Symmetry breaking in plasmonic nanocavities: subradiant LSPR sensing and a tunable Fano resonance

Enhancement of photothermal heat generation by metallodielectric nanoplasmonic clusters

A four-member homogenous quadrumer composed of silver core-shell nanostructures is tailored to enhance photothermal heat generation efficiency in sub-nanosecond time scale. Calculating the plasmonic and photothermal responses of metallic cluster, we show that it is possible to achieve thermal heat flux generation of 64.7 μW.cm^-2 and temperature changes in the range of ΔT = 150 K, using Fano resonant effect. Photothermal heat generation efficiency is even further enhanced by adding carbon nanospheres to the offset gap between particles and obtained thermal heat flux generation of 93.3 μW.cm^-2 and temperature increase of ΔT = 172 K. It is also shown that placement of dielectric spheres gives rise to arise collective magnetic dark plasmon modes that improves the quality of the observed Fano resonances. The presented data attests the superior performance of the proposed metallodielectric structures to utilize in practical tumor and cancer therapies and drug delivery applications.

Simple analytical expression for the peak-frequency shifts of plasmonic resonances for sensing

We derive a closed-form expression that accurately predicts the peak frequency shift and broadening induced by tiny perturbations of plasmonic nanoresonators without critically relying on repeated electrodynamic simulations of the spectral response of nanoresonator for various locations, sizes, or shapes of the perturbing objects. In comparison with other approaches of the same kind, the force of the present approach is that the derivation is supported by a mathematical formalism based on a rigorous normalization of the resonance modes of nanoresonators consisting of lossy and dispersive materials. Accordingly, accurate predictions are obtained for a large range of nanoparticle shapes and sizes used in various plasmonic nanosensors even beyond the quasistatic limit. The expression gives quantitative insight and, combined with an open-source code, provides accurate and fast predictions that are ideally suited for preliminary designs or for interpretation of experimental data. It is also valid for photonic resonators with large mode volumes.

Fano-like resonances in split concentric nanoshell dimers in designing negative-index metamaterials for biological-chemical sensing and spectroscopic purposes

In this study, we investigated numerically the plasmon response of a dimer configuration composed of a couple of split and concentric Au nanoshells in a complex orientation. We showed that an isolated composition of two concentric split nanoshells could be tailored to support strong plasmon resonant modes in the visible wavelengths. After determining the accurate geometric dimensions for the presented antisymmetric nanostructure, we designed a dimer array that shows complex behavior during exposure to different incident polarizations. We verified that the examined dimer was able to support destructive interference between dark and bright plasmon modes, which resulted in a pronounced Fano-like dip. Observation of a Fano minimum in such a simple molecular orientation of subwavelength particles opens new avenues for employing this structure in designing various practical plasmonic devices. Depositing the final dimer in a strong coupling condition on a semiconductor metasurface and measuring the effective refractive index at certain wavelengths, we demonstrate that each one of dimer units can be considered a meta-atom due to the high aspect ratio in the geometric parameters. Using this method, by extending the number of dimers periodically and illuminating the structure, we examined the isotropic, polarization-dependent, and transmission behavior of the metamaterial configuration. Using numerical methods and calculating the effective refractive indices, we computed and sketched corresponding figure of merit over the transmission window, where the maximum value obtained was 42.3 for Si and 54.6 for gallium phosphide (GaP) substrates.

Tunable deep-subwavelength superscattering using graphene monolayers

In this Letter, we theoretically propose for the first time that graphene monolayers can be used for superscatterer designs. We show that the scattering cross-section of the bare deep-subwavelength dielectric cylinder is markedly enhanced by six orders of magnitude due to the excitation of the first-order resonance of graphene plamons. By utilizing the tunability of the plasmonic resonance through tuning graphene's chemical potential, the graphene superscatterer works in a wide range of frequencies from several terahertz to tens of terahertz.

Plasmonic eigenmodes in individual and bow-tie graphene nanotriangles

In classical electrodynamics, nanostructured graphene is commonly modeled by the computationally demanding problem of a three-dimensional conducting film of atomic-scale thickness. Here, we propose an efficient alternative two-dimensional electrostatic approach where all calculation procedures are restricted to the graphene sheet. Furthermore, to explore possible quantum effects, we perform tight-binding calculations, adopting a random-phase approximation. We investigate multiple plasmon modes in 20 nm equilateral triangles of graphene, treating the optical response classically as well as quantum mechanically. Compared to the classical plasmonic spectrum which is "blind" to the edge termination, we find that the quantum plasmon frequencies exhibit blueshifts in the case of armchair edge termination of the underlying atomic lattice, while redshifts are found for zigzag edges. Furthermore, we find spectral features in the zigzag case which are associated with electronic edge states not present for armchair termination. Merging pairs of triangles into dimers, plasmon hybridization leads to energy splitting that appears strongest in classical calculations while splitting is lower for armchair edges and even more reduced for zigzag edges. Our various results illustrate a surprising phenomenon: Even 20 nm large graphene structures clearly exhibit quantum plasmonic features due to atomic-scale details in the edge termination.

