Optimal Aspect Ratio and Excitation Spectral Region of LSPR Sensors Using Individual Au Dimmeric Nanoplates

Plasmonics - Both the excitation wavelength and aspect ratio AR are key parameters controlling the refractive index sensitivity performances of plasmonic sensors of individual Au nanoplate dimmers....     

Triangular Au�CAg Nanoframes with Tunable Surface Plasmon Resonance Signal from Visible to Near-Infrared Region

In recent years, metal hollow nanostructures are intriguing to be synthesized and studied because they exhibit unique surface plasmonic properties. Although many methods for tuning the surface plasmonic absorption peaks of silver nanostructures have been reported, it still remains a great challenge to produce hollow Ag nanostructure with controllable surface plasmon resonance (SPR) via a facile method. In this paper, triangular Au–Ag nanoframes were successfully fabricated using triangular silver nanoplates as templates, through galvanic replacement reaction between the silver nanoplates and HAuCl₄, exhibiting tuneable SPR response from visible (605 nm) to near-infrared region (1,235 nm).     

Search of Extremely Sensitive Near-Infrared Plasmonic Interfaces: A Theoretical Study

The sensitivity of the wavelength position of localized surface plasmon resonance (LSPR) in metal nanostructures to local changes in the refractive index has been widely used for label-free detection strategies. Tuning the optical properties of the nanostructures from the visible to the infrared region is expected to have a drastic effect on the refractive index sensitivity. Here, we theoretically investigate the optical response of a newly designed plasmonic interface to changes in the bulk refractive index by the finite difference time domain method. It consists of a structured interface, where the planar interface is superposed with dielectric pillars 30 nm in height and 125 nm in length with a separation distance of 15 nm. The pillars are covered with U-shaped gold nanostructures of 50 nm in height, 125 nm in length, and 5 nm of gold base thickness. The whole structure is finally covered with a 5-nm thick dielectric layer of n ₂?=?2.63. This plasmonic structure shows bulk refractive index sensitivities up to 1750 nm/RIU (RIU : refractive index unit) in the near infrared (λ?=?2621 nm). The enhanced sensitivity is a consequence of the extremely enhanced electrical field between the gold nanopillars of the plasmonic interface.     

Ag Nanostructures Produced by Glancing Angle Deposition with Remarkable Refractive Index Sensitivity

Glancing angle deposition is a powerful method for direct fabrication of nanostructures on various substrates. In this research, GLAD method has been used to fabricate Ag nanostructures with columnar morphology for refractive index sensing applications. The morphology and plasmonic properties of the nanostructures are controlled by changing deposition parameters such as glancing angle, speed of azimuthal rotation of the substrate, and the height of deposited nanostructures. The results show that increasing the deposition thickness from 200 to 500 nm leads to narrowing the plasmonic peak, which mainly relates to increment of the distance between larger nanostructures. By changing the glancing angle between 86° to 80°, the narrowest plasmonic peak corresponding to the greatest sensitivity has been obtained for the film deposited at the angle of 82°. Also, increment of the rotation speed of the samples leads to narrowing of the plasmonic peaks. By measuring the refractive index sensitivity (RIS) of the nanostructures, a best sensitivity of 154 nm/RIU has been obtained. Finally, we investigated the stability of Ag nanostructures in deionized water by introducing a new stabilizing technique in which a thin Au layer is coated on the Ag nanostructures. This technique has the merits of simultaneously protecting the Ag nanostructures against oxidation and keeping their refractive index sensitivity high enough for long time usages.     

Higher Order Plasmonic Modes Excited in Ag Triangular Nanoplates by an Electron Beam

Ag triangular nanoplates are known to generate strong plasmonic resonances when excited by both light and electron beams. Experimental electron energy-loss spectra (EELS) and maps were acquired using an aberration-corrected JEOL-ARM microscope. The corner, edge and centre modes that are often observed in such structures were also observed in these measurements. In addition, novel higher order internal modes were observed and were found to be well reproduced by theoretical calculations using boundary element method (BEM). These modes are “dark modes” so are not observed in the optical extinction spectra. They are confined surface propagating modes and are analogous to laser cavity modes.     

Plasmonic Biosensor Based on Triangular Au/Ag and Au/Ag/Au Core/Shell Nanoprisms onto Indium Tin Oxide Glass

Gold–silver core–shell triangular nanoprisms (Au/AgTNPs) were grown onto transparent indium tin oxide (ITO) thin film-coated glass substrate through a seed-mediated growth method without using peculiar binder molecules. The resulting Au/AgTNPs were characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction, UV–vis spectroscopy, and cyclic voltammograms. The peak of dipolar plasmonic resonance was located at near infrared region of ～700 nm, which showed the refractive index (RI) sensitivity of 248 nm/RIU. Moreover, thin gold shells were electrodeposited onto the surface of Au/AgTNPs in order to stabilize nanoparticles. Compared with the Au/AgTNPs, this peak of localized surface plasmon resonance (LSPR) was a little red-shift and decreased slightly in intensity. The refractive index sensitivity was estimated to be 287 nm/RIU, which showed high sensitivity as a LSPR sensing platform. Those triangular nanoprisms deposited on the ITO substrate could be further functionalized to fabricate LSPR biosensors. Results of this research show a possibility of improving LSPR sensor by using core–shell nanostructures.     

