Complex Plasmonic Nanostructures Research Articles

Efficient and Rigorous Modeling of Light Scattering From Complex Plasmonic Nanostructures With Multiple Incident Excitations

Optical responses to incident lights with different incident directional angles or polarization angles are often of concern in the analysis of light scattering from plasmonic nanostructures. This leads to a multiple-excitation problem for the modeling and prediction of light scattering. An efficient and rigorous numerical method is presented to deal with this problem. In this method, the hybrid finite element and boundary integral (FEBI) formula is adopted to model the interior complex plasmonic materials and the exterior scattering space rigorously, and a low-rank compressed matrix factorization algorithm based on the FEBI framework is proposed to deal with the multiple-excitation problem efficiently. Numerical experiments are performed to illustrate the effectiveness of the proposed method.

Towards Electrochemiluminescence Microscopy Exploration of Plasmonic-Mediated Phenomena at the Single-Nanoparticle Level.

The rational design of functional plasmonic metasurfaces and metamaterials requires the development of high-throughput characterization techniques compatible with operando conditions and capable of addressing single-nanostructures. In their work, Wei et al. demonstrate the use of electrochemiluminescence microscopy to investigate the mechanism behind plasmon-enhanced luminescence induced by gold nanostructures. The use of gold plasmonic arrays was exploited to achieve the rapid spectroscopic evaluation of all the individual nanostructures, and the correlation of the results with high- resolution electron microscopy analysis, guaranteeing a strict one-to-one correspondence. The authors were able to identify two different mechanisms for the enhancement of [Ru(bpy)3 ]2+ -tri-n-propylamine electrochemiluminescence mediated by single gold nanoparticles and by small plasmonic clusters. In the future, the proposed characterization could be used for the rapid and in situ spectroscopic analysis of more complex plasmonic nanostructures and metasurfaces.

Towards Electrochemiluminescence Microscopy Exploration of Plasmonic‐Mediated Phenomena at the Single‐Nanoparticle Level

AbstractThe rational design of functional plasmonic metasurfaces and metamaterials requires the development of high‐throughput characterization techniques compatible with operando conditions and capable of addressing single‐nanostructures. In their work, Wei et al. demonstrate the use of electrochemiluminescence microscopy to investigate the mechanism behind plasmon‐enhanced luminescence induced by gold nanostructures. The use of gold plasmonic arrays was exploited to achieve the rapid spectroscopic evaluation of all the individual nanostructures, and the correlation of the results with high‐ resolution electron microscopy analysis, guaranteeing a strict one‐to‐one correspondence. The authors were able to identify two different mechanisms for the enhancement of [Ru(bpy)3]2+‐tri‐n‐propylamine electrochemiluminescence mediated by single gold nanoparticles and by small plasmonic clusters. In the future, the proposed characterization could be used for the rapid and in situ spectroscopic analysis of more complex plasmonic nanostructures and metasurfaces.

DNA Nanostructure-Guided Plasmon Coupling Architectures

DNA Nanostructure-Guided Plasmon Coupling Architectures

Enhancement of second-harmonic generation from Fano plasmonic metasurfaces by introducing structural asymmetries.

Resonant plasmonic metasurfaces have attracted much attention for great potential in augmenting nonlinear optical conversion at the nanoscale and thus related sensing and integrated optics applications. In this work, we use the nonlinear scattering theory to numerically investigate enhanced second-harmonic generation (SHG) from Fano metasurfaces which consist of gold asymmetric double-bars. We find that the Fano resonance at the fundamental wavelength boosts the nonlinear response by more than a factor of 60. On this basis, by introducing translational and rotational structural asymmetries, the SHG signal is further amplified because of the broken mirror symmetry. More specifically, under the optimal condition, the previously suppressed SHG component can be greatly released and play a more important role compared to the original existing SHG component in an extra 6-fold enhancement in total SHG intensity. The 360-fold enhancement by tailoring both resonance quality and structural asymmetries indicates the clear and important roles of both linear resonance and local-field distribution in reaching the largest SHG emission. Our results are a step towards enlarging SHG responses of more complex plasmonic nanostructures.

