Tunable localized surface plasmon resonances in one-dimensional h-BN/graphene/h-BN quantum-well structure

The graphene/hexagonal boron-nitride(h-BN) hybrid structure has emerged to extend the performance of graphenebased devices. Here, we investigate the tunable plasmon in one-dimensional h-BN/graphene/h-BN quantum-well structures.The analysis of optical response and field enhancement demonstrates that these systems exhibit a distinct quantum confinement effect for the collective oscillations. The intensity and frequency of the plasmon can be controlled by the barrier width and electrical doping. Moreover, the electron doping and the hole doping lead to very different results due to the asymmetric energy band. This graphene/h-BN hybrid structure may pave the way for future optoelectronic devices.     

Enhanced photon emission by field emission resonances and local surface plasmon in tunneling junction

We investigated the photon emission spectra on Ag (111) surface excited by tunneling electrons using a low temperature scanning tunneling microscope in ultrahigh vacuum. Characteristic plasmon modes were illustrated as a function of the bias voltage. The one electron excitation process was revealed by the linear relationship between the luminescence intensity and the tunneling current. Luminescence enhancement is observed in the tunneling regime for the relatively high bias voltages, as well as at the field emission resonance with bias voltage increased up to 9 V. Presence of a silver (Ag) nanoparticle in the tunneling junction results in an abnormally strong photon emission at the high field emission resonances, which is explained by the further enhancement due to coupling between the localized surface plasmon and the vacuum. The results are of potential value for applications where ultimate enhancement of photon emission is desired.     

Surface plasmon dispersion relation and local field enhancement distribution for a deep sinusoidal grating

Two features of light scattering from a deep lossless metallic sinusoidal grating are considered in the limit g/d → ∞, where g and d are the height and periodicity of the grating, respectively. It is found that the surface plasmon dispersion relation is comprised of two flat bands with a frequency gap Δω/ω_p = (√2 − 1)/2, where ω_p is the volume plasmon frequency. For the local field enhancement distribution at the grating's surface, the results show the existence of two qualitatively distinct domains, i.e., λ ⪢ d and λ ⪡ d, where λ denotes the radiation wavelength. In both domains, however, the local field enhancement is larger at the bottom of the troughts than at the top of the peaks. The dressed Rayleigh expansion is used throughout for the analysis.     

Tunable local surface plasmon resonance in liquid-crystal-coated Ag nanoparticles

The far-field and near-field properties of a spherical nematic liquid crystal (NLC) coated metal nanoparticle (NPs) have been investigated in an external field, basing on the quasistatic theory. The resonant wavelength is tunable by varying metallic material of core, anisotropy extent and thickness of liquid crystals (LCs). The field enhancement is along the incident polarization near the outer surface of the shell. The direction of field is reverse in the inner surface comparing with the one if outer shell. In contrast to isotropy shell, the surface plasmon resonance (SPR) shows an obvious red shift and field enhancement near outer surface of the shell always is stronger.     

Multiple Fano resonances in nanorod and nanoring hybrid nanostructures

Multiple Fano resonances of plasmonic nanostructures have attracted much attention due to their potential applications in multicomponent biosensing. In this paper, we propose a series of hybridized nanostructures consisting of a single nanoring and multiple nanorods to generate multiple Fano resonances. One to three Fano resonances are achieved through tuning the number of nanorods. The interaction coupling process between different components of the nanostructures is recognized as the mechanism of multiple Fano resonances. We also theoretically investigate the applications of the produced multiple Fano resonances in refractive index sensing. The specific properties of multiple Fano resonances will make our proposed nanostructures beneficial to high-sensitivity biosensors.     

Investigation of plasmon resonances in local structures by the discrete sources method

The modification of the Discrete Sources Method, which allows one to perform effective numerical analysis of plasmon resonances in local structures, is proposed and realized. The numerical algorithm allows computation of the near field with a high degree of accuracy and tracking resonances that cause an increase of the near-field intensity by several orders of magnitude.     

