Near-field optical observations of surface plasmon wave interference at subwavelength hole arrays perforated in Au film

We image optical near-field patterns at subwavelength circular hole arrays in Au film by using scanning near-field optical microscopy in near-infrared wavelengths.Periodical oscillation features are found in the near-field images at the air/Au interface and exhibit two typical kinds of standing wave oscillation forms at the wavelengths corresponding to the transmission minimum and maximum in the transmission spectrum,and the latter one originates from the excitation and interference of a surface plasmon wave at the metallic hole arrays.Our work indicates that monitoring optical near-field patterns can help to reveal many interesting properties of surface plasmon waves at metallic nanostructures and understand their underlying physical mechanisms.     

Surface plasmon polariton excitation and manipulation by nanoparticle arrays

Using a vectorial dipolar model for multiple surface plasmon-polariton (SPP) scattering [T. Søndergaard, S.I. Bozhevolnyi, Phys. Rev. B 67 (2003) 165405], we investigate the possibility of simultaneous SPP excitation and in-plane manipulation with square-lattice arrays of nanoparticles. The SPP excitation followed by focusing and/or waveguiding of SPP waves is observed with nanoparticle arrays of different shapes, demonstrating the feasibility of the suggested approach.     

Controlling surface plasmon excitation of pair arrays of metallic nanocylinders

The polyvinyl alcohol (PVA)/halloysite nanotubes (HNTs) solution were prepared with the aid of ultrasonic treatment. The composite films were prepared through casting or coagulating the PVA/HNTs solution. The coagulation process was employed to obtain a composite film without aggregation of HNTs. It is shown that the particle size and distribution of HNTs in the PVA/HNTs solution is independent of the ratio between HNTs and PVA. It is also revealed that the aggregation of HNTs takes place during the drying process of the as cast film. Compared with the film by coagulation method, the HNTs in the as cast film show less profound effect on the nucleation of the crystallization of the PVA. The crystallization temperature initially increases with HNTs loading and overloading of HNTs tend to depress the improvement in the crystallization temperature. The glass transition temperature (T_g) of the composite film decreases with HNTs loading and the aggregation process shows practically no effect on the T_g. Inclusion of HNTs greatly depresses the decomposition of the PVA backbone, while it is not effective for improving the resistance to the abstraction of the side groups. The aggregation process at low concentration of HNTs has more significant effect on the thermal decomposition of composite films compared with that at high concentration. PACS 61.82.Pv; 61.46.Fg; 61.46.-w; 68.37.Lp; 67.80.Gb     

金属光子晶体平板的超强透射及其表面等离子体共振

用全矢量的三维有限差分时域(finite-difference time-domain,简称FDTD)方法,研究了正方形单元结构金属光子晶体平板的增强传输效应以及局域性表面等离子体共振现象.这种增强效应来自于两个不同的等离子体共振机制：由长方形空气孔形成的局域波导共振以及由周期性结构引起的光子晶体共振效应.对于由长方形空气孔形成的局域波导共振模式,其等离子体波全部局域在整个长方形空气孔区域中.而由周期性引起的共振模式,其频率随着金属平板表面周期性的变化而变化,相应的等离子体波分布在长方形空气孔区域的两端.产生的表面等离子体都局域在长方形空气孔区域中,电场强度得到了显著的增强. 关键词： 光子晶体 金属平板 超强透射 表面等离子体     

Analysis of phase shift of surface plasmon polaritons at metallic subwavelength hole arrays

We present the transmission spectra of light transmitting a metallic thin film perforated with differently shaped sub- wavelength hole arrays, which are calculated by a plane-wave-based transfer matrix method. We analyze the transmission peak positions and the phase-shift angles of different surface plasmon polariton （SPP） modes by using the microscopic theoretical model proposed by Haitao Liu and Philippe Lalanne [Liu Haitao, and Lalanne Philippe 2008 Nature 452 728], in which the phase shift properties of the SPPs scattered by the subwavelength hole arrays are considered. The results show that the transmission peak position and the minus phase shift angle of the SPP increase as the hole size increases. On the other hand, the effective dielectric constant of the metallic film can be deduced by the microscopic theoretical model.     

