Localization of the electromagnetic field in the vicinity of gold nanoparticles: Surface modification of different substrates

Theoretical predictions and experimental results for nanosized modification of metal (Au), semiconductor (Si), or dielectric (soda lime glass) substrates using near-electromagnetic field enhancement in the vicinity of gold nanoparticles are presented. The near field properties for the system consisting of an isolated gold nanoparticle or nanoparticle aggregate deposited on the substrates, which is irradiated by electromagnetic wave, are investigated using Finite Difference Time Domain Simulation technique. The influence of the substrate material on the near field distribution characteristics is predicted. The results reveal that the field on the substrate surface is enhanced in the three investigated cases, but its spatial distribution and magnitude depend on the substrate material. In the case of the metal and semiconductor substrate the enhanced near field is strongly localized in the vicinity of the contact point with the particle, in an area with diameter smaller than the particle's one. The intensity of the enhanced field on the glass is more than an order of magnitude lower than the case of using silicon substrate. The properties of the near field on the substrate surface also depend on the particle arrangement. For a two-dimensional gold nanoparticle array, when the particles are closely arrayed, the intensity of the enhanced field on the substrate surface is minimal. With the increase of the interparticle distance the near field intensity increases. The validity of the obtained theoretical results is confirmed experimentally.     

Near field properties of nanoparticle arrays fabricated by laser annealing of thin Au and Ag films

In this work we show the properties of the electromagnetic field in the vicinity of a monolayer nanoparticle array on SiO₂ substrate. The nanoparticle array is produced by a simple experimental procedure, where thin gold and silver films are deposited on a substrate by pulsed laser deposition technique and they are annealed by nanosecond laser pulses. At certain conditions the laser annealing leads to a homogeneous decomposition of the film into nanoparticles with diameters in the range of few tens of nanometers. Using FDTD simulations the near field distribution in array structures taken from SEM images are obtained. The distribution shows presence on “hot spots” where the near field intensity is enhanced more than two orders of magnitude compared to the incident one. The existence of enhanced field intensity is assumed to be the main reason on enhancement of the Raman scattering signal obtained experimentally using the produced structures as active substrates.     

用于飞秒激光纳米加工的TiO_2粒子阵列诱导多种基底表面近场增强

利用纳米粒子辅助对飞秒激光能量进行空间局域化,使其在基底表面诱导产生纳米尺度的近场增强,这对超衍射极限微结构加工具有重要意义.目前对于粒子阵列诱导飞秒激光纳米孔加工的研究仅限于金属Au粒子及低折射率聚苯乙烯介电粒子等,本文提出并开展了应用高折射率TiO_2介电粒子阵列作为辅助诱导激光近场增强从而进行飞秒激光超衍射纳米孔加工的研究.对TiO_2介电粒子阵列在Si,Pt及SiO_2表面的近场强度分布进行了数值模拟,研究其基底表面近场增强的规律及物理过程.研究结果发现,使用硅基底时,阵列与单一TiO_2球形粒子相比其近场增强仅下降约30%;相对于入射激光强度而言,在直径约为100nm的空间范围内获得140倍的近场增强,这一现象可用于百纳米孔的激光加工.同时在其他典型基底的理论计算结果中也表明,几乎在所有金属及介电材料表面均可以实现良好的百纳米空间范围内的近场增强,并且具有近场随着基底折射率变大而增强的规律.这些现象的产生归因于TiO_2粒子中磁四极振荡产生的激光前向场增强及粒子与基底的耦合作用.进一步引入镜像电荷模型对基底光学参数对其表面近场增强的影响规律进行了分析和解释.本文的模拟结果对飞秒激光近场超衍射极限纳米加工的应用有着重要的意义.     

Uniform plasmonic near-field nanopatterning by backward irradiation of femtosecond laser

We present localized optical field distribution properties in the vicinity of gold particles on a silicon substrate by backward and forward irradiation. It is technically difficult to fabricate nanostructures on the surface by a conventional forward laser incident to the substrate because gold nanoparticles easily aggregate to form double-layered particle arrays. We calculated enhanced optical field properties in order to pattern the substrate surface only with a template of the bottom-layered particle arrays in the case that the backward irradiation of a femtosecond laser is used in the system of aggregated double-layered gold nanoparticle arrays. With the backward irradiation, the optical field intensity in the substrate for the double-layered hexagonal arrays is found to be only 30% lower than the mono-layered system. Moreover, a near field cannot be generated with the forward irradiation. As a result, only the backward irradiation scheme is found to be effective for uniform surface nanopatterning at enhanced plasmonic near-field zones.     