Dynamically Tunable by Kerr Effect Multichannel Filter Based on Plasmon Induced Transparencies at Optical Communication Range

Dynamically tunable multichannel filter based on plasmon-induced transparencies (PITs) is proposed in a plasmonic waveguide side-coupled to slot and rectangle resonators system at optical communication range. The slot and rectangle resonators in this system can be regarded as radiative or dark resonators as same as the radiative or dark elements in the metamaterial structure with the help of the evanescent coupling. The multiple PIT responses which can enable the realization of nanoscale filter with four channels are originated from the direct near-field coupling and indirect phase couple through a plasmonic waveguide simultaneously. Moreover, the magnitudes and bandwidths of the filter can be efficiently tuned by controlling of the geometric parameters such as the coupling distances and the pump light-induced refractive index change of the Kerr material which is embedded into the metal-dielectric-metal waveguide between the radiative resonators.     

The Optimal Aspect Ratio of Gold Nanorods for Plasmonic Bio-sensing

The plasmon resonance of metal nanoparticles shifts upon refractive index changes of the surrounding medium through the binding of analytes. The use of this principle allows one to build ultra-small plasmon sensors that can detect analytes (e.g., biomolecules) in volumes down to attoliters. We use simulations based on the boundary element method to determine the sensitivity of gold nanorods of various aspect ratios for plasmonic sensors and find values between 3 and 4 to be optimal. Experiments on single particles confirm these theoretical results. We are able to explain the optimum by showing a corresponding maximum for the quality factor of the plasmon resonance.     

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.     

A Novel Plasmonic Zone Plate Lens Based on Nano-Slits with Refractive Index Modulation

Conventionally, plasmonic lenses introduce a phase delay distribution across their surfaces by modulating the dimensions of nanostructures within a metal film. However, there is very limited modulation of the phase delay due to the small dependence of the mode propagation constant on the structure dimensions. In this paper, a novel design of plasmonic zone plate lenses (PZPL) with both slit width and refractive index modulation is proposed to enable integrating more slits in a fixed lens aperture with the extended phase delay range and, therefore, greatly enhance the performance of the devices. More than three-time enhancement of the light intensity at the focus is achieved compared to the structure with only slit width modulation. Like a conventional immersion system, a PZPL embedded in a dielectric is found to have a further improved focusing performance, where light is focused down to a 0.44λ spot using a PZPL with an aperture of 12λ and a focal length of 6λ. Dispersive light-focusing behaviour is also analysed and the modulation of the focal length by colour has a potential application in stacked image sensors and multi-dimensional optical data storage.     

Optical Properties of Silver Hollow Triangular Nanoprisms

The extinction spectra and electric field distributions of hollow triangular nanoprisms are calculated using the discrete dipole approximation method and compared with those of the solid triangular nanoprisms. When light propagates along the prisms, the main plasmon peaks of hollow triangular nanoprisms red shift compared with those of the solid triangular nanoprisms. At the main plasmon peaks, the hollow triangular nanoprisms provide more hot spots than the solid triangular nanoprisms. Therefore, the hollow triangular nanoprisms are more surface-enhanced Raman scattering active than the solid triangular prisms and can be used to detect small amount of molecules. For the hollow triangular nanoprism, although the local electric field distribution extremely relates to the incident polarization, the extinction spectra are independent of the incident polarization. In addition, the main plasmon peaks red shift linearly with the edge length, while they blue shift exponentially with the increase of the thickness of the hollow triangular nanoprisms. These results could be used to engineer hollow triangular nanoprisms for specific plasmonic applications.     

Characterisation of a Gold Nanorod Sol–Gel Utilising Inter-particle Coupling to Yield High Refractive Index Sensitivity

The development of gold nanorod plasmonic sol?Cgel polymer is presented and characterised with respect to its sensitivity to refractive index change. Structural characterisation of the polymer was conducted using Raman microscopy and energy dispersion spectroscopy while plasmonic function was investigated using UV/VIS spectroscopy. Refractive index sensitivities utilising the peak wavelength shift of the localised surface plasmon resonance were shown to be of the order of 2,338?nm per refractive index unit; in addition, peak absorbance was considered as an alternative measure. Furthermore, demonstration of sensitivity to biomolecule interaction has been shown as a model study. Strong photoluminescence was observed during Raman studies that restricts the potential use of the polymer as a surface-enhanced Raman substrate.     

