Numerical study of guided modes in arrays of metallic nanowires

We numerically investigate the band structure and guided modes within arrays of metallic nanowires. We show that bandgaps appear for a range of array geometries and that these can be used to guide light in these structures. Values of attenuation as low as 1.7 dB/cm are predicted for arrays of silver wires at communications wavelengths. This is more than 100 times smaller than the attenuation of the surface plasmon polariton modes on a single silver nanowire.     

Gap plasmon modes resolved by ultraflat nanogap and linear polarization in terrace-stepped hexagonal boron nitride spacer sandwiched by Ag nanowire and metal substrates

We investigate the nanogap and polarization-resolved excitation of gap plasmon modes using terrace-stepped hexagonal boron nitride (hBN) sandwiched between Ag nanowires and Au substrates for a metal–insulator–metal gap structure. The gap plasmon modes in the proposed hybrid structure are dominantly excited by a P-polarized incident light, which is supported by full-wave numerical simulations. Plasmon mode evolution for various hBN spacer thicknesses ranging from 5 to 90 nm shows that optical signals acquired via unpolarized dark-field mapping spectroscopy are primarily due to the optical scattering of the P-polarized incident light. Moreover, this plasmonic mode changes significantly from gap plasmon mode to Fabry–Perot-type resonance in a hBN thickness of 50–90 nm. Our analysis reveals that the proposed hybrid structure based on Ag nanowires and stepped hBN provides a well-defined gap thickness and is a robust platform for analyzing gap plasmon modes.     

Terahertz plasmon and surface-plasmon modes in cylindrical metallic nanowires

We present a theoretical study on collective excitation modes associated with plasmon and surface-plasmon oscilla- tions in cylindrical metallic nanowires. Based on a two-subband model, the dynamical dielectric function matrix is derived under the random-phase approximation. An optic-like branch and an acoustic-like branch, which are free of Landau damp- ing, are observed for both plasmon and surface-plasmon modes. Interestingly, for surface-plasmon modes, we find that two branches of the dispersion relation curves converge at a wavevector qz = qrnax beyond which no surface-plasmon mode exists. Moreover, we examine the dependence of these excitation modes on sample parameters such as the radius of the nanowires. It is found that in metallic nanowires realized by state-of-the-art nanotechnology the intra- and inter-subband plasmon and surface-plasmon frequencies are in the terahertz bandwidth. The frequency of the optic-like modes decreases with increasing radius of the nanowires, whereas that of the acoustic-like modes is not sensitive to the variation of the radius. This study is pertinent to the application of metallic nanowires as frequency-tunable terahertz plasmonic devices.     

Strong coupling of localized and surface plasmons to microcavity modes

We strongly couple surface plasmon modes on a thin metal layer via localized plasmons of nanowires to photonic microcavity modes. In particular, we place an array of nanowires close to a mirror and position a second mirror at Bragg distance. The coupling becomes evident from an anticrossing of the resonances in the dispersion diagram. We experimentally determine the dispersion by applying external pressure to the microcavity and find excellent agreement with simulations.     

The influence of wire shape on surface plasmon mode distribution

The influence of the nanowire shape on the excitation of surface plasmon polaritons at metallic nanowire arrays is studied numerically. For a system of silver nanowires housed on a polymer substrate, nanowires with rectangular and elliptical cross sections are compared. It was found that in the case of rectangular nanowires the excitation efficiency is higher for surface plasmons at the polymer–metal interface than for surface plasmons at the air–metal interface. Conversely, in the case of elliptical nanowires the air–metal plasmon modes are stronger. Further, it is noted that the nanowire shape directly influences the position of the surface plasmon resonance.     

Effects of shape of terminus on excitation of surface plasmon modes on metal nanowires

The effects of the shape of a nanowire terminus on the excited surface plasmon polariton （SPP） modes are investigated. The conical terminus and terminus cut at a certain angle are studied. For the first time, the quantitative mode decompositions are carried out to derive the full information about excited SPP modes. It is demonstrated that tuning the shape of the terminus provides an effective method to control the composition of excited SPP modes on metal nanowires. It is especially found that some important patterns, such as the pure TM0 mode and the superposition of TM0 and HE＋1 or HE-1 modes, can be generated by some specific shapes of the terminus, whereas there is no way to produce these patterns using flat-end nanowires.     

