Substrate-mediated charge transfer plasmons in simple and complex nanoparticle clusters

Open Cross-gap Gold Nanocubes with Strong, Large-Area, Symmetric Electromagnetic Field Enhancement for On-Particle Molecular-Fingerprint Raman Bioassays

Plasmonic nanoparticles with an externally open nanogap can localize the electromagnetic (EM) field inside the gap and directly detect the target via the open nanogap with surface-enhanced Raman scattering (SERS). It would be beneficial to design and synthesize the open gap nanoprobes in a high yield for obtaining uniform and quantitative signals from randomly oriented nanoparticles and utilizing these particles for direct SERS analysis. Here, we report a facile strategy to synthesize open cross-gap (X-gap) nanocubes (OXNCs) with size- and EM field-tunable gaps in a high yield. The site-specific growth of Au budding structures at the corners of the AuNC using the principle that the Au deposition rate is faster than the surface diffusion rate of the adatoms allows for a uniform X-gap formation. The average SERS enhancement factor (EF) for the OXNCs with 2.6 nm X-gaps was 1.2 × 109, and the EFs were narrowly distributed within 1 order of magnitude for ∼93% of the measured OXNCs. OXNCs consistently displayed strong EM field enhancement on large particle surfaces for widely varying incident light polarization directions, and this can be attributed to the symmetric X-gap geometry and the availability of these gaps on all 6 faces of a cube. Finally, the OXNC probes with varying X-gap sizes have been utilized in directly detecting biomolecules with varying sizes without Raman dyes. The concept, synthetic method, and biosensing results shown here with OXNCs pave the way for designing, synthesizing, and utilizing plasmonic nanoparticles for selective, quantitative molecular-fingerprint Raman sensing and imaging applications.

Uncovering the Evolution of Low-Energy Plasmons in Nanopatterned Aluminum Plasmonics on Graphene

We report adjusting the charge-transfer-plasmon (CTP) resonances of aluminum (Al) bowties on suspended monolayer graphene via controlled nanofabrication and focused electron-beam irradiation. CTP resonances of bowties with a conductive junction blue-shift with an increase in junction width, whereas their 3λ/2 and λ resonances barely red-shift. These plasmon modes are derived and confirmed by an LC circuit model and electromagnetic simulations performed with boundary-element and frequency-domain methods. A monotonic decay of the CTP lifetime is observed, while the junction width is extended. Instead, the lifetimes of 3λ/2 and λ resonances are nearly independent of junction width. When the junction is shrunk by electron-beam irradiation, all antenna resonances red-shift. Having created an electron-beam-induced sub 5 nm gap in bowties, we monitor the unambiguous transition of a CTP into a bonding-type gap mode, which is highly sensitive to the separation distance. Meanwhile, the 3λ/2 and λ resonances evolve into dipolar bright and dipolar dark modes.

Metasurface-Driven Optically Variable Devices

Optically variable devices (OVDs) are in tremendous demand as optical indicators against the increasing threat of counterfeiting. Conventional OVDs are exposed to the danger of fraudulent replication with advances in printing technology and widespread copying methods of security features. Metasurfaces, two-dimensional arrays of subwavelength structures known as meta-atoms, have been nominated as a candidate for a new generation of OVDs as they exhibit exceptional behaviors that can provide a more robust solution for optical anti-counterfeiting. Unlike conventional OVDs, metasurface-driven OVDs (mOVDs) can contain multiple optical responses in a single device, making them difficult to reverse engineered. Well-known examples of mOVDs include ultrahigh-resolution structural color printing, various types of holography, and polarization encoding. In this review, we discuss the new generation of mOVDs. The fundamentals of plasmonic and dielectric metasurfaces are presented to explain how the optical responses of metasurfaces can be manipulated. Then, examples of monofunctional, tunable, and multifunctional mOVDs are discussed. We follow up with a discussion of the fabrication methods needed to realize these mOVDs, classified into prototyping and manufacturing techniques. Finally, we provide an outlook and classification of mOVDs with respect to their capacity and security level. We believe this newly proposed concept of OVDs may bring about a new era of optical anticounterfeit technology leveraging the novel concepts of nano-optics and nanotechnology.

