Tunable Broadband Optical Responses of Substrate-Supported Metal/Dielectric/Metal Nanospheres

Using the image charge theory and finite element methods, we present the first comprehensive study on the optical properties of substrate-supported, three-layer, metal/dielectric/metal nanospheres. By adopting dipolar and quadrupolar approximations of the quasistatic image charge theory, we derive analytical expressions for the polarization-dependent polarizabilities of a three-layer nanosphere near a substrate and use them to find the nanosphere’s plasmon resonance wavelengths as functions of the geometric and material parameters of the nanosphere–substrate system. By calculating the resonance wavelength of substrate-supported gold/silica/gold nanosphere over a sufficiently large domain of the nanosphere’s dimensions, we show that this wavelength can be tuned from visible to infrared regions by altering only the size of the nanosphere’s core. We also show that the resonance position as well as the enhancement and confinement of the near-field can be dynamically tuned over broad ranges by changing the polarization of the excitation light. Of significance for the applicability of our results in practice is that we employ size-dependent permittivity of gold, which allows experimentalists to readily produce these substrate-supported nanospheres with desired optical responses. Upon comparing our analytical results with the results of numerical simulations, we reveal the range of the nanospheres’ outer radii within which the dipolar and quadrupolar approximations adequately describe the nanosphere–substrate interaction. Since majority of the optical functions are realized with light polarized parallel to the substrate, our results allow one to readily engineer the broadband optical responses of substrate-supported metal/dielectric/metal nanospheres for applications in resonance-enhanced sensing, in light harvesting, and in biomedicine.     

Numerical Simulation of Extinction Spectra of Plasmonically Coupled Nanospheres Using Discrete Dipole Approximation: Influence of Compositional Asymmetry

We demonstrate plasmon coupling phenomenon between equivalent (homodimer) and non-equivalent (heterodimer) spherical shape noble metal nanoparticle (Ag, Au and Al). A systematic comparison of surface plasmon resonance (SPR) and extinction properties of various configurations (monomer, homodimer and heterodimer) has been investigated to observe the effect of compositional asymmetry. Numerical simulation has been done by using discrete dipole approximation method to study the optical properties of plasmonically coupled metal nanoparticles (MNPs). Plasmon coupling between similar nanoparticles allows only higher wavelength bonding plasmon mode while both the plasmon modes lower wavelength antibonding mode as well as higher wavelength bonding mode in the case of heterodimer. Au monomer of radius 50 nm shows resonance peak at 518 nm while plasmon coupling between Au-Au homodimer results in a spectral red shift around 609 nm. Au-Ag plasmonic heterodimer (radius 50 nm) reveals two resonant modes corresponding to higher energy antibonding mode (422 nm) as well as lower energy bonding mode (533 nm). Further, we have shown that interparticle edge-to-edge separation is the most significant parameter affecting the surface plasmon resonances of MNPs. As the inter particle separation decreases, resonance wavelength shows red spectral shift which is maximum for the touching condition. It is shown that plasmon coupling is a reliable strategy to tune the SPR.

     

Combination of Nanoholes with Metal Nanoparticles–Fabrication and Characterization of Novel Plasmonic Nanostructures

Small metal nanostructures, especially gold and silver nanoparticles, are known for their interesting optical properties caused by plasmonic effects. Molecular plasmonics, a combination of these optically active nanostructures with the molecular world, opens new possibilities for bioanalytics and (bio-) nanophotonics. Isotropic or anisotropic, homogeneous or heterogeneous metal nanoparticles provide a platform for different, highly defined functional units with interesting optical properties such as plasmon waveguides or molecular beacons. Nanohole arrays in metal layers are another promising component for nanophotonics. New photonic materials were realized from combinations of single metal nanoparticles with individual nanoholes in metals. Atomic force microscopic imaging was used to determine the particle location as well as the lateral dimensions and the topography of the resulting structures. Besides ultramicroscopic characterization of the nanoarrangements, such as nanoparticles positioned in nanoholes, far-field optical methods were also applied to investigate their optical properties.     