Ag/ZnO纳米结构的荧光增强效应

利用化学合成方法制备了Ag纳米线和ZnO量子点。对这两种纳米结构的表面形貌、晶体结构和光学性质分别进行了研究。结果表明：Ag纳米线和ZnO量子点均为单晶结构,平均直径分别为160 nm和5 nm左右。将Ag纳米线混入ZnO量子点可以使其紫外荧光显著增强,其中位于345 nm和383 nm 的荧光分别增强30倍和12倍。这与Ag纳米线和ZnO量子点混合体系的局域表面等离子体共振耦合吸收峰位相一致,说明该体系存在两种共振耦合模式。该研究结果为将来开发ZnO基纳米发光器件提供了一条新的途径。     

Modification of the photoluminescence anisotropy of semiconductor nanowires by coupling to surface plasmon polaritons

We demonstrate efficient modification of the polarized light emission from single semiconductor nanowires by coupling this emission to surface plasmon polaritons on a metal grating. The polarization anisotropy of the emitted photoluminescence from single nanowires is compared for wires deposited on silica, a flat gold film, and a shallow gold grating. By varying the orientation of the nanowire with respect to the grating grooves, the large intrinsic polarization anisotropy can be either suppressed or enhanced. This modification is interpreted by the appearance of an additional emission channel induced by surface plasmon polaritons and their conversion to p-polarized radiation at the grating.     

Waveguide plasmon polaritons in metal-dielectric photonic crystal slabs

The optical properties of arrays of metallic (gold) nanowires deposited on dielectric substrates are studied both theoretically and experimentally. Depending on the substrate, Wood’s anomalies of two types are observed in the transmission spectra of such planar metal-dielectric photonic crystals. One of them is diffraction (Rayleigh) anomalies associated with the opening of diffraction channels to the substrate or air with an increase in the frequency of the incident light. The other type of Wood’s anomaly is resonance anomalies associated with excitation of surface quasi-guided modes in the substrate. Coupling of the quasi-guided modes with individual nanowire plasmons brings about the formation of waveguide plasmon polaritons. This effect is accompanied by a strong rearrangement of the optical spectrum and can be utilized to control the photonic bands of metal-dielectric photonic crystal slabs.     

Optical second harmonic generation with surface plasmons in piezoelectric crystals

We report the first observation of reflected optical second harmonic generation of light due to excitation of fundamental and harmonic surface plasmon modes at the interface between a metallic film and a piezoelectric crystal. Excitation of the fundamental surface plasmon mode produces a large enhancement of the piezoelectric generated reflected harmonic light whereas excitation of the nonlinear mode produces only a small minimum in the background surface generated harmonic light.     

利用重离子径迹模版法制备出的钯纳米线阵列的表面等离激元共振现象研究(英文)

利用电化学沉积在重离子径迹模版中制备出了不同直径的一维钯纳米线。利用扫描电子显微镜、透射电子显微镜和X射线衍射等多多种手段对制得的钯纳米线进行了形貌和结构表征。利用紫外可见光谱仪分析了钯纳米线的光学响应,发现钯纳米线存在表面等离子体共振现象。随着纳米线直径和长度的增加,其表面等离子体共振峰位发生红移;通过改变光谱测试中激发光的入射角度,其表面等离激元共振模式会随着角度的增大而变多,这可能是在横向振动模式的基础上激发了沿纳米线长度方向振动的纵向模式。与此同时,基于时域有限差分法对钯纳米线的表面等离子体共振特性进行数值模拟,结果与实验符合较好。Palladium nanowires with varied diameters were fabricated using ion-track templates coupled with electrochemical deposition. The morphology and crystallographic structure were characterized with Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray diffraction (XRD). The plasmonic responses of the as-prepared nanowires were investigated by UV-Vis-NIR spectroscopy and the simulations based on the finite-difference time-domain algorithm. The results demonstrate that the surface plasmon resonances of Pd nanowire are sensitive to the wire geometry, but also influenced by the incidence angle of light. The frequency of the transverse dipolar plasmon resonance of nanowire arrays shifts within a wide range from visible to near infrared. With increasing of wires' diameter or length, the resonance peak shifts to the red. With increasing of incident angle, a new peak appears, which is possibly assigned to the excitation of the longitudinal resonance. In addition, numerical simulations disclose that propagating surface plasmon polaritons can be excited on the palladium nanowires and the wavelength of the resonance peak is in good agreement with the experimental results.     

Elementary-field modeling of surface-plasmon excitation with partially coherent light

We present a mathematical model and its numerical implementation for the analysis of the interaction of spatially partially coherent electromagnetic fields with micro- and nanostructured objects. The model is based on the decomposition of the incident field into a set of fully coherent but mutually uncorrelated elementary field modes, and the use of the Fourier Modal Method (FMM) with the S-matrix propagation algorithm. We apply the model to studies of the excitation of surface plasmons in thin metallic slabs, nanowires, and resonant structures. We demonstrate, e.g., that the plasmon excitation efficiency is not essentially affected by the degree of spatial coherence. However, certain plasmon interference effects can be efficiently smoothed out by using illumination with reduced coherence.     

Transmission of light through coaxial and non-coaxial double-layer gold slit arrays

We investigate numerically the transmittance of light through gold double-layer structures with periodic coaxial and non-coaxial slits. We attribute the enhanced optical transmission to the surface plasmon resonance collaborating with the localized waveguide resonance. The transmission spectra and the surface electric fields are used to characterize the resonances of both types. For the coaxial system, with the increase of the slit width of the second layer, the resonance peak values of both types increase sharply at first until the two layers have the same slit width and then decrease dramatically; additionally, the center of the localized waveguide resonance peak shifts to shorter wavelength noticeably, but the surface plasmon peak center moves negligibly. As regards the non-coaxial structure, it shows a similar behavior of the localized waveguide resonance peak to that of the coaxial one; however, the surface plasmon resonance peak behaves in a different way. These results may be associated with the surface plasmon coupling modes and the Fabry–Perot cavity modes in the double-layer structure.     