Numerical investigation of plasmonic bowtie nanorings with embedded nanoantennas for achieving high SEIRA enhancement factors

This paper presents the numerical investigation of several complex plasmonic nanostructures — bowtie nanoring and crossed-bowtie nanoring nanoantennas with embedded bowtie nanoantennas and crossed-bowtie nanoantennas — for surface enhanced infrared absorption (SEIRA) spectroscopy-based substrates. The proposed nanostructures exhibit substantially large SEIRA enhancement factor (∼8.1 × 105) compared to previously reported enhancement factor values for bowtie nanoantennas or nanoring antennas. The plasmonic properties of the proposed nanostructures have been studied by the numerical evaluation of the near-field electromagnetic enhancement at resonant plasmon mode excitation wavelengths in the mid-IR spectral regime. The highest SEIRA enhancement of ∼8.1 × 105 occurs at a wavelength of ∼6800 nm (6.8 μm). A substantial electric field enhancement as large as ∼375, corresponding to SEIRA EF of ∼1.4 × 105 is noted even when the minimum gaps between the plasmonic nanostructures is as large as 10 nm, which can easily be fabricated using the conventional nanolithography techniques. The occurrence of several electric field hotspots due to the presence of plasmonic nanoantennas embedded inside the nanorings was observed, as the electric fields are enhanced in the vicinity of the plasmonic nanostructures being proposed. The multiple electric field hotspots in the proposed nanostructures can lead to larger average electric field enhancement as well as the average SEIRA enhancement for these substrates. Moreover, by embedding plasmonic nanoantenna structures inside the bowtie nanorings and crossed-bowtie nanorings, large spectral tunability of plasmon resonance wavelengths is achieved in the spectral regime from 4 μm to 8 μm. This is done by varying a larger number of spectral parameters that are present in these complex nanostructures. This paper also reports a novel configuration of crossed-bowtie nanoring plasmonic structure exhibiting less polarization dependence of the SEIRA enhancement factor. This structure also exhibits tunability of hotspot positions when the direction of the polarization of the incident light is rotated. The proposed structures in this paper can be fabricated by the state-of-the-art nanofabrication technologies. The proposed structures could find potential applications in chemical and biological sensing and biochemical detection of analyte molecules.

Centimeter-Scale Superlattices of Three-Dimensionally Orientated Plasmonic Dimers with Highly Tunable Collective Properties.

The precise organization and orientation of plasmonic molecules on substrates is crucial to their application in functional devices but still remains a grand challenge. This article describes a bottom-up strategy to efficiently fabricate centimeter-scale superlattices of three-dimensionally oriented plasmonic dimers with highly tunable collective optical properties on substrates. The in-plane (i.e., X-Y plane) and out-of-plane (i.e., along Z-axis) orientation of the constituent plasmonic dimers can be precisely controlled by a combination of directional capillary force and supporting polymer film. Our experimental measurements and numerical simulations show that the macroscopic dimer superlattices exhibit polarization-dependent plasmon Fano resonances in air and multimodal surface lattice resonances with high quality factors in a homogeneous medium, owing to the high positional and orientational ordering of the subunits. Our strategy enables the fabrication of complex plasmonic nanostructures with precise configurations for advanced plasmonic devices such as plasmon nanolasing and metamaterials.

Laser-Scanning-Guided Assembly of Quasi-3D Patterned Arrays of Plasmonic Dimers for Information Encryption.

The application of plasmonic dimeric nanostructures in color displays, data storage, and especially metamaterials necessitates the patterning of dimers into ordered arrays, but controllable assembly of plasmonic nanoparticles into patterned dimer arrays on substrates still remains a challenge. Here, a facile laser-scanning-based strategy to fabricate quasi-3D patterned arrays of plasmonic nanoparticle dimers with controlled orientation for plasmonic information encryption is reported. Laser scanning of polymer-covered plasmonic nanoparticle (e.g., gold) arrays selectively exposes the surface of irradiated nanoparticle via localized photothermal heating, guiding the assembly of another type of nanoparticles onto the exposure nanoparticle surface to form dimers on substrates. This combined top-down/bottom-up approach is highly flexible in forming high-resolution patterns of plasmonic dimers from nanoparticles of different sizes and shapes. The z-axis orientation, interparticle spacing, and nanoparticle size and shape of plasmonic dimers can be precisely tuned, enabling the modulation of the coupled resonances of the dimer arrays. Moreover, it is demonstrated that the patterned dimer arrays can be used in information encryption where their plasmonic color can be repeatedly displayed and erased. This work provides an important addition to tools for the fabrication of patterned complex plasmonic nanostructures from as-synthesized nanoparticles with broad applications.