High-intensity near-field generation for silicon nanoparticle arrays with oblique irradiation for large-area high-throughput nanopatterning

We present near-field distributions around an isolated 800-nm silica or silicon nanoparticle, and nanoparticle arrays of 800-nm silica or silicon nanoparticles, on a silicon substrate by the finite-difference time-domain method when 800-nm light is irradiated obliquely to the substrate. Nanopatterning mediated with the nanoparticle system is promising for large-area, high-throughput patterning by using an enhanced localized near-field ablation by the nanoscattered light lens effect. The irradiation area cannot be extended for silica nanoparticles, because the optical field enhancement factor is low. Gold nanoparticles can generate highly enhanced near fields, although at present there are no useful ways to arrange the gold nanoparticles on the substrate at a high throughput. Silicon nanoparticles with high dielectric permittivity have optical characteristics of both silica and gold nanoparticles. The particle arrangement on the Si substrate is technically easy using a wet pulling process. From the calculation, high optical field intensity is acquired with oblique s-polarized irradiation to the substrate under silicon nanoparticle arrays, and the intensity is almost the same as that under gold nanoparticle arrays under the same condition. With this method, high-throughput nanopatterning for a large area would be achievable.     

Nanophotonics of isolated spherical particles

The problem of extreme focusing of an optical beam into the spatial region with wavelength dimensions is considered with the use of the special features of radiation interaction with isolated spherical particles. Results of numerical computations of the optical field intensity at the surface of silver particles of different radii upon exposure to laser radiation with different wavelengths are presented. It is demonstrated that the relative intensity of the plasmon optical field on the nanoparticle surface increases and the field focusing region decreases with increasing particle radius. Results of numerical computations illustrating the influence of the shell of composite nanoparticles comprising a dielectric core and a metal shell on the optical field intensity in the vicinity of the particle are presented. The problem of local optical foci of a transparent microparticle (photonic nanojets) is investigated. It is established that variation of the micron particle size, its optical properties, and laser radiation parameters allows the amplitude and spatial characteristics of the photonic nanojet region to be controlled efficiently.     

Analysis of Trapping Force and Torque for Fiber-optical Trap by the Finite-differential Time-domain (FDTD) Method

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Near field localization mediated by a single gold nanoparticle embedded in transparent matrix: Application for surface modification

In this work the near field properties of a single gold nanoparticle embedded in transparent host medium are investigated theoretically. The analysis of the electromagnetic field in the near field zone is obtained by finite difference time domain (FDTD) simulation technique. The nanoscale system consists of a transparent layer in which a gold particle with diameters of D = 200 or 80 nm is embedded, is situated on a substrate surface. Laser pulse at wavelength of 800 nm irradiates normally this system. It is found that the field in the vicinity of the particle is enhanced, and at a certain condition the zone with the highest enhancement is localized on the substrate surface. Furthermore, the near field characteristics are found to be controllable by the dielectric properties of the host material, substrate, parameters of the incident irradiation and particle size. With the increase of the refractive index of the host medium, both the magnitude of the near field on the substrate and the characteristic size of the field enhanced zone decrease. The influence of the particle size and polarization of the incident laser irradiation on the near filed properties of the system are also presented. The proposed configuration can be applied for a multiple nanoprocessing and an integrated near field source with a spatial resolution of D/3.     

Analysis of Trapping Force and Torque for Fiber-optical Trap by the Finite-differential Time-domain (FDTD) Method

In this paper,the FDTD method is used to calculate the light field distribution in an uncoated fiber probe with a small dielectric particle.By integrating Maxwell′s stress tensor in the FDTD calculation,the time-averaged force exerted on the dielectric particles with different sizes and at different positions is obtained.The calculation results are also verified experimentally.The force near the axis in the vicinity of the tip is about 10-14 N for the incident 10 mW laser of 980 nm.Furthermore,the torque exerting on an ellipsoidal dielectric particle by focusing light field with two fiber probes is calculated and the dependence of this torque on both the declination angle of the ellipsoidal particle and the incident laser power ratio of the two probes is obtained.The detailed intensity map within the particle is also given.     

Femtosecond-laser nanofabrication onto silicon surface with near-field localization generated by plasmon polaritons in gold nanoparticles with oblique irradiation

Nanohole fabrication process with gold nanoparticles irradiated by femtosecond laser at different incident angles is investigated. Nanoparticles with diameter of 200 nm and laser irradiation with center wavelength of 800 nm are used in the present study. The analysis of the electromagnetic field distribution in the near-field zone of the particle is made by simulations based on finite-differential time domain (FDTD) method. It is shown that when gold nanoparticle is irradiated by laser pulse surface plasmon excitation can be induced, and associated with it, high-intensity near field is produced in a limited area around the particle. It is found that the change of the irradiation conditions by means of irradiation from various incident directions gives a possibility of laser nanoprocessing with tunable characteristics. Our results show that enhanced optical intensity is able to be induced on the substrate surface regardless of incident direction of the laser due to the image charge interaction with the substrate. Furthermore, the use of p-polarized laser irradiation at a certain angle gives a minimum of the spatial dimensions of the enhanced zone on the substrate which is about two times smaller than that obtained at normal incidence.     