Parametrical study of left-handed or right-handed propagation by stacking hole arrays

In this paper we analyse the recently reported structure with left-handed (LH) propagation obtained by periodically staking subwavelength hole arrays working at the extraordinary transmission (ET) resonance. A parametrical study that clarifies in more detail the equivalent inverse transmission line approach depicted in a previous work is performed. It is seen that ET is necessary to ensure a strong inductive character of the apertures. Additionally, the metal fraction of the unit cell must be large enough in order to favour the electric coupling between plates. What is more, the series capacitance is further increased by setting the plates closer. This allows considering the stacked structure as a metamaterial in that particular dimension. The experimental transmission results previously published are complemented here with a measurement of the reflection coefficient in a stack with LH propagation. These results can give rise to interesting applications such as novel lenses and other quasioptical structures.     

Near-field optical experiments on low-symmetry split-ring-resonator arrays

Effective symmetric and antisymmetric eigenmodes of coupled plasmonic resonances play a crucial role in many photonic metamaterials. Recently, we discussed a particular arrangement of metallic split-ring resonators that is planar, hence enabling direct experimental access to the different eigenmodes via near-field optical microscopy. In this Letter, corresponding optical experiments are presented and compared with simple theoretical modeling, providing a direct confirmation of our previous, more indirect conclusions.     

Decay times of surface plasmon excitation in metal nanoparticles by persistent spectral hole burning

We describe a new technique to determine the homogeneous linewidths of surface plasmon resonances of metal nanoparticles and thus measure the decay time of this collective electron excitation. The method is based on spectral hole burning and has been applied to supported oblate Ag particles with radii of 7.5 nm. From the experimental results and a theoretical model of hole burning the linewidth of 260 meV corresponding to a decay time of 4.8 fs was extracted. This value is shorter than expected for damping by bulk electron scattering. We conclude that additional damping mechanisms have been observed and reflect confinement of the electrons in nanoparticles with sizes below 10 nm.     

Near-field distribution of broadband antireflective nanostructure arrays

Broadband antireflection (AR) nanostructure arrays patterned directly into a bulk substrate, behaving as the expected performance of high laser induced damage threshold (LIDT), is proposed for the application of laser-transmitting optical elements. These regular periodic arrays known as “moth-eye” structure exhibit particularly noteworthy AR property over a wide spectrum and over a large field of view in addition to overcome efficiently the disadvantages of multilayer AR thin-film. Furthermore, the near-field distribution in nanostructure arrays with respect to the period, the groove depth and incident angle, respectively, was numerically simulated using a rigorous Fourier modal method (FMM). It is interesting that the maximum value of the normalized electric field intensity is obviously distributed inside the air layer upon the nanostructure for the periodicity of 100 nm, which has significant potential to reach to ultra high LIDT for the application of short-pulse high-power laser system. Besides, the variation of the near-field distribution inside the nanostructure arrays exhibits critically dependent on the period of surface structure, but the effect caused by the discrepancy in groove depth and incident angle is unimportant.     

Coupled plasmon modes in an ordered hexagonal monolayer of metal nanoparticles: a direct observation

We report on the experimental observation of STM-induced photon emission in ultrahigh vacuum on a network of 4-nm silver spheres. The spheres are covered by a dielectric, electrically insulating, organic layer and deposited on Au(111). The bias-dependent spatial distribution of the photon emission rates reveals the electric-field distribution of the different coupled plasmon modes in this model.     

Terahertz excitation of the higher-order plasmon modes in field-effect transistor arrays with common and separate two-dimensional electron channels

Using a rigorous electrodynamic approach, the spectra of the terahertz response of periodically ordered arrays of field-effect transistors with two-dimensional electron channel were calculated. It was shown that the higher-order plasmon resonances can be excited in such structures at frequencies above 10 THz.     

Wavelength-dependent optical force on elliptical silver cylinders at plasmon resonance

We use rigorous diffraction theory to analyze the force on elliptical cylinders made from silver as a function of the elongation. We find that, when the wires are illuminated at the plasmon wavelength and placed in a highly focused Gaussian beam, they are attracted toward the optical axis if the waist of the laser is behind the wire and repelled if the waist is before the wire. Also, the force distributions of illumination wavelengths smaller and larger than the plasmon wavelength are analyzed.     

A multiband absorber based on multipolar plasmon excitation

We report a multiband absorber with a top-layer grating structure based on the multipolar plasmon excitation. The simulation results show that the absorber has three distinctive absorption peaks originated from multipolar plasmon excitation at wavelengths λ = 0.576 μm, λ = 0.760 μm and λ = 5.630 μm with the absorption magnitudes more than 0.86, 0.96 and 0.99, respectively. The multipolar plasmon excitation can be described by surface plasmon standing waves.     