Directional Fano resonance in a silicon nanosphere dimer

Fano resonance arising from the interaction between a broad "bright" mode and a narrow "dark" mode has been widely investigated in symmetry-breaking structures made of noble metals such as plasmonic asymmetric oligomers or other well-designed nanostructures. However, Fano resonance in nanoscale all-dielectric dimers has not been experimentally demonstrated so far. We report the first experimental observation of directional Fano resonance in silicon nanosphere dimers (both homodimer and heterodimer) and clarify that the coupling between magnetic and electric dipole modes can easily generate Fano resonance in all-dielectric oligomers, distinctly differing from conventional Fano resonances based on electric responses or artificial optical magnetism. A silicon nanosphere dimer, exhibiting a strong magnetic response inside and an electric enhancement in the gap, is an excellent structure to support magnetic-based Fano scattering. Interactions between magnetic and electric dipoles can suppress backward scattering and enhance forward scattering at Fano wavelengths. This directional scattering is much more prominent than that from a single silicon sphere and shows promising applications in areas such as directional nanoantenna or optical switching, opening up avenues for developing all-dielectric low-loss metamaterials or nanophotonic devices at visible wavelengths.

STEM-EELS analysis of multipole surface plasmon modes in symmetry-broken AuAg nanowire dimers

Surface plasmon coupling in nanowires separated by small gaps generates high field enhancements at the position of the gap and is thus of great interest for sensing applications. It is known that the nanowire dimensions and in particular the symmetry of the structures has strong influence on the plasmonic properties of the dimer structure. Here, we report on multipole surface plasmon coupling in symmetry-broken AuAg nanowire dimers. Our dimers, consisting of two nanowires with different lengths and separated by gaps of only 10 to 30 nm, were synthesized by pulsed electrochemical deposition in ion track-etched polymer templates. Electron energy-loss spectroscopy in scanning transmission electron microscopy allows us to resolve up to nine multipole order surface plasmon modes of these dimers spectrally separated from each other. The spectra evidence plasmon coupling between resonances of different multipole order, resulting in the generation of additional plasmonic modes. Since such complex structures require elaborated synthesis techniques, dimer structures with complex composition, morphology and shape are created. We demonstrate that finite element simulations on pure Au dimers can predict the generated resonances in the fabricated structures. The excellent agreement of our experiment on AuAg dimers with finite integration simulations using CST microwave studio manifests great potential to design complex structures for sensing applications.

Nonlinear chiro-optical amplification by plasmonic nanolens arrays formed via directed assembly of gold nanoparticles

Metal nanoparticle assemblies are promising materials for nanophotonic applications due to novel linear and nonlinear optical properties arising from their plasmon modes. However, scalable fabrication approaches that provide both precision nano- and macroarchitectures, and performance commensurate with design and model predictions, have been limiting. Herein, we demonstrate controlled and efficient nanofocusing of the fundamental and second harmonic frequencies of incident linearly and circularly polarized light using reduced symmetry gold nanoparticle dimers formed by surface-directed assembly of colloidal nanoparticles. Large ordered arrays (>100) of these C∞v heterodimers (ratio of radii R1/R2 = 150 nm/50 nm = 3; gap distance l = 1 ± 0.5 nm) exhibit second harmonic generation and structure-dependent chiro-optic activity with the circular dichroism ratio of individual heterodimers varying less than 20% across the array, demonstrating precision and uniformity at a large scale. These nonlinear optical properties were mediated by interparticle plasmon coupling. Additionally, the versatility of the fabrication is demonstrated on a variety of substrates including flexible polymers. Numerical simulations guide architecture design as well as validating the experimental results, thus confirming the ability to optimize second harmonic yield and induce chiro-optical responses for compact sensors, optical modulators, and tunable light sources by rational design and fabrication of the nanostructures.

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.

Fano resonance in all-dielectric binary nanodisk array realizing optical filter with efficient linewidth tuning

In this research paper, we study the Fano resonance originating from the interaction of in-phased lattice collective resonance and anti-phased lattice collective resonance supported by a binary silicon nanodisk array. Experimental results agree well with the calculations using finite-difference-time-domain method and show a strong dependence of such Fano lineshapes on the radius difference of the particles in the array. Further calculations demonstrate that such binary silicon nanodisk array can be used as an optical filter and offers an efficient way to tune the linewidth simply by changing the radius of the particles, linewidth from 12 nm to 0.7 nm and corresponding Q factor from 72 to 1290 as the radius R(2) increasing from 60 nm to 115 nm. Such scheme possessing the merits of being easily fabricated, simulated, and tuned is very promising for practical applications.

Ultrasensitive biosensors using enhanced Fano resonances in capped gold nanoslit arrays

Nanostructure-based sensors are capable of sensitive and label-free detection for biomedical applications. However, plasmonic sensors capable of highly sensitive detection with high-throughput and low-cost fabrication techniques are desirable. We show that capped gold nanoslit arrays made by thermal-embossing nanoimprint method on a polymer film can produce extremely sharp asymmetric resonances for a transverse magnetic-polarized wave. An ultrasmall linewidth is formed due to the enhanced Fano coupling between the cavity resonance mode in nanoslits and surface plasmon resonance mode on periodic metallic surface. With an optimal slit length and width, the full width at half-maximum bandwidth of the Fano mode is only 3.68 nm. The wavelength sensitivity is 926 nm/RIU for 60-nm-width and 1,000-nm-period nanoslits. The figure of merit is up to 252. The obtained value is higher than the theoretically estimated upper limits of the prism-coupling SPR sensors and the previously reported record high figure-of-merit in array sensors. In addition, the structure has an ultrahigh intensity sensitivity up to 48,117%/RIU.

