Fano interferences solely in terms of the physical properties of the uncoupled nanostructures when available, without the need of additional fitting parameters.

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(res)3/V(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.

Three‐Dimensionally Isotropic Negative Refractive Index Materials from Block Copolymer Self‐Assembled Chiral Gyroid Networks

In 1999, Pendry et al. predicted that specifically engineered artificial materials, that is, metamaterials, would have unusual magnetic responses, for example, negative permeability. Following this work, much effort has been devoted to the design and fabrication of metamaterials with negative refractive index. 3] Such negative index metamaterials have the potential, for example, in the form of superlenses, to overcome the diffraction limit in imaging or to enable novel optical effects, including cloaking. Today most metamaterial fabrication relies on top-down approaches such as lithography techniques, making efficient access to three-dimensionally (3D) isotropic metamaterials challenging thus hindering their practical applications. Recent progress in bottom-up type self-assembly offers promise to overcome some of these limitations. In particular block copolymer (BCP) selfassembly has emerged as a useful designer tool to create nanostructures including 3D continuous morphologies of disparate materials like ceramics and metals. The present paper makes clear theoretical predictions for how to design 3D isotropic materials with negative refraction and circularly polarized light propagation from a class of block copolymer based self-assembled materials not yet rigorously studied in the context of metamaterials. Through theoretical understanding and guidance on materials choices, characteristic length and frequency scales, which are determined by calculations and described in detail here, a “recipe” is provided for the synthesis, fabrication and characterization of these materials. We present calculations of the photonic properties of 3D periodic metallic nanomaterials with co-continuous cubic morphologies as illustrated in Figure 1. Such structures are experimentally accessible through self-assembly of AB diblock copolymers and ABC triblock terpolymers and are referred to as double gyroid (D-GYR) and alternating gyroid (A-GYR). Both of these structures have two 3D continuous cubic and interwoven minority networks separated by a matrix majority network. In the A-GYR the two minority networks are distinguishable leading to chirality while in the D-GYR they are not. We predict for the resulting metallic nanomaterials that the coupled surface plasmon resonance of the two minority networks of the D-GYR induces low frequency light propagation with a negative index of refraction. Due to their cubic symmetry, these materials are 3D isotropic (see Figure 1e). They also show circularly polarized light propagation originating from the chirality of the gyroid networks. We further predict that by tailoring BCP synthesis one can design materials with varying refractive index and frequency at which negative refraction occurs. Finally, in contrast to D-GYR metallic nanomaterials, chiral A-GYR metallic nanomaterials are expected to exhibit a surprising metallic band gap despite their smaller metallic fraction. We Figure 1. Schematic routes to 3-dimensionally co-continuous metamaterials with cubic symmetry and expected optical behavior. a) D-GYR; b) hollow D-GYR; and c) A-GYR metamaterials. For clarity of presentation, specific blocks are represented to be transparent. d) D-GYR metamaterial formed from many unit cells. The two chiral gyroid struts are depicted in different color for clarity. e) Projected images of a DGYR metamaterial unit cell with unit cell length a onto three orthogonal axes. Two struts are cut in different planes for showing full loops. Surface plasmon polaritons f) oscillate on the closed loop of gyroid networks and g) on a 1-dimensional metal/insulator/metal wave-guide.

Strongly confined gap plasmon modes in graphene sandwiches and graphene-on-silicon

We explore the existence of tightly confined gap modes in structures consisting of two infinitely long graphene ribbons vertically offset by a gap. By investigating carefully such a sandwich geometry we find that the gap modes originate from a strong hybridization that gives rise to improved waveguide performance while modifying the guiding behaviour compared to a single ribbon. Our work particularly focuses on the physical origin and description of these plasmon modes, studying the critical parameters of width, gap and operation wavelength. This allows different regimes, coupling mechanisms and mode families to be recognized. Importantly we show that the gap modes also exist when a single graphene sheet is placed on top of a metal or a doped semiconductor—a geometry that is readily achievable experimentally. As an example we report on an unprecedented level of confinement of a terahertz wave of nearly five orders of magnitude when a graphene ribbon is placed on top of a highly doped silicon substrate. Because of their remarkable field distributions and extreme confinement, the families of modes presented here could be the building blocks for both graphene-based integrated optics and ultrasensitive sensing modalities.

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.

Beyond the hybridization effects in plasmonic nanoclusters: diffraction-induced enhanced absorption and scattering.

It is demonstrated herein both theoretically and experimentally that Youngs interference can be observed in plasmonic structures when two or three nanoparticles with separation on the order of the wavelength are illuminated simultaneously by a plane wave. This effect leads to the formation of intermediate-field hybridized modes with a character distinct of those mediated by near-field and/or far-field radiative effects. The physical mechanism for the enhancement of absorption and scattering of light due to plasmonic Youngs interference is revealed, which we explain through a redistribution of the Poynting vector field and the formation of near-field subwavelength optical vortices.

Broadband molecular sensing with a tapered spoof plasmon waveguide

Unambiguous identification of low concentration chemical mixtures can be performed by broadband enhanced infrared absorption (BEIRA). Here we propose and numerically study a corrugated parallel plate waveguide (CPPW) with gradient grooves which is capable of directly converting transmission modes to surface plasmon modes and could hence serve as a powerful chemical sensor. Such a waveguide can be designed to exhibit a wide pass band covering an extended portion of a molecule absorption spectrum. Broadband sensing of toluene and ethanol thin layers is demonstrated by calculating the transmission coefficient of the waveguide and is shown to correspond exactly to their infrared spectra. In addition, the upper limit and the lower limit of the bandgap are mainly dependent on the minimum and maximum groove height, respectively, which provide an effective way of tuning the working frequency of the device in order to support surface plasmon modes within a desired frequency range according to a specific application.

Sub-diffractional, volume-confined polaritons in a natural hyperbolic material: hexagonal boron nitride (Presentation Recording)

Strongly anisotropic media where principal components of the dielectric tensor have opposite signs are called hyperbolic. These materials permit highly directional, volume-confined propagation of slow-light modes at deeply sub-diffractional size scales, leading to unique nanophotonic phenomena. The realization of hyperbolic materials within the optical spectral range has been achieved primarily through the use of artificial structures typically composed of plasmonic metals and dielectric constituents. However, while proof-of-principle experiments have been performed, the high plasmonic losses and inhomogeneity of the structures limit most advances to the laboratory. Recently, hexagonal boron nitride (hBN) was identified as a natural hyperbolic material (NHM), offering a low-loss, homogeneous medium that can operate in the mid-infrared. We have exploited the NHM response of hBN within periodic arrays of conical nanoresonators to demonstrate ‘hyperbolic polaritons,’ deeply sub-diffractional guided waves that propagate through the volume rather than on the surface of a hyperbolic material. We have identified that the polaritons are manifested as a four series of resonances in two distinct spectral bands that have mutually exclusive dependencies upon incident light polarization, modal order, and aspect ratio. These observations represent the first foray into creating NHM building blocks for mid-infrared to terahertz nanophotonic and metamaterial devices. This talk will also discuss potential near-term applications stemming from these developments.