Slab Thickness Dependence of Localized Surface Plasmon Resonance Behavior in Gold Nanorings

We present detailed experimental and numerical studies of plasmonic properties of gold nanoring (NR) arrays with different slab thicknesses from 15 to 125 nm. The hybrid plasmon resonances for the bonding and antibonding modes in gold NRs exhibit a high slab thickness dependence behavior in optical properties. For the thinner slab thickness below 50 nm, both hybrid modes show large spectral tunabilities by varying the slab thickness. Furthermore, for such hollow NR structure, the enhancements of electric field intensities at the inner and outer ring surfaces when reducing the slab thickness are investigated. We observe a significant transition of field distributions for the antibonding mode. All these features can be understood by surface charge distributions from our simulations. The results of this study offer a potential strategy to design a composite plasmonic nanostructure with large field enhancement for numerous applications.     

Near and Far-Field Properties of Nanoprisms with Rounded Edges

Photonic devices can be developed, and their working principle can be understood only by considering the phenomena taking place at the nanoscale level. Optical properties of plasmonic structures depend on their geometric parameters and are sensitive to them. Recently, many advanced methods for the preparation of nanostructures have been proposed; however still, the geometric parameters are inaccurate. Numerical simulations provide a powerful tool for the analysis of plasmonic nanostructures. To the best of our knowledge, there are not many papers on near-field and far-field properties of single nanoprism and nanoprism dimer, the so-called bowtie, with rounded edges. For this purpose, Finite Integration Technique implemented to the CST Microwave Studio was used. Besides the edge rounding, an additional modification of the resonance modes was investigated, achieved by placement of a spherical nanoparticle in the gap between the prisms. Results of numerical simulations indicate that the radius of the curvature edges strongly affects the plasmon peak localization, and this effect cannot be neglected in plasmonic device design. Increase in the radius of edge curvature causes main extinction cross-section peak blueshift in all cases analyzed. Moreover, our calculations imply that the nanoparticle in the gap between prisms strongly influences the dependence of spectral properties on the radius curvature.     

Optical Modeling of Plasmonic Nanoparticles Enhanced Light Emission of Silicon Light-Emitting Diodes

Significant enhancement of radiative efficiency of thin-film silicon light-emitting diodes achieved by placing the active layer in close proximity to silver (Ag) nanoparticles has been observed. In this paper, optical properties including transmission, reflection, and absorption of a random assembly of Ag nanoparticles are theoretically investigated using the effective medium model. Furthermore, the influence of Ag nanoparticles on light emission of silicon light-emitting diodes is studied by an improved effective mode volume model we propose here. The normalized line shape of dipole oscillation is calculated directly using Lorentz–Drude model without using any approximation. Thus, it results in more accurate calculation of the enhanced Purcell factor in comparison with the conventional approach. We show that an enhancement of radiative efficiency of silicon light-emitting diodes can be achieved by localized surface plasmons on metal nanoparticles. The calculated result of optimal Ag nanoparticle size to enhance light emission of silicon light-emitting diodes at 900 nm wavelength is in very good agreement with those obtained from the experimental result. The model is useful for the design of metallic nanoparticles enhanced light emitters.     

Molecular dynamics study of carbon nanotube oscillator on gold surface

We investigated the substrate effect of carbon nanotube (CNT) oscillators using classical molecular dynamics simulations. Double-walled CNT oscillators on {100} gold surface were considered. The nanotube–gold interactions induced the compressive deformations of the outer nanotube and affected the transitional velocity and the energy dissipation of the nanotube oscillator. When the inner nanotube was extruded from the outer nanotube, the central regions of the outer nanotube were compressed by the nanotube–gold interactions and then, these compressive forces pushed out the inner nanotube and finally, the transitional velocity of the inner nanotube was slightly increased at the edges regions. Since the energy dissipation of the nanotube oscillator on gold surface was higher than that in vapor, the decrease of the transitional velocity for the nanotube oscillator on gold surface was greater than that for the nanotube oscillator in vapor.     