Templated fabrication of periodic arrays of metallic and silicon nanorings with complex nanostructures

Here we report a scalable colloidal templating approach for fabricating periodic arrays of metallic and silicon nanorings with complex nanostructures. Non-close-packed monolayer silica colloidal crystal prepared by a simple spin-coating technology is first used as template for making periodic arrays of mushroom-like composite nanostructures consisting of silica spherical caps and polymer stems. Subsequent metal sputtering and reactive ion etching lead to the formation of ordered asymmetric nickel nanorings which can be further utilized as etching masks for patterning periodic arrays of symmetric silicon nanorings. Moreover, periodic arrays of metallic and silicon concentric double nanorings can be fabricated by using the asymmetric nickel nanorings as templates. We have also demonstrated that gold concentric double nanorings show strong surface-enhanced Raman scattering (SERS) with a SERS enhancement factor of ∼9.5 × 10(7) from adsorbed benzenethiol molecules. The SERS enhancement and the electric field amplitude distribution surrounding gold concentric double nanorings have been calculated by using finite element electromagnetic modeling. This new colloidal templating technique is compatible with standard microfabrication and enables wafer-scale production of a variety of periodic nanorings with hierarchical structures that could find important technological applications in plasmonic and magnetic devices.

Tailoring the negative-refractive-index metamaterials composed of semiconductor-metal-semiconductor gold ring/disk cavity heptamers to support strong Fano resonances in the visible spectrum

In this study, we investigated numerically the plasmon response of a planar negative-index metamaterial composed of symmetric molecular orientations of Au ring/disk nanocavities in a heptamer cluster. Using the plasmon hybridization theory and considering the optical response of an individual nanocluster, we determined the accurate geometrical sizes for a ring/disk nanocavity heptamer. It is shown that the proposed well-organized nanocluster can be tailored to support strong and sharp Fano resonances in the visible spectrum. Surrounding and filling the heptamer clusters by various metasurfaces with different chemical characteristics, and illuminating the structure with an incident light source, we proved that this configuration reflects low losses and isotropic features, including a pronounced Fano dip in the visible spectrum. Technically, employing numerical methods and tuning the geometrical sizes of the structure, we tuned and induced the Fano dip in the visible range, while the dark and bright plasmon resonance extremes are blueshifted to shorter wavelengths dramatically. Considering the calculated transmission window, we quantified the effective refractive index for the structure, while the substance of the substrate material was varied. Using Si, GaP, and InP semiconductors as substrate materials, we calculated and compared the corresponding figure of merit (FOM) for different regimes. The highest possible FOM was obtained for the GaP-Au-GaP negative-refractive-index metamaterial composed of ring/disk nanocavity heptamers as 62.4 at λ∼690  nm (arounnd the position of the Fano dip). Despite the outstanding symmetric nature of the suggested heptamer array, we provided sharp Fano dips by the appropriate tuning of the geometrical and chemical parameters. This study yields a method to employ ring/disk nanocavity heptamers as a negative-refractive-index metamaterial in designing highly accurate localization of surface plasmon resonance sensing devices and biochemical sensors.

Nanosphere-in-a-nanoegg: damping the high-order modes induced by symmetry breaking

We study the optical properties of the nanosphere-in-a-nanoegg structure (NSNE) by the three-dimensional finite difference time domain method. We demonstrate the suppression of the high-order plasmon modes in NSNE, which is induced by the plasmon interaction between the inner nanosphere and the outer nanoegg shell. A two-layer plasmon hybridization model is presented to explain this mechanism. The results we showed for plasmon mode suppression would be important to the design of the metal plasmonic devices. In addition, due to high tunable plasmon resonances in the near-infrared region (700 to 1,300 nm) with sub-100-nm size, NSNE can serve as a good substitute for the Au-silica-Au multilayer nanoshells in biological applications. Furthermore, compared with the Au-silica-Au nanoshells, NSNE has the advantage that the strong field enhancement can be achieved at the outer surface of the Au shell.

Silver nano-dendritic crystal film: a rapid dehydration SERS substrate of totally new concept

This work provides a rapid dehydration SERS substrate with the potential of rapid, convenient and real-time SERS detection for practical application.

Experimental demonstration of sharp Fano resonance within binary gold nanodisk array through lattice coupling effects

Fano resonance originating from the interaction of anti-phased and in-phased lattice collective resonances supported by a binary gold nanodisk array is investigated both numerically and experimentally. As the magnitude of particle radii in the two arrays approaches gradually, the Q factor of such resonance increases while the spectral contrast reduces. In the experiment, a high spectral contrast of 0.7 and a large Q factor of 14 could be achieved at the same time due to the excitation of the pure dipole resonance of each particle resulting from the weak near-field coupling. Optimal calculation gives rise to a higher Q factor of up to 85 and a corresponding spectral contrast of 0.86, which is very promising in refractive sensors. The sensitivity and the figure of merit are 337  nm/RIU and 116, respectively.

Fabrication of Mn2O3 nanorods: an efficient catalyst for selective transformation of alcohols to aldehydes

Self-assembled high surface area Mn₂O₃ nanorods have been fabricated through an effective polymer–surfactant interaction and their outstanding catalytic property for the selective transformation of alcohols to aldehydes has been discovered.