Vavilov-Cherenkov like effect in metal nanofilms

The so-called surface plasmon polaritons, i.e., natural waves with a low phase velocity (much lower than the speed of light in a vacuum), exist in silver, gold, and copper nanofilms and nanowires. Electrons that are relatively slow in comparison with those that emit Cherenkov light in a homogeneous medium produce plasmons. The dispersion relations for the corresponding plasmons and the emission angles of plasmons with corresponding frequencies are calculated. It is shown that devices based on detecting Cherenkov light in nanofilms and nanowires can be used to detect low-energy electrons.     

Probing the electronic structure of ZnO nanowires by valence electron energy loss spectroscopy

Valence electron energy loss spectroscopy in a transmission electron microscope is employed to investigate the electronic structure of ZnO nanowires with diameter ranging from 20 to 100 nm. Its excellent spatial resolution enables this technique to explore the electronic states of a single nanowire. We found that all of the basic electronic structure characteristics of the ZnO nanowires, including the 3.3 eV band gap, the single electron interband transitions at approximately = 9.5, approximately = 13.5,and approximately = 21.8 eV, and the bulk plasmon oscillation at approximately 18.8 eV, resemble those of the bulk ZnO. Momentum transfer resolved energy loss spectra suggest that the 13.5 eV excitation is actually consisted of two weak excitations at approximately = 12.8 and approximately = 14.8 eV, which originate from transitions of two groups of the Zn 3d electrons to the empty density of states in the conduction band, with a dipole-forbidden nature. The energy loss spectra taken from single nanowires of different diameters show several size-dependent features, including an increase in the oscillator strength of the surface plasmon resonance at approximately = 11.5 eV, a broadening of the bulk plasmon peak, and splitting of the O 2s transition at approximately = 21.8 eV into two peaks, which coincides with a redshift of the bulk plasmon peak, when the nanowire diameter decreases. All these observations can be well explained by the increased surface/volume ratio in nanowires of small diameter.     

涂覆石墨烯的非对称并行电介质纳米线波导的模式特性分析

采用多级展开方法,对涂覆石墨烯的非对称并行电介质纳米线波导的模式特性进行了分析.首先对这种波导中的表面等离子模式进行分类,然后对七种低阶模式的有效折射率和传播长度随工作频率、几何结构参数和石墨烯费米能的依赖关系进行详细的分析.结果表明,通过改变工作频率、几何结构参数和石墨烯的费米能,可以在较大范围内调节模式的特性.与有限元法进行的对比表明,基于多级方法的半解析结果与有限元法的数值结果非常符合.研究结果可为涂覆石墨烯的非对称并行电介质纳米线的设计和制作提供一定的理论基础.     

Silver nanowires for photonics applications

Although silver nanowires as plasmonic components have been investigated extensively in both theoretical and experimental studies, a systematic study is still lacking. In this work, a review is given to explain some basic features of experimentally prepared nanowires and their optical properties in different situations, such as waveguides, resonators, and antennas. The review also lists several possible applications of nanowires for enhanced light‐emitting, photonic device fabrication, sensors, lasers, and nonlinear optics. Combined with the merits of both nanowires and surface plasmon polaritons, silver nanowires are certain to show their potential in photonics in the near future.     

Nonlinear spoof surface plasmon polariton phenomena based on conductor metamaterials

We present nonlinear phenomena produced from spoof surface plasmon polariton (SSPP) modes. Below the THz spectrum, artificially textured conducting metastructures on a subwavelength scale generate surface-bound modes and are called SSPP modes, similar to surface plasmon polariton (SPP) modes in the visible spectrum. Even though nonlinear effects in the THz domain are negligible, subwavelength metallic gap structures are ideal candidates to realize nonlinear behavior in the THz domain because of slow light propagation, strong electromagnetic confinement, and a high quality factor Q. In particular, when SSPP structures are combined with Kerr nonlinear materials, nonlinear-bistable curves can be observed below the THz spectrum.     

Chiral surface plasmon polaritons on metallic nanowires

Chiral surface plasmon polaritons (SPPs) can be generated by linearly polarized light incident at the end of a nanowire, exciting a coherent superposition of three specific nanowire waveguide modes. Images of chiral SPPs on individual nanowires obtained from quantum dot fluorescence excited by the SPP evanescent field reveal the chirality predicted in our theoretical model. The handedness and spatial extent of the helical periods of the chiral SPPs depend on the input polarization angle and nanowire diameter as well as the dielectric environment. Chirality is preserved in the free-space output wave, making a metallic nanowire a broad bandwidth subwavelength source of circular polarized photons.