Excitation Of Localized Surface Plasmons Research Articles

Fabrication of plasmon length-based surface enhanced Raman scattering for multiplex detection on microfluidic device.

The length of bioreceptors plays an important role in signal enhancement of surface-enhanced Raman scattering (SERS) due to amplification of electromagnetic fields generated by the excitation of localized surface plasmons. Herein, intact antibodies (IgG) and Fab fragments conjugated onto gold nanostar were used to fabricate two kinds of immunosensors for measurement of their SERS signals. Using CA125 as the antigen and Rhodamine-6G (R6G)-conjugated immunogolds, a SERS immunosensor was self-assembled by antigen-antibody interaction. The results showed that the SERS signal from the Fab immunosensor was 2.4 times higher than that of the IgG immunosensor. Furthermore, increased hot-spots by silver atom deposition onto the IgG and Fab immunosensor showed 2.1 and 1.4 times higher signals than before enhancement, respectively. For application, based on the Fab immunosensor, a SERS-compatible microfluidic system was designed for multiplex assays to overcome the drawbacks of conventional assays. This system can measure biological specimens directly from bio fluids instead of using a complex microfluidic device containing separation and detection elements. Four approved biomarkers of breast cancer, including cancer antigen (CA125), HER2, epididymis protein (HE4), and Eotaxin-1, were detected from patient-mimicked serum with limits of 15 fM, 17 fM, 21 fM, and 6.5 fM, respectively. The results indicated that the lengths and geometry of the bioreceptors determined the intensity of SERS signal from the interface and cavity of the sandwich immunosensor. Silver atom deposition at the cavity of the immunosensor increased the SERS signal. Finally, the SERS immunosensor built-in microfluidic system improved the performance of multiplex diagnostics.

Photochemical Growth of Highly Densely Packed Gold Nanoparticle Films for Biomedical Diagnostics

Utilizing ultraviolet photochemical reduction of gold(III) chloride trihydrate (HAuCl4), a new kind of synthesis of a highly dense gold nanoparticle film on a p-type silicon wafer was conducted. Through scanning electron microscopy and atomic force microscopy, the gold nanoparticle film was confirmed to be 90 nm thick, with an average gold nanoparticle size of 125 nm in diameter. To explore applications in surface enhanced Raman spectroscopy (SERS), a protein model of streptavidin and pegylated biotin functionalized to the surface of gold nanoparticle films was employed. Observations indicate that there was a proximity induced excitation of localized surface plasmons due to the densely packed gold nanoparticle film. Excitation of the localized surface plasmons leading to hot spots of SERS activity on the gold nanoparticle film allowing it to act as an eco-friendly and highly sensitive SERS substrate for biomedical diagnostic applications.

Perturbations of Dipole Decay Dynamics Induced by Plasmonic Nano-Antennas — A Study within the Discrete Dipole Approximation

We report a discrete dipole approximation approach to analyse the perturbations induced by silver nano-particles on the decay dynamics of a point-like emitter placed in their proximity. Due to the excitation of localized surface plasmons, metallic nano-particles behave like optical antennas and are able to convert localized fields into free-propagating optical radiation, and vice versa. Field localization and enhancement induce strong changes on the decay dynamics of dipoles located in the perturbed electromagnetic environment, and these can be faithfully quantified within the framework of classical electromagnetism in terms of total, radiative and non-radiative decay rates. The method is tested on benchmark cases, i.e., nanospheres and nano-shells, and it is then applied to analytically-unsolvable shapes such as sharp nano-cones and oxide-covered small nano-antennas. Numerical results reveal 105-order enhancements in the total decay rate of the dipole when located very near to the sharp tip, both with and without a thin Ag2O layer. Moreover, the counter-intuitive behaviour of the cone response in relation to the distance between the metal and the source of the radiation is discussed. Applications span from strong coupling studies to time-resolved fluorescence spectroscopy.