Review of Some Interesting Surface Plasmon Resonance-enhanced Properties of Noble Metal Nanoparticles and Their Applications to Biosystems

Noble metal, especially gold (Au) and silver (Ag) nanoparticles exhibit unique and tunable optical properties on account of their surface plasmon resonance (SPR). In this review, we discuss the SPR-enhanced optical properties of noble metal nanoparticles, with an emphasis on the recent advances in the utility of these plasmonic properties in molecular-specific imaging and sensing, photo-diagnostics, and selective photothermal therapy. The strongly enhanced SPR scattering from Au nanoparticles makes them useful as bright optical tags for molecular-specific biological imaging and detection using simple dark-field optical microscopy. On the other hand, the SPR absorption of the nanoparticles has allowed their use in the selective laser photothermal therapy of cancer. We also discuss the sensitivity of the nanoparticle SPR frequency to the local medium dielectric constant, which has been successfully exploited for the optical sensing of chemical and biological analytes. Plasmon coupling between metal nanoparticle pairs is also discussed, which forms the basis for nanoparticle assembly-based biodiagnostics and the plasmon ruler for dynamic measurement of nanoscale distances in biological systems.     

A lectin-based gold nanoparticle assay for probing glycosylation of glycoproteins

We report a glycoanalysis method in which lectins are used to probe the glycans of therapeutic glycoproteins that are adsorbed on gold nanoparticles. A model mannose-presenting glycoprotein, ribonuclease B (RNase B), and the therapeutic monoclonal antibody (mAb) rituximab, were found to adsorb spontaneously and non-specifically to bare gold nanoparticles such that glycans were accessible for lectin binding. Addition of a multivalent binding lectin, such as concanavalin A (Con A), to a solution of the modified gold nanoparticles resulted in cross-linking of the nanoparticles. This phenomenon was evidenced within 1 min by a change in the hydrodynamic diameter, D(H), measured by dynamic light scattering (DLS) and a shift and increase in absorbance of the plasmon resonance band of the gold nanoparticles. By combining the sugar-binding specificity and the cross-linking capabilities of lectins, the non-specific adsorption of glycoproteins to gold surfaces, and the unique optical reporting properties of gold nanoparticles, a glycosylation pattern of rituximab could be generated. This assay provides advantages over currently used glycoanalysis methods in terms of short analysis time, simplicity of the conjugation method, convenience of simple spectroscopic detection, and feasibility of providing glycan characterization of the protein drug product by using a variety of binding lectins.     

Tuning the Dipolar Plasmon Hybridization of Multishell Metal-Dielectric Nanostructure: Gold Nanosphere in a Gold Nanoshell

Because of the interaction between dipole resonances of the inner gold sphere and the outer gold shell, gold-dielectric-gold multishells with sub-50 nm diameter may at most have three hybridization modes of surface plasmon resonance (SPR). Theoretical calculations based on quasi-static theory indicate that there are blending and splitting of SPR bands in the absorption spectra, which makes the number of absorption peak tunable by changing the radius of inserted gold sphere, thickness of gold shell, dielectric constant of middle dielectric shell or outer environment. The two absorption peaks at longer wavelength, which correspond to the hybridization from the bonding shell plasmon and the sphere plasmon, are usually intense and well tunable. The absorption peak at shorter wavelength, which corresponds to the symmetric coupling between the anti-bonding shell plasmon and the sphere plasmon, is relative weak and only occurs with large dielectric constant of the middle shell, small dielectric constant of the outer surrounding, large inner radius of the gold shell, and small radius of the inner gold sphere. Furthermore, the physical origin of these plasmon hybridizations in gold-dielectric-gold multishells nanostructure has also been illuminated by analyzing the local electric field distributions.     

