Plasmonic analogue of electromagnetically induced transparency at the Drude damping limit

Fanoshells: nanoparticles with built-in Fano resonances

A nanoparticle consisting of a dielectric (SiO(2)) and metallic (Au) shell layer surrounding a solid Au nanoparticle core can be designed with its superradiant and subradiant plasmon modes overlapping in energy, resulting in a Fano resonance in its optical response. Synthesis of this nanoparticle around an asymmetric core yields a structure that possesses additional Fano resonances as revealed by single particle dark field microspectroscopy. A mass-and-spring coupled oscillator model provides an excellent description of the plasmon interactions and resultant optical response of this nanoparticle.

Transition from isolated to collective modes in plasmonic oligomers

We demonstrate the transition from isolated to collective optical modes in plasmonic oligomers. Specifically, we investigate the resonant behavior of planar plasmonic hexamers and heptamers with gradually decreasing the interparticle gap separation. A pronounced Fano resonance is observed in the plasmonic heptamer for separations smaller than 60 nm. The spectral characteristics change drastically upon removal of the central nanoparticle. Our work paves the road toward complex hierarchical plasmonic oligomers with tailored optical properties.

Infrared perfect absorber and its application as plasmonic sensor

We experimentally demonstrate a perfect plasmonic absorber at lambda = 1.6 microm. Its polarization-independent absorbance is 99% at normal incidence and remains very high over a wide angular range of incidence around +/-80 degrees. We introduce a novel concept to utilize this perfect absorber as plasmonic sensor for refractive index sensing. This sensing strategy offers great potential to maintain the performance of localized surface plasmon sensors even in nonlaboratory environments due to its simple and robust measurement scheme.

Simple model for the hybridization of surface plasmon resonances in metallic nanoparticles

An "electrostatic" eigenmode method based on the coupling of evanescent electric fields is presented for modeling the hybridization of localized surface plasmon resonances in metallic nanoparticles of arbitrary shape. The method yields simple analytical expressions for the hybridized energies and excitation amplitudes of nanoparticle ensembles. Because of its ease of applicability and simple conceptual basis, we anticipate that the method will be of value in understanding and predicting the effects of interacting plasmonic nanoparticles.

Understanding the electric and magnetic response of isolated metaatoms by means of a multipolar field decomposition

We introduce a technique to decompose the scattered near field of two-dimensional arbitrary metaatoms into its multipole contributions. To this end we expand the scattered field upon plane wave illumination into cylindrical harmonics as known from Mie's theory. By relating these cylindrical harmonics to the field radiated by Cartesian multipoles, the contribution of the lowest order electric and magnetic multipoles can be identified. Revealing these multipoles is essential for the design of metamaterials because they largely determine the character of light propagation. In particular, having this information at hand it is straightforward to distinguish between effects that result either from the arrangement of the metaatoms or from their particular design.

Symmetry breaking and strong coupling in planar optical metamaterials

We demonstrate narrow transmission resonances at near-infrared wavelengths utilizing coupled asymmetric split-ring resonators (SRRs). By breaking the symmetry of the coupled SRR system, one can excite dark (subradiant) resonant modes that are not readily accessible to symmetric SRR structures. We also show that the quality factor of metamaterial resonant elements can be controlled by tailoring the degree of asymmetry. Changing the distance between asymmetric resonators changes the coupling strength and results in resonant frequency tuning due to resonance hybridization.

Plasmonic electromagnetically-induced transparency in symmetric structures

A broken symmetry is generally believed to be a prerequisite for plasmonic electromagnetically-induced transparency (EIT), since the asymmetry allows the excitation of the otherwise forbidden dark mode. Nevertheless, according to the picture of magnetic plasmon resonance (MPR)-mediated plasmonic EIT, we show that plasmonic EIT can be achieved even in symmetric structures based on the second-order MPR. This not only sharpens our understanding of the existing concept, but also provides a profound insight into the plasmonic coherent interference in the near-field zone.