Enhanced detection sensitivity of higher-order vibrational modes of gold nanodisks on top of a GaN nanorod array through localized surface plasmons

We report a method that enables the excitation of localized surface plasmons (LSPs) in a gold nanodisk array by placing each nanodisk on top of a GaN nanorod. When the rod length was much longer than the plasmon penetration depth inside the nanorod, the plasmonic field was found to be localized, and coupling between neighboring gold nanodisks was eliminated. The interaction between LSPs and acoustic vibrations in gold nanodisks was then investigated. Owing to the strong localization of the plasmonic field, weak, higher-order vibrational modes of gold nanodisk could be optically observed. Furthermore, such an LSP-based acoustic sensor could be operated at any angle of incident light. Our study not only provides an approach to excite LSPs in high-density metallic arrays, but also opens one of the possible solutions for the development of highly sensitive sub-terahertz hypersonic sensors with high angle tolerance of incident light.

Block copolymer self assembly for design and vapor-phase synthesis of nanostructured antireflective surfaces

The authors combine block copolymer self assembly with vapor-phase synthesis for design of antireflective thin film coatings. The nanometer-scale features in patterns formed by cylindrical phase block copolymers provide surface topography for vapor-phase growth of semiconductors and metals by oblique angle physical vapor deposition. The authors control the dimensions and density of the synthesized nanotextures through selection of copolymer molecular weight. A layer of aligned, densely packed germanium wire arrays with diameters much smaller than optical wavelengths acts as an effective optical medium, significantly reducing reflections and improving light coupling into a silicon substrate. A synthesized layer of uniformly sized silver nanoparticles provides antireflection instead through optical excitation of localized surface plasmons. The block copolymer-based synthesis approach allows control of particle shape anisotropy, tuning the frequency of plasmon resonances and expanding the spectral range of antireflection.

Рефлектометричні дослідження нанопористих плівок з масивом наночастинок золота

The influence of conditions occurring at the pulsed laser deposition of films with gold nanoparticles on the film porosity has been studied, by using the X-ray reflectometry. The films of two types were obtained by depositing particles (i) from the direct high-energy flow of erosion-torch particles and (ii) from the backward low-energy one. In both cases, the films were deposited either at the residual air pressure p = 10^−2 Pa or in the argon atmosphere with the pressures pAr = 5–100 Pa. In case (i), the film porosity was 0.1% at p = 10^−2 Pa and 1% at pAr ≤ 5 Pa. The plasmon properties of those films are associated with the propagation of surface plasmon-polariton waves. As the argon pressure grew further up to 100 Pa, the porosity increased to approximately 30%. In case (ii), the porosity of films deposited at pAr = 5–100 Pa onto substrates located in the target plane equaled 30 to 70% and depended on the distance from the film to the torch axis. All films with the porosity higher than 20% turned out nanocomposite structures with arrays of gold nanoparticles, which enabled us to observe the excitation of local surface plasmons.

Optofluidic microvalve-on-a-chip with a surface plasmon-enhanced fiber optic microheater.

We present an optofluidic microvalve utilizing an embedded, surface plasmon-enhanced fiber optic microheater. The fiber optic microheater is formed by depositing a titanium thin film on the roughened end-face of a silica optical fiber that serves as a waveguide to deliver laser light to the titanium film. The nanoscale roughness at the titanium-silica interface enables strong light absorption enhancement in the titanium film through excitation of localized surface plasmons as well as facilitates bubble nucleation. Our experimental results show that due to the unique design of the fiber optic heater, the threshold laser power required to generate a bubble is greatly reduced and the bubble growth rate is significantly increased. By using the microvalve, stable vapor bubble generation in the microchannel is demonstrated, which does not require complex optical focusing and alignment. The generated vapor bubble is shown to successfully block a liquid flow channel with a size of 125 μm × 125 μm and a flow rate of ∼10 μl/min at ∼120 mW laser power.

Optical and magneto—optical properties of periodic Co double layer film

Co double layer film (CoDLF) consisting of a disk-array layer and an antidot-array layer, both with square order, was investigated. Both the reflectivity and Kerr spectra of CoDLF show anisotropic effects when the azimuthal angle of incident light changes. From the simulation result of surface plasmon polaritons (SPPs), we attribute the reflectivity minima and Kerr angle maxima in the spectra mainly to the excitation of different diffractional orders' SPPs. More interestingly, the Kerr angle changes sign at specific wavelengths. We attribute these phenomena to the excitation of SPPs and localized surface plasmons (LSPs), and the interaction between them.