Functional Charge Transfer Plasmon Metadevices

Reducing the capacitive opening between subwavelength metallic objects down to atomic scales or bridging the gap by a conductive path reveals new plasmonic spectral features, known as charge transfer plasmon (CTP). We review the origin, properties, and trending applications of this modes and show how they can be well-understood by classical electrodynamics and quantum mechanics principles. Particularly important is the excitation mechanisms and practical approaches of such a unique resonance in tailoring high-response and efficient extreme-subwavelength hybrid nanophotonic devices. While the quantum tunneling-induced CTP mode possesses the ability to turn on and off the charge transition by varying the intensity of an external light source, the excited CTP in conductively bridged plasmonic systems suffers from the lack of tunability. To address this, the integration of bulk plasmonic nanostructures with optothermally and optoelectronically controllable components has been introduced as promising techniques for developing multifunctional and high-performance CTP-resonant tools. Ultimate tunable plasmonic devices such as metamodulators and metafilters are thus in prospect.

The Role of Electron Transfer in the Nonlinear Response of Ge2Sb2Te5-Mediated Plasmonic Dimers

Here, we study the possibility of exquisitely selective harmonic generation based on the concept of charge transfer plasmons (CTPs) in bridged nanoparticle assemblies. By choosing plasmonic dimer nanoantenna, as a fundamental member of the nanocluster family, and bridging the capacitive gap space between the proximal nanoparticles with an optothermally controllable substance, we judiciously showed that variations in the generation of third harmonic light in the visible regime can be possible by considering distinct states of the functional bridge. To this end, the conductive connection between the nanoparticles is mediated with Ge2Sb2Te5 (GST) with inherently opposite optical and electrical properties below (dielectric, amorphous state) and above 477 °C (conductive, crystalline state). This helped to actively control the transition of charges across the bridge and thereby control the excitation of CTP resonances and provide a switching feature between dipolar and CTP modes. This versatile approach also allowed for production of the intended harmonic signal at different wavelengths depending on the conductivity of the interparticle nanojunction.

Charge transfer interactions of pyrazine with Ag12 clusters towards precise SERS chemical mechanism

We have synthesized Ag12 nanoclusters (NCs) with mercaptosuccinic acid (H2SMA) as the ligand. This cluster is found to be water-soluble and has satisfactory stability with [Ag12(HSMA)6Na6]2+, as determined by high-resolution mass spectrometry. Interestingly, it is noted that both the H2SMA ligand and Ag12 clusters do not display interference Raman signals, suggesting that this material is a good candidate as a substrate for surface-enhanced Raman spectroscopy (SERS). As a result, we observe enhanced Raman activity of pyrazine molecules adsorbed on Ag12 NCs along with a large red-shift up to ∼27 cm-1. To fully demonstrate the charge transfer interactions between pyrazine and Ag12 clusters, by utilizing first-principles calculations, we estimate polarizability tensor and conduct electronic natural population analysis (NPA), natural bond orbital (NBO) analysis, deformation density analysis (DDA) and charge decomposition analysis (CDA). In view of the minimized contribution from local surface plasmon resonance (LSPR), such a comprehensive study of metal NCs, which are free of Raman interference, provides a modelling method towards the long-debated chemical mechanism in SERS theory.

Morphology dependent near-field response in atomistic plasmonic nanocavities

In this work we examine how the atomistic morphologies of plasmonic dimers control the near-field response by using an atomistic electrodynamics model. At large separations, the field enhancement in the junction follows a simple inverse power law as a function of the gap separation, which agrees with classical antenna theory. However, when the separations are smaller than 0.8 nm, the so-called quantum size regime, the field enhancement is screened and thus deviates from the simple power law. Our results show that the threshold distance for the deviation depends on the specific morphology of the junction. The near field in the junction can be localized to an area of less than 1 nm2 in the presence of an atomically sharp tip, but the separation distances leading to a large confinement of near field depend strongly on the specific atomistic configuration. More importantly, the highly confined fields lead to large field gradients particularly in a tip-to-surface junction, which indicates that such a plasmonic structure favors observing strong field gradient effects in near-field spectroscopy. We find that for atomically sharp tips the field gradient becomes significant and depends strongly on the local morphology of a tip. We expect our findings to be crucial for understanding the origin of high-resolution near-field spectroscopy and for manipulating optical cavities through atomic structures in the strongly coupled plasmonic systems.