Plasmon resonance hybridization in self-assembled copper nanoparticle clusters: efficient and precise localization of surface plasmon resonance (LSPR) sensing based on Fano resonances

In this work, we have investigated the hybridization of plasmon resonance modes in completely copper (Cu)-based subwavelength nanoparticle clusters from simple symmetric dimers to complex asymmetric self-assembled structures. The quality of apparent bonding and antibonding plasmon resonance modes for all of the clusters has been studied, and we examined the spectral response of each one of the proposed configurations numerically using the finite-difference time domain (FDTD) method. The effect of the geometric sizes of nanoparticles used and substrate refractive index on the cross-sectional profiles of each of the studied structures has been calculated and drawn. We proved that Fano-like resonance can be formed in Cu-based heptamer clusters as in analogous noble metallic particles (e.g., Au and Ag) by determining the coupling strength and interference between sub-radiant and super-radiant resonance modes. Employing certain Cu nanodiscs in designing an octamer structure, we measured the quality of the Fano dip formation along the scattering diagram. Accurate tuning of the geometric sizes for the Cu-based octamer yields an opportunity to observe isotropic, deep, and narrow Fano minima along the scattering profile that are in comparable condition with the response of other plasmonic metallic substances. Immersing investigated final Cu-based octamer in various liquids with different refractive indices, we determined the sensing accuracy of the cluster based on the performance of the Fano dip. Plotting a linear diagram of plasmon energy differences over the refractive index variations as a figure of merit (FoM), which we have quantified as 13.25. With this method, the precision of the completely Cu-based octamer is verified numerically using the FDTD tool. This study paves the way toward the use of Cu as an efficient, low-cost, and complementary metal-oxide semiconductor (CMOS)-compatible plasmonic material with optical properties that are similar to analogous plasmonic substances.

Plasmon-induced hot carrier science and technology

The discovery of the photoelectric effect by Heinrich Hertz in 1887 set the foundation for over 125 years of hot carrier science and technology. In the early 1900s it played a critical role in the development of quantum mechanics, but even today the unique properties of these energetic, hot carriers offer new and exciting opportunities for fundamental research and applications. Measurement of the kinetic energy and momentum of photoejected hot electrons can provide valuable information on the electronic structure of materials. The heat generated by hot carriers can be harvested to drive a wide range of physical and chemical processes. Their kinetic energy can be used to harvest solar energy or create sensitive photodetectors and spectrometers. Photoejected charges can also be used to electrically dope two-dimensional materials. Plasmon excitations in metallic nanostructures can be engineered to enhance and provide valuable control over the emission of hot carriers. This Review discusses recent advances in the understanding and application of plasmon-induced hot carrier generation and highlights some of the exciting new directions for the field.

Prospects for plasmonic hot spots in single molecule SERS towards the chemical imaging of live cells

Single molecule surface enhanced Raman scattering (SM-SERS) is a highly local effect occurring at sharp edges, interparticle junctions and crevices or other geometries with a sharp nanoroughness of plasmonic nanostructures (“hot spots”) for an analyte detection.

Systematic study of antibonding modes in gold nanorod dimers and trimers

Using on-wire lithography to synthesize well-defined nanorod dimers and trimers, we report a systematic study of the plasmon coupling properties of such materials. By comparing the dimer/trimer structures to discrete nanorods of the same overall length, we demonstrate many similarities between antibonding coupled modes in the dimers/trimers and higher-order resonances in the discrete nanorods. These conclusions are validated with a combination of discrete dipole approximation and finite-difference time-domain calculations and lead to the observation of antibonding modes in symmetric structures by measuring their solution-dispersed extinction spectra. Finally, we probe the effects of asymmetry and gap size on the occurrence of these modes and demonstrate that the delocalized nature of the antibonding modes lead to longer-range coupling compared to the stronger bonding modes.

Periodicity-induced symmetry breaking in a Fano lattice: hybridization and tight-binding regimes

We investigate experimentally and theoretically the role of periodicity on the optical response of dolmen plasmonic arrays that exhibit a Fano line shape. Contrary to previous works on single nanostructures, this study deals with the in-plane near-field coupling between adjacent unit cells. By making an analogy to the electronic properties of atoms in the tight-binding model, specific behaviors of photonic states are investigated numerically as a function of the structural asymmetry for different coupling directions. These predictions are verified experimentally with dark-field measurements on nanostructure arrays which exhibit high tunability and fine control of their spectral features as a function of the lattice constants. These effects, originated from symmetry-breaking and selective excitation of the subradiant mode, provide additional degree of freedom for tuning the spectral response and can be used for the sensitive detection of local perturbations. This study provides a general understanding of the near-field interactions in Fano resonant lattices that can be used for the design of plasmonic nanostructures and planar metamaterials.

Boosting Fano resonances in single layered concentric core-shell particles

Efficient excitation of Fano resonances in plasmonic systems usually requires complex nano-structure geometries and some degree of symmetry breaking. However, a single-layered concentric core-shell particle presents inherent Fano profiles in the scattering spectra when sphere and cavity modes spectrally overlap. Weak hybridization and suitable choice of core and shell materials gives rise to strong electric dipolar Fano resonances in these systems and retardation effects can result in resonances of higher multipolar order or of magnetic type. Furthermore, suitable tailoring of illumination conditions leads to an enhancement of the Fano resonance by quenching of unwanted electromagnetic modes. Overall, it is shown that single layered core-shell particles can act as robust Fano resonators.