Nanoporous Au/SnO/Ag heterogeneous films for ultrahigh and uniform surface-enhanced Raman scattering

There is a strong interest in plasmonic nanostructures that uniformly enhance Raman signals of chemical and biological molecules using surface-enhanced Raman spectroscopy (SERS) for trace detection. Although the resonant excitation of localized surface plasmons of single or assembled metallic nanoparticles can generate large electromagnetic fields, their SERS effects suffer from poor reproducibility and uniformity, limiting their highly reliable and stable applications. Here, we report self-supported large-scale nanoporous hybrid films with high density and uniform hot spots, produced by the implantation of SnO nanoparticles into nanoporous Au/Ag bimetallic films (NP Au/SnO/Ag) for the trace detections of both resonant and non-resonant molecules. The NP Au/SnO/Ag films exhibit extraordinary SERS enhancements, which increase with the increasing density of Au/SnO/Ag sandwich protrusions, as a result of the formation of abundant and uniform hot spots. The nanogaps in their wrinkled films further improve the capability to detect molecules at single molecular levels, making the hybrid films promising SERS-active substrates with superior reproducibility and reliability for applications in life science and environment protection.

A generalized non-local optical response theory for plasmonic nanostructures.

Metallic nanostructures exhibit a multitude of optical resonances associated with localized surface plasmon excitations. Recent observations of plasmonic phenomena at the sub-nanometre to atomic scale have stimulated the development of various sophisticated theoretical approaches for their description. Here instead we present a comparatively simple semiclassical generalized non-local optical response theory that unifies quantum pressure convection effects and induced charge diffusion kinetics, with a concomitant complex-valued generalized non-local optical response parameter. Our theory explains surprisingly well both the frequency shifts and size-dependent damping in individual metallic nanoparticles as well as the observed broadening of the crossover regime from bonding-dipole plasmons to charge-transfer plasmons in metal nanoparticle dimers, thus unravelling a classical broadening mechanism that even dominates the widely anticipated short circuiting by quantum tunnelling. We anticipate that our theory can be successfully applied in plasmonics to a wide class of conducting media, including doped semiconductors and low-dimensional materials such as graphene.

In-Situ Surface-Enhanced Raman Scattering Observation of Intermediate Species at Plasmon-Induced Water Oxidation Process Using Au Nanostructures on Titanium Oxide Single Crystal

Recent attempts to use localized surface plasmon resonance (LSPR) for photoexcitation of molecules suggests the possibility to modify the excitation processes and the selection rule of electrons [1, 2]. Among several choices of the applications of LSPR, the system of Au-nanostructured titanium dioxide (TiO2) is one of promising approaches for plasmon-enhanced photoenergy conversion under illumination of visible and near infrared light . It has been suggested that photo-induced electron injection from the Au structure into the TiO2 conduction band occurs by localized surface plasmons excitation. In this system, Au-nanostructured TiO2 enabled efficient water oxidation owing to the excitation of multiple electrons and holes generated by near-infrared light, and the evolution of O2 via four-electron oxidation of water. In this work, size-controlled Au-nanostructures was fabricated on a TiO2 single crystalline by means of angle-resolved nanosphere lithography (AR-NSL) to measure surface-enhanced Raman scattering (SERS) from the intermediate species at water oxidation process at the electrode-electrolyte interface under near-infrared light irradiation. SERS spectra were obtained from the size-controlled Au nano-dimer (Au-NSL) arrays on a single crystalline TiO2 immersed in 0.1 M NaF solution. For comparison, we also measured SERS spectra from Au-NSL on indium tin oxide (ITO). Observed spectral feature at ~580 cm-1 can be attributed to Au-O stretching vibrations of adsorbates. This wavenumber suggests that adsorbed hydroxide anion with Au-O bonding are produced as intermediate of water oxidation reaction at Au/TiO2. It should be noteworthy that Au-O vibration was observed at more negative potential on Au/TiO2 than that of Au/ITO. Negative shift is attributed to the water oxidation on Au/TiO2 is assisted by electrons and holes generated by plasmon-induced excitation.