Tunable multiband plasmonic response of indium antimonide touching microrings in the terahertz range

In this paper, we propose a terahertz (THz) plasmonic structure that supports three resonance modes, including the charge transfer plasmon (CTP), the bonding dipole-dipole plasmon, and the antibonding dipole-dipole plasmon, which can be strongly tuned by geometrical parameters, passively, and the temperature, actively. The structure exhibits a considerable thermal sensitivity of more than 0.01 THz/K. The introduced multiband and tunable THz plasmonic structures offer important applications in thermal switches, thermo-optical modulators, broadband filters, design of multifunctional molecules originating from the multiband specification of the proposed structure, and improvement in plasmonic sensor applications stemming from a detailed study of the CTP mode.

Single-Particle Plasmon Voltammetry (spPV) for Detecting Anion Adsorption

Nanoparticle and thin film surface plasmons are highly sensitive to electrochemically induced dielectric changes. We exploited this sensitivity to detect reversible electrochemical potential-driven anion adsorption by developing single-particle plasmon voltammetry (spPV) using plasmonic nanoparticles. spPV was used to detect sulfate electroadsorption to individual Au nanoparticles. By comparing both semiconducting and metallic thin film substrates with Au nanoparticle monomers and dimers, we demonstrated that using Au film substrates improved the signal in detecting sulfate electroadsorption and desorption through adsorbate modulated thin film conductance. Using single-particle surface plasmon spectroscopic techniques, we constructed spPV to sense sulfate, acetate, and perchlorate adsorption on coupled Au nanoparticles. spPV extends dynamic spectroelectrochemical sensing to the single-nanoparticle level using both individual plasmon resonance modes and total scattering intensity fluctuations.

Hybrid plasmonic gap modes in metal film-coupled dimers and their physical origins revealed by polarization resolved dark field spectroscopy

Plasmonic gap modes sustained by metal film-coupled nanostructures have recently attracted extensive research attention due to flexible control over their spectral response and significantly enhanced field intensities at the particle-film junction. In this work, by adopting an improved dark field spectroscopy methodology - polarization resolved spectral decomposition and colour decoding - we are able to "visualize" and distinguish unambiguously the spectral and far field radiation properties of the complex plasmonic gap modes in metal film-coupled nanosphere monomers and dimers. Together with full-wave numerical simulation results, it is found that while the monomer-film system supports two hybridized dipole-like plasmon modes having different oscillating orientations and resonance strengths, the scattering spectrum of the dimer-film system features two additional peaks, one strong yet narrow resonant mode corresponding to a bonding dipolar moment and one hybridized higher order resonant mode, both polarized along the dimer axis. In particular, we demonstrate that the polarization dependent scattering radiation of the film-coupled nanosphere dimer can be used to optically distinguish from monomers and concurrently determine the spatial orientation of the dimer with significantly improved accuracy at the single-particle level, illustrating a simple yet highly sensitive plasmon resonance based nanometrology method.

Polarization-Independent Multiple Fano Resonances in Plasmonic Nonamers for Multimode-Matching Enhanced Multiband Second-Harmonic Generation

Plasmonic oligomers composed of metallic nanoparticles are one class of the most promising platforms for generating Fano resonances with unprecedented optical properties for enhancing various linear and nonlinear optical processes. For efficient generation of second-harmonic emissions at multiple wavelength bands, it is critical to design a plasmonic oligomer concurrently having multiple Fano resonances spectrally matching the fundamental excitation wavelengths and multiple plasmon resonance modes coinciding with the harmonic wavelengths. Thus far, the realization of such a plasmonic oligomer remains a challenge. This study demonstrates both theoretically and experimentally that a plasmonic nonamer consisting of a gold nanocross surrounded by eight nanorods simultaneously sustains multiple polarization-independent Fano resonances in the near-infrared region and several higher-order plasmon resonances in the visible spectrum. Due to coherent amplification of the nonlinear excitation sources by the Fano resonances and efficient scattering-enhanced outcoupling by the higher-order modes, the second-harmonic emission of the nonamer is significantly increased at multiple spectral bands, and their spectral positions and radiation patterns can be flexibly manipulated by easily tuning the length of the surrounding nanorods in the nonamer. These results provide us with important implications for realizing ultrafast multichannel nonlinear optoelectronic devices.