Preparation and Optical Characterization of Core–Shell Bimetal Nanoparticles

Chemical approaches allow for the synthesis of highly defined metal heteronanostructures, such as core–shell nanospheres. Because the material in the metal nanoparticles determines the plasmon resonance-induced absorption band, control of particle composition results in control of the position of the absorption band. Metal deposition on gold or silver nanoparticles yielded core–shell particles with modified optical properties. UV–vis spectroscopy on solution-grown, as well as surface-grown, particles was conducted and provided ensemble measurements in solution. Increasing the layers of a second metal leads to a shift in the absorption band. A shell diameter comparable to the original particle diameter leads to a predominant influence by the shell material. Extent of shell growth could be controlled by reaction time or the concentration of metal salt or reducing agent. Besides optical characterization, the utilization of atomic force microscopy, scanning electron microscopy, and transmission electron microscopy yielded important information about the ultrastructure of nanoparticle complexes. Surface-grown core–shell particles were superior in terms of achievable shell thickness, because of difficulties encountered with solution-grown particles due to salt-induced aggregation.     

Optical Absorption in Overcoats of Nanoparticle Arrays on a Metallic Substrate

Surface plasma oscillations in metallic particles as well as in thin metallic films have been studied extensively in the past decades. New features regarding surface plasma excitations are, however, constantly discovered, leading, for example, to surface-enhanced Raman scattering studies and enhanced optical transmission though metal films with nanohole arrays. In the present work, the role of a metallic substrate is examined in two cases, one involving an overcoat of dielectric nanoparticles and the other an overcoat of metallic nanoparticles. Theoretical results are obtained by modeling the nanoparticles as forming a two-dimensional, hexagonal lattice of spheres. The scattered electromagnetic field is then calculated using a variant of the Green function method. Comparison with experimental results is made for nanoparticles of tungsten oxide and tin oxide deposited on either gold or silver substrates, giving qualitative agreement on the extra absorption observed when the dielectric nanoparticles are added to the metallic surfaces. Such absorption would be attributed to the mirror image effects between the particles and the substrate. On the other hand, calculations of the optical properties of silver or gold nanoparticle arrays on a gold or a silver substrate demonstrate very interesting features in the spectral region from 400 to 1,000 nm. Interactions between the nanoparticle arrays surface plasmons and their images in the metallic substrate would be responsible for the red shift observed in the absorption resonance. Moreover, effects of particle size and ambient index of refraction are studied, showing a great potential for applications in biosensing with structures consisting of metallic nanoparticle arrays on metallic substrates.     

Large and Ultrafast Optical Response of a One-Dimensional Plasmonic–Photonic Cavity

The resonant coupling of a localized surface plasmon mode and a cavity mode in a photonic crystal has been recently shown to strongly tailor the stationary optical response of gold nanoparticles. Here, we demonstrate that this can be further exploited for controlling light on an ultrashort time scale. The stationary and ultrafast optical responses of such a plasmonic–photonic cavity are investigated numerically. We show that the transient photo-induced change of the optical transmittance of a bare nanocomposite thin film can be amplified up to 60 times once resonantly coupled to the cavity mode in the hybrid device, despite the degradation of this mode due to absorption losses. In addition, different all-optical, ultrafast, efficient, and reversible photonic functions (increase or decrease of the signal intensity, transient spectral shift of the cavity mode) can be achieved depending on the spectral position of the transmitted mode tuned by varying the angle of incidence. The transient modification of the signal intensity is predicted to reach about 300 % after a subpicosecond rise time when the defect mode matches the plasmon resonance.     