Experimental Study on Localized Surface Plasmon Mode Hybridization in the Near and Mid Infrared

We investigate plasmon excitations within a regular grating of double-layered gold/insulator nanoparticles in the infrared and visible spectral region. Provided a flat gold film as substrate, strong coupling between the localized surface plasmon modes and their image-like excitations in the metal is observed. The interaction results in a strong red shift of the plasmon mode as well as the splitting of the modes into levels of different angular momenta, often referred to as plasmon hybridization. The diameters of the nanoparticles are designed in a way that the splitting of the resonances occurs in the spectral region between 0.1 and 1 eV, thus being accessible using an infrared microscope. Moreover, we investigated the infrared absorption signal of gratings that contain two differently sized nanoparticles. The interaction between two autonomous localized surface plasmon excitations is investigated by analyzing their crossing behavior. In contrast to the interaction between localized surface plasmons and propagating plasmon excitations which results in pronounced anticrossing, the presented structures show no interaction between two autonomous localized surface plasmons. Finally, plasmon excitations of the nanostructured surfaces in the visible spectral region are demonstrated through photographs acquired at three different illumination angles. The change in color of the gratings demonstrates the complex interaction between propagating and localized surface plasmon modes.

Theoretical analysis of Bloch mode propagation in an integrated chain of gold nanowires

The eigenmodes analysis of Bloch modes in a chain of metallic nanowires (MNWs) provides a significant physical understanding about the light propagation phenomena involved in such structures. However, most of these analyses have been done above the light line in the dispersion relation, where the Bloch modes can only be excited with radiative modes. By making use of the Fourier modal method, in this paper we rigorously calculate the eigenmode and mode excitation of a chain of MNWs via the fundamental transverse magnetic (TM) mode of a dielectric waveguide. Quadrupolar and dipolar transversal Bloch modes were obtained in an MNW chain embedded in a dielectric material. These modes can be coupled efficiently with the fundamental TM mode of the waveguide. Since the eigenmodes supported by the integrated plasmonic structure exhibit strong localized surface plasmon (LSP) resonances, they could serve as a nanodevice for sensing applications. Also, the analysis opens a direction for novel nanostructures, potentially helpful for the efficient excitation of LSPs and strong field enhancement.

Gold nanoparticle coated U-bend fibre optic probe for localized surface plasmon resonance based detection of explosive vapours

In this study, we report a chemical sensor utilizing localized surface plasmon resonance (LSPR) of gold nanoparticles (GNP) for vapour phase detection of explosives like 2,4,6-trinitrotoluene (TNT) and cyclotrimethylenetrinitramine (RDX). The GNP were immobilized on a U-bend fibre optic sensor probe of 200μm core diameter and bend diameter of 1.5mm for evanescent field based excitation of localized surface plasmons of the GNP. The immobilized GNP were functionalized with receptor molecules viz. 4-mercaptobenzoic acid (4-MBA), l-cysteine and cysteamine to provide the binding sites for the nitro-based explosive molecules. Binding of the explosive analytes to the surface moieties of the GNP was found to elicit refractive index changes in the environment surrounding the nanoparticles. This led to changes in the absorbance characteristics of the GNP-LSPR spectrum. Furthermore, the GNP coated probes modified with l-cysteine and cysteamine exhibited a high degree of selectivity towards TNT. The detection limit of the LSPR fibre optic probe for TNT vapours was found to be in the lower parts per billion (ppb) with further scope for improvement.

Extraordinary terahertz transmission through subwavelength spindle-like apertures in NbN film

We studied numerically the temperature dependent extraordinary terahertz transmission through niobium nitride (NbN) film perforated with subwavelength spindle-like apertures. Both the resonant frequency and intensity of extraordinary terahertz transmission peaks can be greatly modified by the transition of NbN film from the normal state to the superconducting state. An enhancement of the (±1,0) NbN/magnesium oxide (MgO) peak intensity as high as 200% is demonstrated due to the combined contribution of both the superconducting transition and the excitation of localized surface plasmons (LSPs) around the apertures. The extraordinary terahertz transmission through spindle-like hole arrays patterned on the NbN film can pave the way for us to explore novel active tuning devices.