Single protein sensing with asymmetric plasmonic hexamer via Fano resonance enhanced two-photon luminescence

Fano resonances in plasmonic systems have been proved to facilitate various sensing applications in the nanoscale. In this work, we propose an experimental scheme to realize a single protein sensing by utilizing its two-photon luminescence enhanced by a plasmonic Fano resonance system. The asymmetric gold hexamer supporting polarization-dependent Fano resonances and plasmonic modes without in-plane rotational symmetry is used as a referenced spatial coordinate for bio-sensing. We demonstrate via the full-vectorial three-dimensional simulation that the moving direction and the spatial location of a protein can be detected via its two-photon luminescence, which benefits from the resonant near-field interaction with the electromagnetic hot-spots. The sensitivity to changes in position of our method is substantially better compared with the conventional linear sensing approach. Our strategy would facilitate the sensing, tracking and imaging of a single biomolecule in deep sub-wavelength scale and with a small optical extinction cross-section.

Double Fano resonances in plasmonic nanocross molecules and magnetic plasmon propagation

Double Fano resonances in optical frequency are investigated in an artificial plasmonic molecule consisting of seven identical nanocrosses. These two Fano resonances are found to originate from different physical mechanisms. One is caused by the excitation of the inherent quadrupole dark mode supported by a single nanocross, and the other is attributed to the magnetic plasmon mode due to the generation of antiphase ring currents in adjacent fused tetramers. The two Fano resonances can either be tuned simultaneously or independently within a wide spectral range by adjusting the geometrical parameters of the nanocrosses. The excitation of the magnetic plasmon in a chain made of coupled nanoparticles allows for subwavelength guiding of optical energy with low radiative losses. The field decay length is as long as 2.608 μm, which is comparable to that of the magnetic plasmon waveguides and far surpasses the value achieved in electric plasmon counterparts. Because of the special shape of the nanocross, a Mach-Zehnder interferometer can be built to steer optical beams. These results show that the proposed plasmonic nanostructures have potential applications in biochemical sensing, narrow line-shape engineering and on-chip optical signal propagation in nanoscale integrated optics.

Adjusting diffraction spectrum of an echelon-like grating influenced by surface plasmon of nanomaterials

This article aims to study the varying diffraction spectrum phenomenon influenced by surface plasmon of nanomaterials. Experiments used silver nanoparticles to build the echelon-like grating as test samples, where the grating spacing of line pattern is 10 μm and within the width of silver nanoparticles line is 5 μm alternately. In this work, the silver stripes with gradient thickness were first formed line pattern alternately with glancing angle deposition and photolithography, and then annealed at temperature of 250 °C for 3 mins to fabricate the silver nanoparticles as bowl-like forms. Thicknesses of the silver nanoparticles in the echelon-like grating increase from 0 nm to 40 nm and their diameters grow from 0 nm to 35 nm with quasilinear increment simultaneously. Analyzing the varying diffraction spectrum was focused on observing the first order diffraction by changing the beam size and the probe position of the incident light. The significant results show that the bigger the incident light beam size, the larger the peak wavelength difference (Δλ) of the two first order diffractions. Moreover, the peak spectrum is shifted to long wavelength and Δλ is diminished by probing area with big size of nanoparticles.

Switching plasmon coupling through the formation of dimers from polyaniline-coated gold nanospheres

Active modulation of the plasmon coupling in homodimers of polyaniline (PANI)-coated Au nanospheres is achieved by changing the proton-doping state of the PANI shell. Such a PANI-enabled modulation of the plasmon coupling in the dimers gives rise to remarkable spectral shifts, which show an exponential dependence on the interparticle gap distance. For the dimer with a 10 nm PANI shell thickness and a 0.5 nm gap distance, the shift of the stronger scattering peak in response to the active modulation reaches 231 nm. Electrodynamic simulations reveal that the shift of the dipolar bonding plasmon mode dominates the position variation of the stronger scattering peak for the dimers with different gap distances. Moreover, the quadrupolar bonding plasmon mode can be turned on and off by controlling the proton-doping state of the dimers with gap distances of less than ∼3 nm. These results are of high importance for fundamentally understanding the sensitivity of coupled plasmon resonance modes to the dielectric environment, as well as for designing active plasmonic devices with high modulation performances.