Fano resonances in THz metamaterials composed of continuous metallic wires and split ring resonators

We demonstrate theoretically and experimentally that Fano resonances can be obtained in terahertz metamaterials that are composed of periodic continuous metallic wires dressed with periodic split ring resonators. An asymmetric Fano lineshape has been found in a narrow frequency range of the transmission curve. By using a transmission line combined with lumped element model, we are able to not only fit the transmission spectra of Fano resonance which is attributed to the coupling and interference between the transmission continuum of continuous metallic wires and the bright resonant mode of split ring resonators, but also reveal the capacitance change of the split ring resonators induced frequency shift of the Fano resonance. Therefore, the proposed theoretical model shows more capabilities than conventional coupled oscillator model in the design of Fano structures. The effective parameters of group refractive index of the Fano structure are retrieved, and a large group index more than 800 is obtained at the Fano resonance, which could be used for slow light devices.

Plasmonic interactions: from molecular plasmonics and Fano resonances to ferroplasmons

Plasmon interactions are a subject of great interest from both the technological as well as the fundamental points of view. In this Perspective, we outline the great variety of physical phenomena that are produced by the interactions of localized surface plasmon resonance with molecular excitons; with other plasmonic nanostructures, particularly the Fano effect; and with nonplasmonic nanoparticles, such as the just-reported interaction with ferromagnetic nanoparticles. The theoretical as well as experimental challenges remaining to be elucidated are discussed.

Lensfree optofluidic plasmonic sensor for real-time and label-free monitoring of molecular binding events over a wide field-of-view

We demonstrate a high-throughput biosensing device that utilizes microfluidics based plasmonic microarrays incorporated with dual-color on-chip imaging toward real-time and label-free monitoring of biomolecular interactions over a wide field-of-view of >20 mm(2). Weighing 40 grams with 8.8 cm in height, this biosensor utilizes an opto-electronic imager chip to record the diffraction patterns of plasmonic nanoapertures embedded within microfluidic channels, enabling real-time analyte exchange. This plasmonic chip is simultaneously illuminated by two different light-emitting-diodes that are spectrally located at the right and left sides of the plasmonic resonance mode, yielding two different diffraction patterns for each nanoaperture array. Refractive index changes of the medium surrounding the near-field of the nanostructures, e.g., due to molecular binding events, induce a frequency shift in the plasmonic modes of the nanoaperture array, causing a signal enhancement in one of the diffraction patterns while suppressing the other. Based on ratiometric analysis of these diffraction images acquired at the detector-array, we demonstrate the proof-of-concept of this biosensor by monitoring in real-time biomolecular interactions of protein A/G with immunoglobulin G (IgG) antibody. For high-throughput on-chip fabrication of these biosensors, we also introduce a deep ultra-violet lithography technique to simultaneously pattern thousands of plasmonic arrays in a cost-effective manner.

Magnetic-based Fano resonance of hybrid silicon-gold nanocavities in the near-infrared region

Direct interference between the orthogonal electric and magnetic modes in a hybrid silicon-gold nanocavity is demonstrated to induce a pronounced asymmetric magnetic-based Fano resonance in the total scattering spectrum at near-infrared frequencies. Differing from the previously reported magnetic-based Fano resonances in metal nanoparticle clusters, the narrow discrete mode provided by the silicon magnetic dipole resonance can be directly excited by external illumination, and greatly enhanced electric and magnetic fields are simultaneously obtained at the Fano dip.

Unveiling the correlation between nanometer-thick molecular monolayer sensitivity and near-field enhancement and localization in coupled plasmonic oligomers

Metal nanoclusters, sometimes called metamolecules or plasmonic oligomers, exhibit interesting optical properties such as Fano resonances and optical chirality. These properties promise a variety of practical applications, particularly in ultrasensitive biochemical sensing. Here we investigate experimentally the sensitivities of plasmonic pentamers and quadrumers to the adsorption of self-assembled nanometer-thick alkanethiol monolayers. The monolayer sensitivity of such oligomers is found to be significantly higher than that of single plasmonic nanoparticles and depends on the nanocluster arrangement, constituent nanoparticle shape, and the plasmon resonance wavelength. Together with full-wave numerical simulation results and the electromagnetic perturbation theory, we unveil a direct correlation between the sensitivity and the near-field intensity enhancement and spatial localization in the plasmonic "hot" spots generated in each nanocluster. Our observation is beyond conventional considerations (such as optimizing nanoparticle geometry or narrowing resonance line width) for improving the sensing performance of metal nanoclusters-based biosensors and opens the possibilities of using plasmonic nanoclusters for single-molecule detection and identification.