Phase-coupled plasmon-induced transparency

We demonstrate the existence of electromagnetically-induced-transparency (EIT-)like spectral response in a system of nanoscale plasmonic resonator antennas coupled by means of a single-mode silicon waveguide. Our proposed scheme exploits the phase of the coupling between the antennas in contrast with the existing plasmonic approaches that rely on the strength of direct, near-field coupling of nanometallic elements. Quality factors of over 100 and group indices of over 10 are readily achieved at near-infrared frequencies by a single unit in ≈1  μm2 of total device footprint, representing a more than two orders size reduction over corresponding dielectric EIT structures. By obviating the need for a near-field interaction, the phase-coupling scheme also facilitates an improved access to the coupling medium between the resonators thereby paving the way toward dynamic control of their sharp EIT-like spectral response.

Mode imaging and selection in strongly coupled nanoantennas

The number of eigenmodes in plasmonic nanostructures increases with complexity due to mode hybridization, raising the need for efficient mode characterization and selection. Here we experimentally demonstrate direct imaging and selective excitation of the "bonding" and "antibonding" plasmon mode in symmetric dipole nanoantennas using confocal two-photon photoluminescence mapping. Excitation of a high-quality-factor antibonding resonance manifests itself as a two-lobed pattern instead of the single spot observed for the broad "bonding" resonance in accordance with numerical simulations. The two-lobed pattern is observed due to the fact that excitation of the antibonding mode is forbidden for symmetric excitation at the feedgap, while concomitantly the mode energy splitting is large enough to suppress excitation of the "bonding" mode. The controlled excitation of modes in strongly coupled plasmonic nanostructures is mandatory for efficient sensors, in coherent control as well as for implementing well-defined functionalities in complex plasmonic devices.

Polarization angle control of coherent coupling in metamaterial superlattice for closed mode excitation

A superlattice structure of planar metamaterial is fabricated, where the orientation of double-split ring resonators is altered in a periodic way. A time-domain terahertz transmission spectrum shows an enhanced Q-factor resonance appears when a closed mode is selectively excited by angular tuning of polarization direction. The polarization-angle selective resonance in metamaterial superlattice has a potential application in the selective field enhancement for spectroscopy.

Light scattering by an array of electric and magnetic nanoparticles

Light scattering by an array of alternating electric and magnetic nanoparticles is analyzed in detailed. Specific geometrical conditions are derived, where such an array behaves like double-negative particles, leading to a suppression of the backscattered intensity. This effect is very robust and could be used to produce double-negative metamaterials using single-negative components.

Temperature control of Fano resonances and transmission in superconducting metamaterials

Losses are the main evil that limits the use of metamaterials in practical applications. While radiation losses may be controlled by design, Joule losses are hereditary to the metamaterial structures. An exception is superconducting metamaterials, where Joule losses can be uniquely controlled with temperature in a very wide range. We put this in use by demonstrating temperature-dependent transmission in the millimeter-wave part of the spectrum in high-Tc superconducting cuprate metamaterials supporting sub-radiant resonances of Fano type.

Planar metamaterial analogue of electromagnetically induced transparency for plasmonic sensing

We experimentally demonstrate a planar metamaterial analogue of electromagnetically induced transparency at optical frequencies. The structure consists of an optically bright dipole antenna and an optically dark quadrupole antenna, which are cut-out structures in a thin gold film. A pronounced coupling-induced reflectance peak is observed within a broad resonance spectrum. A metamaterial sensor based on these coupling effects is experimentally demonstrated and yields a sensitivity of 588 nm/RIU and a figure of merit of 3.8.

Resonant mode coupling of optical resonances in stacked nanostructures

We propose a method to obtain the resonance frequencies of coupled optical modes for a stack of two periodically corrugated slabs. The method is based on the modes in each slab, which are derived by the Fourier modal method in combination with the optical scattering matrix theory. We then use the resonant mode approximation of the scattering matrices to develop a linear eigenvalue problem with dimensions equal to the number of resonant modes. Its solutions are the resonance frequencies of the coupled system and exhibit a good agreement with exact solutions. We demonstrate the capabilities of this method for pairs of planar waveguides and gratings of one-dimensional wires.

Experimental realization of subradiant, superradiant, and fano resonances in ring/disk plasmonic nanocavities

Subradiant and superradiant plasmon modes in concentric ring/disk nanocavities are experimentally observed. The subradiance is obtained through an overall reduction of the total dipole moment of the hybridized mode due to antisymmetric coupling of the dipole moments of the parent plasmons. Multiple Fano resonances appear within the superradiant continuum when structural symmetry is broken via a nanometric displacement of the disk, due to coupling with higher order ring modes. Both subradiant modes and Fano resonances exhibit substantial reductions in line width compared to the parent plasmon resonances, opening up possibilities in optical and near IR sensing via plasmon line shape design.