金纳米棒的光学性质及其在生物医学领域的应用

简要介绍金纳米棒的光学性质和合成方法,重点阐述其在生物医学领域研究的最新进展,并对其今后的研究方向进行展望．金纳米棒是一种胶囊状的金纳米颗粒,具有一个横向等离子共振吸收峰和一个纵向等离子共振吸收峰,分别对应其横轴和纵轴两个特征尺寸．通过调节金纳米棒的长径比,纵向等离子共振吸收峰可由可见光区跨越至近红外光区．金纳米棒这一独特的光学性质在生物和化学传感方面有着广泛而重要的应用前景．     

Förster Energy Transfer in the Vicinity of Two Metallic Nanospheres (Dimer)

We present a detailed theoretical analysis of the Förster energy transfer process when a pair of molecules (donor and acceptor) is located nearby a cluster of two metallic nanospheres (dimer). We consider the case in which plasmonic resonances are within the overlap between the donor emission and acceptor absorption spectra, as well as the case that excludes such resonances from the aforementioned spectral overlap. Moreover, we explore the dependence of the Förster energy transfer rate on different dimer configurations (size and separation of nanospheres) and several dipole orientations of molecules. The dimer perturbs strongly the Förster energy transfer rate when plasmons are excited, donor dipole is oriented along the longitudinal axis of the dimer, and the radii of nanospheres and the sphere-gap distance are on the order of a few nanometers. In case of plasmonic excitation, the Förster energy transfer rate is degraded as the sphere-gap distance and size of the nanoparticles increase due to the dephasing of electronic motion arising from ohmic losses of metal. Also, we study the Förster efficiency influenced by the dimer, finding that the high efficiency region (delimited by the Förster radius curve) is reduced as a consequence of significant enhancement of the direct donor decay rate. Our study could impact applications that involve Förster energy transfer.     

Optical investigation of the electron transfer protein azurin-gold nanoparticle system

The hybrid system obtained by conjugating the protein azurin, which is a very stable and well-described protein showing a unique interplay among its electron transfer and optical properties, with 20-nm sized gold nanoparticles has been investigated. Binding of azurin molecules to gold nanoparticle surface results in the red shift of the nanoparticle resonance plasmon band and in the quenching of the azurin single tryptophan fluorescence signal. These findings together with the estimate of the hydrodynamic radius of the composite, obtained by means of Dynamic Light Scattering, are consistent with the formation of a monolayer of protein molecules, with preserved natural folding, on nanoparticle surface. The fluorescence quenching of azurin bound molecules is explained by an energy transfer from protein to metal surface and it is discussed in terms of the involvement of the Az electron transfer route in the interaction of the protein with the nanoparticle.     

Optimization of immunolabeled plasmonic nanoparticles for cell surface receptor analysis

Noble metal nanoparticles hold great potential as optical contrast agents due to a unique feature, known as the plasmon resonance, which produces enhanced scattering and absorption at specific frequencies. The plasmon resonance also provides a spectral tunability that is not often found in organic fluorophores or other labeling methods. The ability to functionalize these nanoparticles with antibodies has led to their development as contrast agents for molecular optical imaging. In this review article, we present methods for optimizing the spectral agility of these labels. We discuss synthesis of gold nanorods, a plasmonic nanoparticle in which the plasmonic resonance can be tuned during synthesis to provide imaging within the spectral window commonly utilized in biomedical applications. We describe recent advances in our group to functionalize gold and silver nanoparticles using distinct antibodies, including EGFR, HER-2 and IGF-1, selected for their relevance to tumor imaging. Finally, we present characterization of these nanoparticle labels to verify their spectral properties and molecular specificity.     

Multiple Surface Plasmon Resonances in Compound Structure with Metallic Nanoparticle and Nanohole Arrays

The optical properties of a compound structure with metallic nanoparticle and nanohole arrays are numerically investigated by the means of finite-difference time domain method. We report on the observation of multi-valleys in the reflection spectra due to the excitation of surface plasmon (SP) resonant modes of the compound structure. Simulation results show that multiple SP resonances consist of surface plasmon polaritons on the gold film, localized surface plasmons on the nanoparticles, and coupling mode between them. These findings are important for applications utilizing multiple surface plasmon resonances.     