Ultranarrow band absorbers based on surface lattice resonances in nanostructured metal surfaces

Nanostructured metals have received a significant amount of attention in recent years due to their exciting plasmonic and photonic properties enabling strong field localization, light concentration, and strong absorption and scattering at their resonance frequencies. Resonant plasmonic and metamaterial absorbers are of particular interest for applications in a wide variety of technologies including photothermal therapy, thermophotovoltaics, heat-assisted magnetic recording, hot-electron collection, and biosensing. However, it is rather challenging to realize ultranarrow absorption bands using plasmonic materials due to large optical losses in metals that decrease the quality factor of optical resonators. Here, we theoretically and experimentally demonstrate an ultranarrow band absorber based on the surface lattice resonances (SLRs) in periodic nanowire and nanoring arrays on optically thick, reflecting metallic films. In experiments, we observed ultranarrow band resonant absorption peaks with a bandwidth of 12 nm and absorption amplitude exceeding 90% at visible frequencies. We demonstrate that the resonance absorption wavelength, amplitude of the absorption peak, and the bandwidth can be controlled by tuning the periodicity and the thickness of nanoring and nanowire arrays. Unlike conventional plasmonic absorbers utilizing common metal–insulator–metal stacks, our narrow band absorber consists solely of metals, facilitating stronger optical interaction between the SLR of periodic nanostructures and the highly reflective film. Moreover, by introducing asymmetry to the nanoring/nanowire hybrid system, we observe the spectral evolution of resonance splitting enabled by strong coupling between two individual SLRs arising from nanoring and nanowire arrays. Designing such all-metallic nanostructure arrays is a promising route for achieving ultranarrow band absorbers which can be used as absorption filters, narrow band thermal emitters in thermophotovoltaics, and plasmonic biosensors.

Broadband cross polarization converter using plasmon hybridizations in a ring/disk cavity

In this paper, we report on the design, fabrication and subsequent investigation of a broad band cross polarization converter based on a C2-symmetric ring/disk cavity. Different plasmon hybridization modes are excited in the ring/disk cavity and enable the polarization manipulations. The designed cross polarization converter can convert the x or y polarized incident wave to its cross polarized wave in the frequency range from 9.65 to 14.16 GHz with a bandwidth of ~38% of the central wavelength and an efficiency higher than 80%. At 9.25 GHz and 14.35 GHz, the x (y) polarized incident wave is converted to a left (right) handed and right (left) handed circularly polarized wave, respectively.

From Fano-like interference to superscattering with a single metallic nanodisk

Superscattering was theoretically proposed to significantly enhance the scattering cross-section of a subwavelength nanostructure, far exceeding its single-resonance limit by employing resonances of multiple plasmonic modes. By numerical simulation, we design a subwavelength nanodisk as a simple candidate to achieve superscattering. Due to the phase retardation, the subradiant mode can be excited and interact with the superradiant mode in both spatial and frequency domains. By changing the height and diameter of the nanodisk, we show high tunability of the mode interaction and evolution of the resulting spectral features from Fano-like resonance to superscattering. A model of two-driven coupled oscillators is proposed to quantitatively analyze the spectral evolution. We find that the evolution is caused by not only alignment of the resonant wavelengths of related plasmonic modes, but also reasonably high loss. We show that superscattering doubles the near-field intensity, potentially enhancing the signal 16 times for SERS and 4 times for SEIRS, and doubles the far-field intensity and decreases the peak linewidth, improving the figure of merit for plasmonic refractometric sensing. Our study provides quantitative physical insight into understanding superscattering and Fano-like resonances in a single nanoparticle.

Nanoring structure, spacing, and local dielectric sensitivity for plasmonic resonances in Fano resonant square lattices

Lattices of plasmonic nanorings with particular geometries exhibit singular, tunable resonance features in the infrared. This work examined effects of nanoring inner radius, wall thickness, and lattice constant on the spectral response of single nanorings and in Fano resonant square lattices, combining use of the discrete and coupled dipole approximations. Increasing nanoring inner radius red-shifted and broadened the localized surface plasmon resonance (LSPR), while wall thickness modulated the LSPR wavelength and decreased absorption relative to scattering. The square lattice constant was tuned to observe diffractively-coupled lattice resonances, which increased resonant extinction 4.3-fold over the single-ring LSPR through Fano resonance. Refractive index sensitivities of 760 and 1075 nm RIU(-1) were computed for the plasmon and lattice resonances of an optimized nanoring lattice. Sensitivity of an optimal nanoring lattice to a local change in dielectric, useful for sensing applications, was 4 to 5 times higher than for isolated nanorings or non-coupling arrays. This was attributable to the Fano line-shape in far-field diffractive coupling with near-field LSPR.

Determination of the absorption and radiative decay rates of dark and bright plasmonic modes

When two degenerate surface plasmon polariton (SPP) modes couple, in addition to the creation of plasmonic band gap, their respective decay rates are modified as well, resulting in the formation of a pair of dark and bright modes. We combine temporal coupled mode theory, finite-difference time-domain simulation, and angle- and polarization-resolved reflectivity spectroscopy to study the absorption and radiative decay rates of this pair in periodic system. One-dimensional metallic groove arrays are served as an example here. We find for arrays with small groove width, when approaching to the coupling of -1 and + 1 SPP modes, while the radiative decay rate of the high energy mode tends to become zero, the absorption rate decreases as well, forming a "cold" dark mode. At the same time, both the absorption and radiative decay rates of the low energy mode increase, yielding a "hot" bright mode. The situation is completely reversed when groove width increases, turning the high energy mode into a "cold" bright mode and vice versa for the low energy mode. We attribute such modifications to the interplay between the real and imaginary parts of the complex coupling constant, which are found to be highly geometry dependent. Further numerical simulations show the hybridized modes exhibits distinctive electric and magnetic field symmetries, giving rise to different surface charge distributions and Poynting vector profiles, which significantly affect the resulting absorption and radiation losses. Finally, we have measured the decay rates and the complex coupling constant of the hybridized modes and the experimental results are consistent with the analytic and numerical results.