Sodium-Based Surface Plasmon Resonances for High-Performance Optical Sensing in the Near Infrared

Surface plasmon resonance (SPR) owing to its extreme sensitivity to the refractive index of surrounding medium has been widely applied to chemical and biological sensing. However, it is difficult to realize SPR sensing simultaneously with large sensitivity and figure of merit (FOM) in a conventional prism coupling structure. In this paper, we numerically study the SPRs in a polymer-protected sodium thin film and sinusoidal nanograting based on a prism Kretschmann configuration using finite element method (FEM). In the near-infrared wavelengths, both angular and wavelength interrogation are investigated and high sensitivity (14.9 μm/RIU and 750 deg/RIU for the thin film and nanograting, respectively) and FOM (480.6 /RIU and 255.6 /RIU for the thin film and nanograting, respectively) can be achieved simultaneously. It should be noticed that the large FOM mainly originates from the narrow resonant linewidth. Additionally, our nanograting structure can support a self-reference sensing scheme, which is beneficial to realize accurate sensing. Our FEM simulations are in good agreement with the calculation by the transfer matrix method (TMM) and rigorous coupled wave analysis (RCWA). Our proposed sodium-based SPR structures do not require complex plasmonic nanostructures and are compatible with a thermo-assisted spin-coating process, which shows great potential in realizing high-performance optical sensing beyond noble metals and expands the application scope of sodium-based plasmonic devices.

Enhancing hyperspectral EELS analysis of complex plasmonic nanostructures with pan-sharpening.

Nanoscale hyperspectral techniques-such as electron energy loss spectroscopy (EELS)-are critical to understand the optical response in plasmonic nanostructures, but as systems become increasingly complex, the required sampling density and acquisition times become prohibitive for instrumental and specimen stability. As a result, there has been a recent push for new experimental methodologies that can provide comprehensive information about a complex system, while significantly reducing the duration of the experiment. Here, we present a pan-sharpening approach to hyperspectral EELS analysis, where we acquire two datasets from the same region (one with high spatial resolution and one with high spectral fidelity) and combine them to achieve a single dataset with the beneficial properties of both. This work outlines a straightforward, reproducible pathway to reduced experiment times and higher signal-to-noise ratios, while retaining the relevant physical parameters of the plasmonic response, and is generally applicable to a wide range of spectroscopy modalities.

Tailored Optical Functionality by Combining Electron‐Beam and Focused Gold‐Ion Beam Lithography for Solid and Inverse Coupled Plasmonic Nanostructures

AbstractPlasmonics is a field uniquely driven by advances in micro‐ and nanofabrication. Many design ideas pose significant challenges in their experimental realization and test the limits of modern fabrication techniques. Here, the combination of electron‐beam and gold ion‐beam lithography is introduced as an alternative and highly versatile route for the fabrication of complex and high fidelity plasmonic nanostructures. The capability of this strategy is demonstrated on a selection of planar as well as 3D nanostructures. Large area and extremely accurate structures are presented with little to no defects and errors. These structures exhibit exceptional quality in shape fidelity and alignment precision. The combination of the two techniques makes full use of their complementary capabilities for the realization of complex plasmonic structures with superior optical properties and functionalities as well as ultra‐distinct spectral features which will find wide application in plasmonics, nanooptics, metasurfaces, plasmonic sensing, and similar areas.