角页岩雕刻器的微痕实验研究

本文以丁村遗址角页岩为原料打制实验标本,设计展开雕刻器微痕实验,并对其中部分标本按照序贯试验原理进行分阶段实验。实验结果表明,角页岩雕刻器雕刻刃的使用微痕基本特征为:轻度到重度磨圆,片疤数量较少,多小中型片疤,终端形态多为羽状和阶梯状,以分散分布为主,大片疤常单独出现。分阶段实验结果表明,磨圆是以从快到慢速度生成的,片疤在不同阶段发生改变。本文旨在通过对角页岩雕刻器使用微痕的研究,为探讨考古标本功能提供可参照的数据与图像。     

Improved Absorbance and Near-Infrared Dispersion of AuGe Nanoparticles over Au Nanoparticles Prepared with Similar Thermal Annealing Environment

The difficulty in patterning the structures at sub-wavelength range leads to employ the bottom-up approach to form nanostructures of metals as well as dielectric components that disperse them in host media. The optical properties of nanoparticles are studied with UV-Vis 750 (lambda) NIR spectroscopy and fit with empirical relations. The refractive index is about the volume fraction of particles. The AuGe nanoparticles demonstrate improved absorbance, lower refractive index, and higher extinction than Au nanoparticles formed with similar thermal process. Surface plasmon resonance (SPR) phenomena are highly sensitive to the bonding between atoms, atomic structure, and the electronic configuration in atoms of the given material. If one takes into account the structure of materials, then the literature on eutectic alloys predicts that alloying gold with germanium (AuGe) with varying compositions will also change the x-ray diffraction peak positions of gold itself. The peak shift can be interpreted as the change in grain size or shift in grain boundaries implying a corresponding change in material’s atomic arrangement within lattice structure. As a result, there will be a change in the charge distribution of free electron cloud in original gold ultimately affecting a change in the plasmon resonance frequency and thereby modulating the various optical phenomena such as absorbance, reflectance, and refractive index. This alloying also brings a change in the dielectric constant of the material such that the plasmonic behavior may shift among different regions (UV, visible, NIR, MWIR, and LWIR). Metal semiconductor eutectic alloy which is widely popular as a soldering material would have scope in futuristic photonic applications due to its tuneable optical properties. In this work, we study the effects of Au and AuGe nanoparticle deposition on GaAs films grown by molecular beam epitaxy (MBE). Au and AuGe thin films (12-nm thick) were annealed in the temperature ranges of 400–800 and 300–700 °C, respectively, to form Au and AuGe nanoparticles. The formation of these nanoparticles was confirmed by scanning electron microscopy (SEM) measurements. Optical absorption spectroscopy measurements showed plasmon resonance peaks at around 670 and 535 nm for the AuGe-deposited 300 °C-annealed sample and Au-deposited 600 °C-annealed sample on sapphire, respectively, thereby confirming the plasmonic effect. Correlation of Raman spectroscopy measurement results with X-ray diffraction measurement results reveal that the transverse optical mode intensity and full width at half maximum of the GaAs (400) peak increased with an increase in annealing temperature, indicating degradation of the crystalline properties of GaAs film at higher annealing temperatures. The highest increments of the photoluminescence (PL) intensities in comparison to that of the bare GaAs film were observed to be 37 and 77% for the Au-deposited 600 °C-annealed and AuGe-deposited 300 °C-annealed samples, respectively. These enhancements of PL spectra are an indication of the significant scattering of photons by Au and AuGe nanoparticles, and they are attributed mainly to the contribution of the local surface plasmon resonance of these nanoparticles. A comparative analysis of PL enhancements revealed that AuGe nanoparticles induced a greater enhancement than Au nanoparticles. The calculated activation energies of the Au-deposited 600 °C-annealed sample, AuGe-deposited 300 °C-annealed sample, and bare GaAs sample were around 18, 24, and 33 meV, respectively. We found one-order increment in peak responsivity of AuGe plasmonic-based trilayer InAs quantum dot detector in comparison to as-grown detector at 80 K. Therefore, this study is expected to be very useful in the realization of high-performance plasmonic-based optoelectronic and sensing devices.