Independently tunable double Fano resonances in asymmetric MIM waveguide structure

In this paper, an asymmetric plasmonic structure composed of a MIM (metal-insulator-metal) waveguide and a rectangular cavity is reported, which can support double Fano resonances originating from two different mechanisms. One of Fano resonance originates from the interference between a horizontal and a vertical resonance in the rectangular cavity. And the other is induced by the asymmetry of the plasmonic structure. Just because the double Fano resonances originate from two different mechanisms, each Fano resonance can be well tuned independently by changing the parameters of the rectangular cavity. And during the tuning process, the FOMs (figure of merit) of both the Fano resonances can keep unchanged almost with large values, both larger than 650. Such, the transmission spectra of the plasmonic structure can be well modulated to form transmission window with the position and the full width at half maximum (FWHM) can be tuned freely, which is useful for the applications in sensors, nonlinear and slow-light devices.

Observation of Fano resonances in all-dielectric nanoparticle oligomers

It is well-known that oligomers made of metallic nanoparticles are able to support sharp Fano resonances originating from the interference of two plasmonic resonant modes with different spectral width. While such plasmonic oligomers suffer from high dissipative losses, a new route for achieving Fano resonances in nanoparticle oligomers has opened up after the recent experimental observations of electric and magnetic resonances in low-loss dielectric nanoparticles. Here, light scattering by all-dielectric oligomers composed of silicon nanoparticles is studied experimentally for the first time. Pronounced Fano resonances are observed for a variety of lithographically-fabricated heptamer nanostructures consisting of a central particle of varying size, encircled by six nanoparticles of constant size. Based on a full collective mode analysis, the origin of the observed Fano resonances is revealed as a result of interference of the optically-induced magnetic dipole mode of the central particle with the collective mode of the nanoparticle structure. This allows for effective tuning of the Fano resonance to a desired spectral position by a controlled size variation of the central particle. Such optically-induced magnetic Fano resonances in all-dielectric oligomers offer new opportunities for sensing and nonlinear applications.

Mode Conversion of Propagating Surface Plasmons in Nanophotonic Networks Induced by Structural Symmetry Breaking

Nanophotonic plasmon circuits may play important roles in next-generation information technology as semiconductor-based electronics is approaching the physical limit. The functions of such circuits rely on the rigorous control of plasmon propagation. One important aspect of such control is controlling the conversion of different plasmon modes for designed plasmon routing in complex nanophotonic networks. Here, for the first time, we experimentally prove that the conversion of plasmon modes occurs widely in metallic nanowire waveguides, the basic components of plasmonic circuits, by introducing local structural symmetry breaking. In further simulations for the structure of a nanowire with a particle in its proximity, it is shown that the mode conversions originate from the redistribution of electric field on the wave front which is caused by the scattering of localized modes in the nanogap and on the nanoparticle. This mode conversion effect can be applied to flexibly control the plasmon propagation behavior in plasmonic nanowire networks.

Fluorescence enhancement of molecules inside a gold nanomatryoshka

Metallic nanoparticles exhibiting plasmonic Fano resonances can provide large enhancements of their internal electric near field. Here we show that nanomatryoshkas, nanoparticles consisting of an Au core, an interstitial nanoscale SiO2 layer, and an Au shell layer, can selectively provide either a strong enhancement or a quenching of the spontaneous emission of fluorophores dispersed within their internal dielectric layer. This behavior can be understood by taking into account the near-field enhancement induced by the Fano resonance of the nanomatryoshka, which is responsible for enhanced absorption of the fluorophores incorporated into the nanocomplex. The combination of compact size and enhanced light emission with internal encapsulation of the fluorophores for increased biocompatibility suggests outstanding potential for this type of nanoparticle complex in biomedical applications.

Mode parity-controlled Fano- and Lorentz-like line shapes arising in plasmonic nanorods

We present the experimental observation of spectral lines of distinctly different shapes in the optical extinction cross-section of metallic nanorod antennas under near-normal plane wave illumination. Surface plasmon resonances of odd mode parity present Fano interference in the scattering cross-section, resulting in asymmetric spectral lines. Contrarily, modes with even parity appear as symmetric Lorentzian lines. Finite element simulations are used to verify the experimental results. The emergence of either constructive or destructive mode interference is explained with a semianalytical 1D line current model. This simple model directly explains the mode-parity dependence of the Fano-like interference. Plasmonic nanorods are widely used as half-wave optical dipole antennas. Our findings offer a perspective and theoretical framework for operating these antennas at higher-order modes.

Plasmonic Fano resonances in nanohole quadrumers for ultra-sensitive refractive index sensing

Plasmonic Fano resonances arising from electromagnetic interactions in metallic nanostructures exhibit spectral characteristics analogous to those from the electron waves in oligomer molecules. Though a great deal of research interest has been attracted to study the optical properties and explore the associated applications of metallic nanoparticle oligomers, the plasmonic response of their complementary structures--nanohole clusters--remains largely unexplored. Here we show numerically by a full-wave finite element method that a nanohole quadrumer can sustain two Fano resonances when the incident electric field is oriented along the long-axis of the quadrumer system. The underlying physical mechanisms responsible for the Fano resonance formation are revealed explicitly by spectrally deconstructing the Fano lineshape, spatially decomposing the structure configuration and mapping the electric field profile and charge distribution, which collectively demonstrate a strong mode coupling between either two antiparallel dipolar modes or dipole-quadruple modes in the nanohole quadrumer. We further show that the spectral profile of the Fano resonance including the resonance linewidth and spectral contrast can be engineered flexibly by adjusting the geometrical parameters of the nanohole cluster, including the nanohole diameter, film thickness and interhole distance. With an optimized and realistic geometrical configuration, the nanohole quadrumer system exhibits an overall sensing figure of merit up to 14.25, far surpassing the value reported for conventional nanoparticle oligomers.