Rational Selection of Nanorod Plasmons: Material, Size, and Shape Dependence Mechanism for Optical Sensors

The localized surface plasmon resonance dependence on surrounding medium refractive index of Ag, Al, Au, and Cu nanoparticles is examined by electrodynamic approach. The refractive index sensitivity and sensing figure of merit (FOM) dependence of selected metal nanoparticles with similar geometry shows that although, sensing relevant parameters are shape (i.e., aspect ratio), and material dependent below the width 20 nm, but above this size these parameters are material independent under similar geometrical conditions. We have concluded that at optimum size, however, Al shows much higher refractive index sensitivity (RIS) in comparison to Au, Cu, and Ag, but FOM is higher for Ag in comparison to other metals. The observed sensing behavior is expected due to parameters like surface scattering, dynamic depolarization, radiation damping, and interband transitions, which may influence the nanorod plasmons.     

A MIM Filter Based on a Side-Coupled Crossbeam Square-Ring Resonator

In this paper, a surface plasmon polarition filter based on a side-coupled crossbeam square-ring resonator is presented and the transmission characteristics of the filter are analyzed by using the finite difference time domain method. The simulation results indicate that the proposed resonator supports multiple resonant modes, and these resonant modes can be adjusted all together by varying the length and refractive index of the outer square ring or partially adjusted by changing the width and refractive index of the crossbeam. By adding two coupled waveguides to the structure, we further demonstrate that a multiple wavelength download filter can be achieved via different coupled waveguides. The proposed structure has potential applications in plasmonic integrated circuits.     

Second-order approximations for selection coefficients at polygenic loci

I determine the second-order approximation for the phenotypic distribution of a quantitative trait, ignoring the effects of epistasis and linkage disequilibrium, conditioned on the presence of a specified genotype at one underlying locus of small effect. I demonstrate that this approximation has an error that is third order in the allelic or genotypic effects, independent of the form of the phenotypic distribution. I also show that the approximation of analogous form for the phenotypic distribution conditioned on the presence of a specified allele at a single locus is also correct to second order. Both approximations allow for dominance and are consistent in the sense that computing marginal fitnesses from approximations based on genotypic deviations and those based on average allelic effect yield the same answers. Surprisingly, the second-order approximations derived here yield the same approximation for dynamics at a single locus as first-order approximations used earlier thus justifiying earlier stability computations based on these first-order approximations.     

Tunable Terahertz Filters Based on Graphene Plasmonic All-Dielectric Metasurfaces

A tunable terahertz filter based on graphene plasmonic all-dielectric metasurfaces is proposed and investigated numerically by using the finite-difference time-domain (FDTD) method. Especially, hybrid all-dielectric metasurfaces are used to make a whole single-sheet graphene forms two different conductivity patterns with the same gate voltage. The simulated results show that resonance wavelength is shifted significantly with the change of gate voltage. Besides, the transmittance spectra are also shifted with the change of the width of SiC, and the filter shows a polarization-dependent modulation property for the length and the width of SiC being 480 and 320 nm, respectively. In addition, the filter can be applied for refractive sensing because the transmittance spectra are shifted with the change of the background refractive index. The study could provide availability for versatile tunable terahertz graphene plasmonic metasurfaces.     

Design and Analysis of Infrared Tunable All-Optical Filters Based on Plasmonic Hybrid Nanostructure Using Periodic Nanohole Arrays

A tunable high transmission optical bandpass filter based on a plasmonic hybrid nanostructure, composed of a periodic array of nanocircles and nanoholes combining two isolated waveguides is introduced in this paper. The presented design improves the coupling, which results in a higher transmission peak. To study the filtering operation, different topologies are investigated. The transmission properties and the resonance wavelengths are adjusted by sweeping various geometrical parameters. A multimode spectrum for each of the topologies is obtained. A tunable bandgap and bandwidth can be obtained by adjusting the refractive index of the periodic nanostructure. We have reached a maximum quality factor and a small full width at half-maximum bandwidth with the maximum transmission values greater than 80%. The advantages of the presented structures which include the benefits of both plasmonic and periodic nanostructures are tunability, high detection resolution, and integrability at the nanoscale for optical applications.

     

四川缙云山森林群落林窗边缘效应的研究

 本文运用Shannon—Wiener物种多样性指数、Simpson生态优势度指标以及边缘效应强度指数,对缙云山亚热带森林群落林窗的边缘效应现象进行了初步研究。29个林窗样地不同部位上述指标的测定表明用物种多样性指数拟合边缘效应测度通式,缙云山亚热带森林群落林窗的边缘效应强度值为1—3左右;而用生态优势度值拟合边缘效应强度测式,则上述边缘效应强度值为0.1一1.2左右。研究表明林窗边缘区由于边缘效应的作用有增大物种多样性的趋势。而林窗面积、所处坡向以及林窗所在的森林群落类型,都对处于不同发育阶段的林窗的边缘效应强度具有影响。在此基础上,本文对林窗边缘效应强度的变化规律、效应性质、测定意义以及对森林营造和优化管理上的作用进行了一定探讨。