A broadband cross-polarization conversion anisotropic metasurface based on multiple plasmon resonances

A compact broadband cross-polarization conversion metasurface functioning in the microwave regime is realized and experimentally demonstrated. The metasurface consists of a two-dimensional periodic arrangement of anisotropic doubleslit split-ring-resonator-based unit cells printed on top of a dielectric substrate, backed by metallic cladding. The proposed metasurface converts an x- or y- polarized wave into its orthogonal polarization over a fractional bandwidth of 100% from 5-15 GHz, both for normal as well as oblique incidence. Moreover, the sub-wavelength unit-cell size, thin dielectric substrate,and unique unit-cell design collectively make the response of the metasurface same for both polarizations and insensitive to the incidence angle. The designed structure is fabricated and tested. The measurement and simulation results are found to be consistent with each other.     

长程表面等离子体的增强效应

研究了双层金属薄膜构型中构型参数对长程表面等离子体的影响,并发现了衰减全反射激发方法下长程表面等离子体的增强效应.以特征矩阵算法为基础,通过数值计算构型的反射谱,研究构型参数的变化对反射谱的影响.发现由于衰减全反射激发方法中耦合器的存在导致的非对称特性,会使双层金属薄膜构型中的长程表面等离子体拥有本征模式特性以外的有趣特性,如长程模式得到增强而另一支受到抑制,从而使能量更为集中在希望被激发的一支.研究结果对非对称激发构型中的长程表面等离子体研究具有启发意义.     

Cumulative plasmon field enhancement in finite metal particle chains

The dramatic field enhancement at the extremity of finite chains of strongly coupled gold nanoparticles illuminated under total internal reflection is investigated numerically. We demonstrate that high enhancement factors can be achieved by exploiting the in-plane forward scattering of the particles, with geometries achievable by state-of-the-art lithographic techniques.     

Tunable anticrossing of gated and ungated plasma resonances and enhancement of interlayer terahertz electric field in an asymmetric bilayer of density-modulated two-dimensional electron gases

We study theoretically the terahertz (THz) response of a bilayer of density-modulated two-dimensional electron gases, which we employ to model the actual double-quantum-well electron channel of a grid-gated field-effect transistor in which strong THz photoresponse was recently observed. We have shown that such a system can be driven into the anticrossing regime between gated and ungated plasma resonances by tuning the gate voltage. The amplitude of the interlayer THz electric field in the ungated (double-layered) portions of the channel increases dramatically in the anticrossing regime. This strong interlayer THz electric field may strongly affect interlayer electron tunneling which, in turn, may contribute to the physical mechanism underlying the strong THz photoresponse observed in recent experiments.     

Shape-dependent plasmon resonances of Ag nanostructures

Different silver nanostructures have been rapidly synthesized under microwave irradiation from a solution of silver nitrate (AgNO₃) and β$\beta$-D glucose; neither additional reducing nor capping agent were required in this soft green solution approach. Not only spherical nanoparticles, but also necklace and wires have been synthesized. The plasmon resonances of the synthesized silver nanostructures were tuned by varying the irradiation time and hence by changing size and morphology of nanostructures. The obtained nanostructures were characterized by X-Ray diffraction (XRD), Uv–Vis spectroscopy (Uv–Vis), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The change of peak position and the shape of the absorption spectra were clearly observed during the whole reaction process; in fact, it was evidenced that initially Ag nanoparticles were formed, which, as reaction time elapsed, self-assembled and fused with each other to yield nanowires.     

Quantum theory for plasmon-assisted local field enhancement

We applied quantum theory for nonlocal response and plasmon-assisted field enhancement near a small metallic nanoscale antenna in the limit of weak incoming fields. A simple asymmetric bio-inspired design of the nanoantenna for polarization-resolved measurement is proposed. The spatial field intensity distribution was calculated for different field frequencies and polarizations. We have shown that the proposed design the antenna allows us to resolve the polarization of incoming photons.     

Effect of a local field on the spectra of autoionization resonances

The change of the spectral parameters of a single autoionization resonance with concentration in gaseous media is studied in terms of the isotropic Lorentz-Lorenz model. It is shown that the effect of a local field may result in a shift, narrowing, or broadening of the resonance, as well as in a change in its amplitude and the character of interference of the discrete state with the continuum. By appropriate selection or variation of the partial pressures of substances in the mixture, the separation of a particular regime is possible. An analogy to the laser-induced narrowing of autoionization resonances is drawn.