Recent advances in plasmonic sensors

Plasmonic sensing has been an important multidisciplinary research field and has been extensively used in detection of trace molecules in chemistry and biology. The sensing techniques are typically based on surface-enhanced spectroscopies and surface plasmon resonances (SPRs). This review article deals with some recent advances in surface-enhanced Raman scattering (SERS) sensors and SPR sensors using either localized surface plasmon resonances (LSPRs) or propagating surface plasmon polaritons (SPPs). The advances discussed herein present some improvements in SERS and SPR sensing, as well as a new type of nanowire-based SPP sensor.

Multipole plasmon resonances in self-assembled metal hollow-nanospheres

Recently, multipole plasmonic mode resonances in metal hollow structures, such as dipole, quadrupole, and octupole modes, have been widely investigated by researchers with the aim for potential applications in bio-sensing, fluorescence, nanolasers or nonlinear nano-photonics. Here, in this work, the multipole plasmon resonances in self-assembled metal hollow-nanospheres (HNSs) are theoretically and experimentally demonstrated and the hot spots originating from the higher order mode plasmonic resonance and interparticle coupling effect are proposed to be used for Raman scattering enhancements. Dipole, quadrupole, octupole and hexadecapole mode plasmonic resonances were clearly resolved in the extinction spectra of these Ag HNS arrays showing good agreement with the theoretical simulation results. Strong regular hot spots were obtained around the surface and in the gaps of the Ag HNSs through the higher order mode plasmonic resonances and corresponding interparticle coupling effect between the HNSs. Maximum local field intensity was accomplished by optimizing the size of as well as the coupling distance between the HNSs and then it was applied to SERS sensing. Raman mapping also demonstrated these self-assembled plasmonic cavity arrays to be a stable and uniform SERS-active substrate.

An analytic approach to modeling the optical response of anisotropic nanoparticle arrays at surfaces and interfaces

Anisotropic nanoparticle (NP) arrays with useful optical properties, such as localized plasmon resonances (LPRs), can be grown by self-assembly on substrates. However, these systems often have significant dispersion in NP dimensions and distribution, which makes a numerical approach to modeling the LPRs very difficult. An improved analytic approach to this problem is discussed in detail and applied successfully to NP arrays from three systems that differ in NP metal, shape and distribution, and in substrate and capping layer. The materials and anisotropic NP structures that will produce LPRs in desired spectral regions can be determined using this approach.

Fast tuning of double Fano resonance using a phase-change metamaterial under low power intensity

In this work, we numerically demonstrate an all-optical tunable Fano resonance in a fishnet metamaterial(MM) based on a metal/phase-change material(PCM)/metal multilayer. We show that the displacement of the elliptical nanoholes from their centers can split the single Fano resonance (FR) into a double FR, exhibiting higher quality factors. The tri-layer fishnet MMs with broken symmetry accomplishes a wide tuning range in the mid-infrared(M-IR) regime by switching between the amorphous and crystalline states of the PCM (Ge₂Sb₂Te₅). A photothermal model is used to study the temporal variation of the temperature of the Ge₂Sb₂Te₅ film to show the potential for switching the phase of Ge₂Sb₂Te₅ by optical heating. Generation of the tunable double FR in this asymmetric structure presents clear advantages as it possesses a fast tuning time of 0.36 ns, a low pump light intensity of 9.6 μW/μm², and a large tunable wavelength range between 2124 nm and 3028 nm. The optically fast tuning of double FRs using phase change metamaterials(PCMMs) may have potential applications in active multiple-wavelength nanodevices in the M-IR region.

High-Q terahertz Fano resonance with extraordinary transmission in concentric ring apertures

We experimentally demonstrate a polarization-independent terahertz Fano resonance with extraordinary transmission when light passes through two concentric subwavelength ring apertures in the metal film. The Fano resonance is enabled by the coupling between a high-Q dark mode and a low-Q bright mode. We find the Q factor of the dark mode ranges from 23 to 40, which is 3~6 times higher than Q of bright mode. We show the Fano resonance can be tuned by varying the geometry and dimension of the structures. We also demonstrate a polarization dependent Fano resonance in a modified structure of concentric ring apertures.

Direct excitation of dark plasmonic resonances under visible light at normal incidence

Dark plasmon resonance modes are optical modes that have small scattering cross-sections and are thus difficult to excite directly by light at normal incidence. In this paper, we propose to excite quadrupolar and higher-order modes with normal incident light (in the visible regime) on a continuous plasmonic metallic surface covering a dielectric pillar array, hence resulting in narrow-band perfect absorption. Different from the general electromagnetic means of inducing dark modes, our dark modes are due to charge densities that are electrically induced by the standing-wave resonance of current on the thin metal sidewall of pillars. This new means of exciting dark modes can significantly improve the excitation efficiency and also provides an easy way to excite strong higher-order modes.