Simulation of light scattering and electron energy loss (EELS) by small spatially dispersive plasmonic nanodimers

Despite the tremendous progress in the field of nanoplasmonics, complex plasmonic nanostructures such as dimers still present a significant challenge for proper numerical characterization. In particular, for the particles which size is smaller than electron mean free path in metals it is necessary to account for effects of spatial nonlocality in simulations. To address these problems a new mathematical model based on the Discrete Sources Method was developed and corresponding computer model was implemented. It enables one to successfully resolve the problem of accounting for spatial dispersion effects, which is especially important in the systems with very small gap and particle size, and it retains all key features of the Discrete Sources numerical approach including flexibility and a posteriori error estimation. We compute such scattering characteristics as electron energy loss probability and total scattered field intensity of nanodimers as one typically has to rely on methods of both optical and electron spectroscopy to experimentally investigate these structures. The developed model also allows to simultaneously consider several external excitations of the plasmonic system, including both electron and plane wave, in order to obtain more information about the system and to significantly speed up computational performance.

All-Dielectric Chiral Metasurfaces Based on Crossed-Bowtie Nanoantennas.

Circular dichroism spectroscopy is a technique used to discriminate molecular chirality, which is essential in fields like biology, chemistry, or pharmacology where different chiral agents often show different biological activities. Nevertheless, due to the inherently weak molecular-chiroptical activity, this technique is limited to high concentrations or large analyte volumes. Finding novel ways to enhance the circular dichroism would boost the performance of these techniques. So far, the enhancement of light–matter interaction mediated by plasmons is the most common way to develop chiral plasmonic structures with extraordinarily strong chiroptical responses. However, absorptive losses of metals at optical frequencies has hindered its practical use in many scenarios. In this work, we propose an all-dielectric low-loss chiral metasurface with unit cells built by high-refractive-index crossed-bowtie nanoantennas. These unit cells, built of silicon, strongly increase the chiroptical effect through the simultaneous interaction of their electric and magnetic modes, which in contrast to other recent proposals shows at the same time a high concentration of the electric field in its gap that leads to the presence of hotspots. The proposed structure exhibits a circular dichroism spectra up to 3-fold higher than that of previous proposals that use complex plasmonic or hybrid nanostructures, making it a clear alternative to develop low-loss metasurfaces with potential applications in chiral target sensing/biosensing. For completeness, single triangular shaped and symmetric (achiral) bowtie nanostructures were also studied as possible candidates for a detection up to the single-molecule level due the lack of a circular dichroism background of the nanostructures themselves.

Highly Plasmonic Titanium Nitride by Room-Temperature Sputtering

Titanium nitride (TiN) has recently emerged as an attractive alternative material for plasmonics. However, the typical high-temperature deposition of plasmonic TiN using either sputtering or atomic layer deposition has greatly limited its potential applications and prevented its integration into existing CMOS device architectures. Here, we demonstrate highly plasmonic TiN thin films and nanostructures by a room-temperature, low-power, and bias-free reactive sputtering process. We investigate the optical properties of the TiN films and their dependence on the sputtering conditions and substrate materials. We find that our TiN possesses one of the largest negative values of the real part of the dielectric function as compared to all other plasmonic TiN films reported to date. Two-dimensional periodic arrays of TiN nanodisks are then fabricated, from which we validate that strong plasmonic resonances are supported. Our room-temperature deposition process can allow for fabricating complex plasmonic TiN nanostructures and be integrated into the fabrication of existing CMOS-based photonic devices to enhance their performance and functionalities.

A Single-Crystalline Silver Plasmonic Circuit for Visible Quantum Emitters.

Plasmonic waveguides are key elements in nanophotonic devices, serving as optical interconnects between nanoscale light sources and detectors. Multimode operation in plasmonic two-wire transmission lines promises important degrees of freedom for near-field manipulation and information encoding. However, highly confined plasmon propagation along gold nanostructures is typically limited to the near-infrared region due to ohmic losses, excluding all visible quantum emitters from plasmonic circuitry. We report on the top-down fabrication of complex plasmonic nanostructures in single-crystalline silver plates. We demonstrate the controlled remote excitation of a small ensemble of fluorophores by a set of waveguide modes and the emission of the visible luminescence into the waveguide with high efficiency. This approach opens up the study of a nanoscale light-matter interaction between complex plasmonic waveguides and a large variety of quantum emitters available in the visible spectral range.