Plasmonic polymers unraveled through single particle spectroscopy

Plasmonic polymers are quasi one-dimensional assemblies of nanoparticles whose optical responses are governed by near-field coupling of localized surface plasmons. Through single particle extinction spectroscopy correlated with electron microscopy, we reveal the effect of the composition of the repeat unit, the chain length, and extent of disorder on the energies, intensities, and line shapes of the collective resonances of individual plasmonic polymers constructed from three different sizes of gold nanoparticles. Our combined experimental and theoretical analysis focuses on the superradiant plasmon mode, which results from the most attractive interactions along the nanoparticle chain and yields the lowest energy resonance in the spectrum. This superradiant mode redshifts with increasing chain length until an infinite chain limit, where additional increases in chain length cause negligible change in the energy of the superradiant mode. We find that, among plasmonic polymers of equal width comprising nanoparticles with different sizes, the onset of the infinite chain limit and its associated energy are dictated by the number of repeat units and not the overall length of the polymer. The intensities and linewidths of the superradiant mode relative to higher energy resonances, however, differ as the size and number of nanoparticles are varied in the plasmonic polymers studied here. These findings provide general guidelines for engineering the energies, intensities, and line shapes of the collective optical response of plasmonic polymers constructed from nanoparticles with sizes ranging from a few tens to one hundred nanometers.

Optical response of threaded chain plasmons: from capacitive chains to continuous nanorods

We present a detailed theoretical analysis of the optical response of threaded plasmonic nanoparticle strings, chains of metallic nanoparticles connected by cylindrical metallic bridges (threads), based on full-electrodynamic calculations. The extinction spectra of these complex metallic nanostructures are dominated by large resonances in the near infrared, which are associated with charge transfer along the entire string. By analysing contour plots of the electric field amplitude and phase we show that such strings can be interpreted as an intermediate situation between metallic nanoparticle chains and metallic nanorods, exhibiting characteristics of both. Modifying the dielectric environment, the number of nanoparticles within the strings, and the dimensions of the threads, allows for tuning the optical response of the strings within a very broad region in the visible and near infrared.

Proximal gap-plasmon nanoresonators in the limit of vanishing inter-cavity separation

Nanostructured metal-insulator-metal (MIM) metasurfaces supporting gap-plasmons (GPs) show great promise due to their ability to manipulate or concentrate light at the nanoscale, which is of importance to a broad palette of technologies. The interaction between individual, proximal GP nanoresonators, reaching the point of first electrical connection, can have unexpected, important consequences and remains unexplored. Here we study the optical properties of a GP-metasurface in the limit of diminishing spacing between GP nanocavities and show that it maintains its nanoresonator array character, with negligible GP interaction, even at extremely close proximity between cavities. Then, at the point where inter-cavity electrical connection is first established, the surface abruptly transforms into a patterned metal-insulator-metal waveguide. We report detailed experimental evidence and explain the underlying physics through computational modeling and based on the properties and inherent symmetries of the electromagnetic field of the investigated metasurface. The novel phenomenon explored here can have a host of potential applications in the field of active plasmonic metamaterials, plasmonic photocatalysis and ultra-sensitive sensors.

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.

Ultrafast nonlinear control of progressively loaded, single plasmonic nanoantennas fabricated using helium ion milling

We demonstrate milling of partial antenna gaps and narrow conducting bridges with nanometer precision using a helium ion beam microscope. Single particle spectroscopy shows large shifts in the plasmonic mode spectrum of the milled antennas, associated with the transition from capacitive to conductive gap loading. A conducting bridge of nanometer height is found sufficient to shift the antenna from the capacitive to the conductive coupling regime, in agreement with circuit theory. Picosecond pump-probe spectroscopy reveals an enhanced nonlinear response for partially milled antennas, reaching an optimum value for an intermediate bridge height. Our results show that manipulation of the antenna load can be used to increase the nonlinear response of plasmonic antennas.