Strong three-dimensional field localization and enhancement on deep sinusoidal gratings with two-dimensional periodicity

The study of total light absorption due to excitation of localized surface plasmons on deep metallic crossed gratings having a sinusoidal profile with a two-dimensional periodicity shows a very strong increase in the electric field intensity, reaching 800 times the incident intensity. The region with high intensity is strongly localized at the groove top and is characterized by a volume much smaller than the diffraction limit, both in transverse direction along the grating plane, and in longitudinal direction when going away from the grating surface. The field enhancement and its localization are much more pronounced than in shallow gratings.

A localized surface plasmon resonance-based optical fiber sensor with sub-wavelength apertures

An optical fiber refractive index sensor based on localized surface plasmon (LSP) resonances within resonant subwavelength apertures in a gold film is demonstrated. The excitation of LSPs within the apertures in the film on the optical fiber endfaces provided the basis of a compact refractive index sensor for liquids. A sensitivity of 755 ± 12 nm per refractive index unit was demonstrated for this sensor.

Efficiency enhancement of polymer solar cells by localized surface plasmon of Au nanoparticles

We demonstrate the improvement of power conversion efficiency (PCE) in bulk heterojunction polymer solar cells based on blended poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester by introducing 40 nm Au nanoparticles (NPs) with various concentrations. The Au NPs were deposited on indium-tin-oxide (ITO) substrates by spin-coating from colloidal solution prior to deposition of poly (3,4-ethylene dioxythiophene:poly (styrene sulfonate) (PEDOT:PSS) buffer layer. It has been found that both short-circuit current density and PCE increase after incorporating Au NPs between ITO and PEDOT:PSS layer, and a suitable area density of Au NPs is required to achieve a maximum enhancement of device efficiency. The PCE of solar cells has been increased from 3.50% to 3.81% with 0.9 wt. % Au NPs. The PCE enhancement is attributed to the localized surface plasmon excitation of Au NPs. The method employed herein is a kind of simple and convenient solution process, and it has great potential in future practical applications.

Plasmonic fractals: ultrabroadband light trapping in thin film solar cells by a Sierpinski nanocarpet

Plasmonic Sierpinski nanocarpet as back structure for a thin film Si solar cell is investigated. We demonstrate that ultra-broadband light trapping can be obtained by placing square metallic nanoridges with Sierpinski pattern on the back contact of the thin film solar cell. The multiple-scale plasmonic fractal structure allows excitation of localized surface plasmons and surface plasmon polaritons in multiple wavelengths leading to obvious absorption enhancements in a wide frequency range. Full wave simulations show that 109 % increase of the short-circuit current density for a 200 nm thick solar cell, is achievable by the proposed fractal back structure. The amount of light absorbed in the active region of this cell is more than that of a flat cell with semiconductor thickness of 1,000 nm.

An Electrically Excited Nanoscale Light Source with Active Angular Control of the Emitted Light

We report on the angular distribution, polarization, and spectrum of the light emitted from an electrically controlled nanoscale light source. This nanosource of light arises from the local, low-energy, electrical excitation of localized surface plasmons (LSP) on individual gold nanoparticles using a scanning tunneling microscope (STM). The gold nanoparticles (NP) are chemically synthesized truncated bitetrahedrons. The emitted light is collected through the transparent substrate and the emission characteristics (angular distribution, polarization, and spectrum) are analyzed. These three observables are found to strongly depend on the lateral position of the STM tip with respect to the triangular upper face of the gold NP. In particular, the resulting light emission changes orientation when the electrical excitation via the STM tip is moved from the base to the vertex of the triangular face. On the basis of the comparison of the experimental observations with an analytical dipole model and finite-difference time-domain (FDTD) calculations, we show that this behavior is linked to the selective excitation of the out-of-plane and in-plane dipolar LSP modes of the NP. This selective excitation is achieved through the lateral position of the tip with respect to the symmetry center of the NP.