Symmetry breaking in gold-silica-gold multilayer nanoshells

We present a computational study of the plasmonic properties of gold-silica-gold multilayer nanoshells with the core offset from the center. Symmetry breaking, due to the core offset, makes plasmon resonances that are dark in concentric geometries visible. Applying plasmon hybridization theory, we explain the origin of these resonances from the interactions of an admixture of both primitive and multipolar modes between the core and the shell. The interactions introduce a dipole moment into the higher order modes and significantly enhance their coupling efficiency to light. To elucidate the symmetry breaking effect, we link the geometrical asymmetry to the asymmetrical distribution of surface charges and demonstrate illustratively the diminishing multipolar characteristic and increasing dipolar characteristic of the higher order modes. The relative amplitudes of the modes are qualitatively related by visual examination of the dipolar component in the surface charge distributions. Using polarization-dependent surface charge plots, we illustrate two distinct mode configurations despite their spectral similarities. We further demonstrate a trend of increasing absorption relative to scattering as the resonant wavelength red shifts in response to a larger core offset.

Heterodimers: plasmonic properties of mismatched nanoparticle pairs

Heterodimers-two closely adjacent metallic nanoparticles differing in size or shape-exemplify a simple nanoscale geometry that gives rise to a remarkably rich set of properties. These include Fano resonances, avoided crossing behavior, and a surprising dependence of the scattering spectrum on the direction of excitation, known as the "optical nanodiode" effect. In a series of studies, we experimentally probe and theoretically analyze these properties in heterodimer nanostructures, where nanoparticle size and plasmon resonance frequency are varied systematically. Polarization-dependent dark-field microspectroscopy on individual heterodimer structures fabricated using a novel electromigration assembly method allows us to examine these properties in detail. These studies expand our understanding of the range of physical effects that can be observed in adjacent metallic nanoparticle pairs.

Frequency-domain simulations of a negative-index material with embedded gain

We solve the equations governing light propagation in a negative-index material with embedded nonlinearly saturable gain material using a frequency-domain model. We show that available gain materials can lead to complete loss compensation only if they are located in the regions where the field enhancement is maximal. We study the increased enhancement of the fields in the gain composite as well as in the metal inclusions and show analytically that the effective gain is determined by the average near-field enhancement.

Effect of retardation on localized surface plasmon resonances in a metallic nanorod

The localized surface plasmon resonances in a metallic nanorod are determined using the "electrostatic approximation" and by a finite-difference time-domain numerical solution of Maxwell's equations. The difference between the two methods is related to the effects of re-radiation, or retardation, which is not included in the electrostatic formulation. It is shown that high-order modes in a metallic nanorod can be modeled by both methods, even beyond the point where the electrostatic method is supposed to fail. This suggests that the simple analytical expressions derived from the electrostatic approximation are valid for describing the large range of resonant modes associated with metallic nanoparticles, including dark modes.

Lattice modes mediate radiative coupling in metamaterial arrays

We show that a resonant response with very high quality factors can be achieved in periodic metamaterials by radiatively coupling their structural elements. The coupling is mediated by lattice modes and can be efficiently controlled by tuning the lattice periodicity. Using a recently developed terahertz (THz) near-field imaging technique and conventional far-field spectroscopy together with numerical simulations we pinpoint the underlying mechanisms. In the strong coupling regimes we identify avoided crossings between the plasmonic eigenmodes and the diffractive lattice modes.

Highly selective terahertz bandpass filters based on trapped mode excitation

We present two types of metamaterial-based spectral bandpass filters for the terahertz (THz) frequency range. The metamaterials are specifically designed to operate for waves at normal incidence and to be independent of the field polarization. The functional structures are embedded in films of benzocyclobutene (BCB) resulting in large-area, free-standing and flexible membranes with low intrinsic loss. The proposed filters are investigated by THz time-domain spectroscopy and show a pronounced transmission peak with over 80% amplitude transmission in the passband and a transmission rejection down to the noise level in the stopbands. The measurements are supported by numerical simulations which evidence that the high transmission response is related to the excitation of trapped modes.