Assembly of Gold Nanorods into Chiral Plasmonic Metamolecules Using DNA Origami Templates.

The inherent addressability of DNA origami structures makes them ideal templates for the arrangement of metal nanoparticles into complex plasmonic nanostructures. The high spatial precision of a DNA origami-templated assembly allows controlling the coupling between plasmonic resonances of individual particles and enables tailoring optical properties of the constructed nanostructures. Recently, chiral plasmonic systems attracted a lot of attention due to the strong correlation between the spatial configuration of plasmonic assemblies and their optical responses (e.g., circular dichroism [CD]). In this protocol, we describe the whole workflow for the generation of DNA origami-based chiral assemblies of gold nanorods (AuNRs). The protocol includes a detailed description of the design principles and experimental procedures for the fabrication of DNA origami templates, the synthesis of AuNRs, and the assembly of origami-AuNR structures. In addition, the characterization of structures using transmission electron microscopy (TEM) and CD spectroscopy is included. The described protocol is not limited to chiral configurations and can be adapted for the construction of various plasmonic architectures.

Full-visible multifunctional aluminium metasurfaces by in situ anisotropic thermoplasmonic laser printing

A new level of anisotropic plasmonic laser printing for aluminum multifunctional metasurfaces.

An aligned octahedral core in a nanocage: synthesis, plasmonic, and catalytic properties.

Plasmonic metal nanostructures with complex morphologies provide an important route to tunable optical responses and local electric field enhancement at the nanoscale for a variety of applications including sensing, imaging, and catalysis. Here we report a high-concentration synthesis of gold core-cage nanoparticles with a tethered and structurally aligned octahedral core and examine their plasmonic and catalytic properties. The obtained nanostructures exhibit a double band extinction in the visible-near infrared range and a large area electric field enhancement due to the unique structural features, as demonstrated using finite difference time domain (FDTD) simulations and confirmed experimentally using surface enhanced Raman scattering (SERS) tests. In addition, the obtained structures had a photoelectrochemical response useful for catalyzing the CO2 electroreduction reaction. Our work demonstrates the next generation of complex plasmonic nanostructures attainable via bottom-up synthesis and offers a variety of potential applications ranging from sensing to catalysis.

STM Imaging of Localized Surface Plasmons on Individual Gold Nanoislands.

An optically modulated scanning tunneling microscopy technique developed for measurement of single-molecule optical absorption is used here to image the light absorption by individual Au nanoislands and Au nanostructures. The technique is shown to spatially map, with nanometer resolution, localized surface plasmons (LSPs) excited within the nanoislands. Electrodynamic simulations demonstrate the correspondence of the measured images to plasmonic near-field intensity maps. The optical STM imaging technique captures the wavelength, polarization, and geometry dependence of the LSP resonances and their corresponding near-fields. Thus, we introduce a tool for real-space, nanometer-scale visualization of optical energy absorption, transport, and dissipation in complex plasmonic nanostructures.

Understanding the Collective Optical Properties of Complex Plasmonic Vesicles

AbstractVesicular assembly of small plasmonic nanoparticles, or plasmonic vesicle, is a promising multifunctional theranostic platform for photothermal therapy, near infrared (NIR) light‐responsive drug release, and rapid clearance of small inorganic particles from the body. Wide ranges of optical properties are reported including characteristic absorption peak in the visible or NIR ranges, or broadband absorption. It is unclear how the complex interaction among a large number of small gold nanoparticles contribute to the collective optical property of a plasmonic vesicle. In this study, the collective optical properties of plasmonic vesicles are examined and four characteristic regimes, namely the isolated nanoparticle regime, Coulomb interaction regime, black gold regime, and nanoshell regime, are revealed. Small plasmonic nanoparticles need to be very close or weakly overlap to give a broadband absorption (i.e., black gold regime) or form a NIR plasmon peak. Furthermore, smaller gold nanoparticle or larger core size leads to higher NIR peak shift and photothermal conversion efficiency. It is anticipated that this study provides design guidelines and thus have a significant impact on further design and development of complex plasmonic nanostructures and vesicles for biomedical applications.