     

Interplay of plasmon resonances in binary nanostructures

By introducing the difference permittivity ratio η=(ε ₂?ε ₀)/(ε ₁?ε ₀), the Green matrix method for computing surface plasmon resonances is extended to binary nanostructures. Based on the near field coupling, the interplay of plasmon resonances in two closely packed nanostrips is investigated. At a fixed wavelength, with varying η the resonances exhibit different regions: the dielectric effect region, resonance chaos region, collective resonance region, resonance flat region, and new branches region. Simultaneously, avoiding crossing and mode transfer phenomena between the resonance branches are observed. These findings will be helpful to design hybrid plasmonic subwavelength structures.     

Near-field second-harmonic generation induced by local field enhancement

The field near a sharp metal tip can be strongly enhanced if irradiated with an optical field polarized along the tip axis. We demonstrate that the enhanced field gives rise to local second-harmonic (SH) generation at the tip surface thereby creating a highly confined photon source. A theoretical model for the excitation and emission of SH radiation at the tip is developed and it is found that this source can be represented by a single on-axis oscillating dipole. The model is experimentally verified by imaging the spatial field distribution of strongly focused laser modes.     

纳米尺度下的局域场增强研究进展

金属纳米颗粒的等离激元共振引起的局域场增强效应,对显微成像、光谱学、半导体器件、非线性光学等诸多领域都具有极大的应用潜力。尤其是在光学纳米材料领域,通过亚波长金属纳米颗粒与电介质的组合引起局域场增强效应,提高了纳米材料的光学性能,并促进纳米材料在光学领域的应用。本文主要综述几种常见纳米结构所产生的局域场增强效应及其应用,详细介绍并总结了金属纳米材料的不同结构参数与局域场增强的关系及局域场增强在非线性光学、光谱学、半导体器件等领域的应用。未来,随着对金属纳米材料的研究愈发深入,局域场增强的应用将更加广泛,这将对诸多领域的发展产生重要影响。     

Nonlinear polarizability and plasmon resonances in terahertz photoconductivity

The nonlinear polarization of an electron plasma in an incident terahertz (THz) field is examined in an iterated shielded potential approximation. It is found to exhibit resonant behavior when the incident terahertz frequency matches the plasma frequency, with concomitant resonant increase of the dielectric function. Such resonant behavior substantially reduces the effectiveness of the screened impurity scattering potentials, admitting resonant increase of conduction when the THz frequency matches the plasma frequency.     

Understanding plasmon resonances of metal-coated colloidal crystal monolayers

Metal films deposited over two-dimensional colloidal crystals (MFoCC) constitute a low-cost periodic structure with interesting photonic and plasmonic properties. It has previously been shown that this structure exhibits a behaviour similar to the well-known Extraordinary Optical Transmission (EOT) of metallic hole arrays in planar films. Here, we explore the transmission characteristics of AgFoCC by systematic comparison with that of the bare CC. Furthermore with additional reflectivity measurements we evaluate the AgFoCC overall plasmonic response, which, notably, exhibits a strong plasmon absorption band at wavelengths larger than those of the transmitted maximum. By corroborating these results with finite-difference time-domain electromagnetic simulations, we identify a hybrid metal-dielectric propagative mode in the transmission mechanism. On the contrary a strongly localized mode is responsible for the maximum light absorption by this structure. These results shed new light on the current understanding of this highly promising plasmonic structure, being useful for the design of surface-enhanced Raman scattering and enhanced fluorescence substrates.     

Topological structure effect on far-infrared spectra in a GaAs/InAs nanoring

On the basis of the growth mechanism of a GaAs/InAs nanoring,we propose a fine model which reflects the confinement details of real nanoring.Through calculations of the two-electron energy and far-infrared(FIR) spectra,we find that the ring topological structure and electron-electron interaction have great influence on the FIR spectra.The two unknown transition peaks in the experiment are determined theoretically.The theoretical results are in good agreement with the experiments.     

End and central plasmon resonances in linear atomic chains

The existence and nature of end and central plasmon resonances in a linear atomic chain, the 1D analog to surface and bulk plasmons in 2D metals, has been predicted by ab initio time-dependent density functional theory. Length dependence of the absorption spectra shows the emergence and development of collectivity of these resonances. It converges to a single resonance in the longitudinal mode, and two transverse resonances, which are localized at the ends and center of the atom chains. These collective modes bridge the gaps, in concept and scale, between the collective excitation of atomic physics and nanoplasmonics. It also outlines a route to atomic-scale engineering of collective excitations.