Dielectric resonances of ordered passive arrays

The electrical and optical properties of ordered passive arrays, constituted of inductive and capacitive components, are usually deduced from Kirchhoffs rules. Under the assumption of periodic boundary conditions, comparable results may be obtained via an approach employing transfer matrices. In particular, resonances in the dielectric spectrum are demonstrated to occur if all eigenvalues of the transfer matrix of the entire array are unity. The latter condition, which is shown to be equivalent to the habitual definition of a resonance in impedance for an array between electrodes, allows for a convenient and accurate determination of the resonance frequencies, and may thus be used as a tool for the design of materials with a specific dielectric response. For the opposite case of linear arrays in a large network, where periodic boundary condition do not apply, several asymptotic properties are derived. Throughout the article, the derived analytic results are compared to numerical models, based on either Exact Numerical Renormalisation or the spectral method.     

椭圆截面金纳米管的局域表面等离激元共振特性研究

基于时域有限差分方法研究了几何形状、入射电场偏振方向、管壁厚度及内核和包埋介质的变化对椭圆截面金纳米管局域表面等离激元共振特性的影响.研究发现,当长轴固定时,短轴的减小将导致纳米管消光峰红移;入射电场偏振方向与椭圆长轴夹角的增大将导致消光峰红移;当颗粒整体尺寸不变时,管壁厚度减小同样会使得消光峰红移.此外,内核及包埋介质介电常数的增大均导致消光峰红移.利用等离激元杂化理论及自由电子和振荡电子竞争机理对上述现象进行了理论分析. 关键词： 消光光谱 局域表面等离激元共振 金纳米管 时域有限差分方法     

Chemical interface damping of surface plasmon excitation in metal nanoparticles: a study by persistent spectral hole burning

As demonstrated recently, persistent spectral hole burning in the optical spectra of metal nanoparticles makes possible the determination of the ultrafast dephasing time T₂ of surface plasmon excitation. Here, the influence of the chemical environment on the dephasing process is investigated under ultrahigh vacuum conditions by depositing an adsorbate on the nanoparticles. By comparing the optical spectra and the widths of the generated holes of the nanoparticles with and without adsorbate coverage, the influence of chemical interface damping on the dephasing process is examined. The potential of the novel procedure is demonstrated for silver nanoparticles on sapphire and quartz substrates using SO₂ as the adsorbate. A drastic decrease of T₂ for the adsorbate-covered nanoparticles is observed and explained by dynamic charge transfer of electrons from the particles into and out of adsorbate states of the SO₂ molecules. PACS 78.67.Bf; 61.46.+w; 71.45.Gm; 73.22.Lp; 68.43.-h     

Surface-enhanced Raman spectroscopy based on ordered nanocap arrays

We fabricated the Ag cap array for surface-enhanced Raman scattering (SERS) by Ag deposition onto two dimensional polystyrene colloid sphere templates, and 4-mercaptopyridine (4-MPy) was used as the probing molecule. When the colloids with different size were chosen as the substrate for 20 nm Ag deposition, the film on 100 nm colloids gave the significant enhancement. SERS intensity increased with the increase of Ag thickness. When 20 nm Ag film was coated by Ta, the SERS signals decreased with the increase of Ta thickness, indicating the main effect from the top of cap structure. When Co layer was added under the Ag film, the SERS intensity decreased with the increase of Co thickness because the Co layer affects electromagnetic and plasmon resonance.     

The influence of hole shape on enhancing transmission through subwavelength hole arrays

The extraordinary light transmission through a 200-nm thick gold film when passing through different subwavelength hole arrays is observed experimentally. The sample is fabricated by electron beam lithography and reactive ion etching system. A comparison between light transmissions shows that the hole shape changing from rectangular to diamond strongly affects the transmission intensity although both structures possess the same lattice constant of 600,nm. Moreover, the position of the transmission maximum undergoes a spectral red-shift of about 63,nm. Numerical simulations by using a transfer matrix method reproduce the observed transmission spectrum quite well.     

Near-field mechanism of photoluminescence excitation in quantum well heterostructures

We report a study into the process of energy transfer between quantum wells divided by 30-nm-thick opaque barriers. It was experimentally observed that the intensity of a photoluminescence signal from a quantum well increased by 15% under resonant excitation of exciton transition in the adjacent quantum well. The quantum wells were 30 nm apart. A radiative mechanism of energy transfer in the near-field region of emitting exciton is proposed. Within this theoretical model, the efficiency of the energy transfer decreases by a power law with greater distance between the quantum wells. The theory is found to be in qualitative agreement with the experimental results.