Evolution of terahertz conductivity in ZnSe nanocrystal investigated with optical-pump terahertz-probe spectroscopy

The unique optical properties of nanoparticles are highly sensitive in respect to particle shapes, sizes, and localization on a sample. This demands for a fully controlled fabrication process. The use of femtosecond laser pulses to generate and transfer nanoparticles from a bulk target towards a collector substrate is a promising approach. This process allows a controlled fabrication of spherical nanoparticles with a very smooth surface. Several process parameters can be varied to achieve the desired nanoparticle characteristics. In this paper, the influence of two of these parameters, i.e. the applied pulse energy and the laser beam shape, on the generation of Si nanoparticles from a bulk Si target are studied in detail. By changing the laser intensity distribution on the target surface one can influence the dynamics of molten material inducing its flow to the edges or to the center of the focal spot. Due to this dynamics of molten material, a single femtosecond laser pulse with a Gaussian beam shape generates multiple spherical nanoparticles from a bulk Si target. The statistical properties of this process, with respect to number of generated nanoparticles and laser pulse energy are investigated. We demonstrate for the first time that a ring-shaped intensity distribution on the target surface results in the generation of a single silicon nanoparticle with a controllable size. Furthermore, the generated silicon nanoparticles presented in this paper show strong electric and magnetic dipole resonances in the visible and near-infrared spectral range. Theoretical simulations as well as optical scattering measurements of single silicon nanoparticles are discussed and compared.     

Optical trapping of metal-dielectric nanoparticle clusters near photonic crystal microcavities

We predict the formation of optically trapped, metal-dielectric nanoparticle clusters above photonic crystal microcavities. We determine the conditions on particle size and position for a gold particle to be trapped above the microcavity. We then show that strong field redistribution and enhancement near the trapped gold nanoparticle results in secondary trapping sites for a pair of dielectric nanoparticles.     

Influence of local environment on the intensity of the localized surface plasmon polariton of Ag nanoparticles

A combined Ag nanoparticle with an insulating or conductive layer structure has been designed for molecular detection using surface enhanced Raman scattering microscopy. Optical absorption studies revealed localized surface plasmon resonance, which shows regular red shift with increasing environmental dielectric constant. With the combined structure of surface enhanced Raman scattering substrates and rhodamine 6G as a test molecule, the results in this paper show that the absorption has a linear relationship with the local electromagnetic field for insulating substrates, and the electrical property of the substrate has a non-negligible effect on the intensity of the local electromagnetic field and hence the Raman enhancement.     

复合Ag/SiO_2正弦光栅基底SERS特性分析

当前微流控表面增强拉曼散射(SERS)检测领域常用的贵金属纳米颗粒溶胶单位体积内热点区域数量有限且热点区域范围较小,而贵金属纳米三维阵列结构加工时间长,成本高昂并存在"记忆效应"。本文提出了集成到微流道的复合Ag/SiO_2正弦光栅SERS基底结构,可以利用激光干涉光刻技术进行制备,无需预制掩膜版,可实现大面积、低成本SERS基底简易快速制备。利用严格耦合波分析方法(RCWA)建立了复合正弦光栅表面电场增强数学评估模型,推导了表面等离子体共振(SPP)耦合吸收率数学模型,分析了入射光、复合正弦光栅结构与外界环境介电常数的优化匹配关系,得到了入射光785 nm条件下的最佳复合正弦光栅结构。通过制备加工并实验验证了复合正弦光栅的SERS性能,SERS增强因子(EF)能够达到10~4。     

退火法制备石墨烯-银纳米粒子及其增强拉曼实验

针对目前SERS基底上金属颗粒制备过程中存在的分布不均匀、易氧化和稳定性差等缺点,通过热蒸镀和高温退火获得分布均匀的SERS基底;同时结合石墨烯优良的光学性能、化学惰性、荧光猝灭以及本身的SERS增强等优点,制备了稳定的石墨烯-银纳米颗粒（GE/AgNPs）复合结构SERS基底。通过GE/AgNPs复合结构的拉曼光谱稳定性试验证明了石墨烯在GE/AgNPs结构中起到隔绝银纳米颗粒与空气直接接触及催化氧化银脱氧的作用,有利于SERS基底的时间稳定性。(1) 石墨烯、Ag纳米颗粒及其复合结构的制备。首先采用热蒸镀和高温退火的方法使Ag纳米颗粒均匀地沉积在SiO₂/Si基底上,再采用化学气相沉积法在Cu箔上制备少层石墨烯,并用湿法转移法将石墨烯转移到目标基底上,并实验研究了以不同的退火顺序对GE/AgNPs基底造成的影响。(2) 石墨烯、Ag纳米颗粒及其复合基底的表征。分别采用光学显微镜、扫描电子显微镜和拉曼光谱进行表征,得到转移后的纯石墨烯较完整地覆盖在SiO₂/Si基底上面,表面比较平整,但在少数地方仍然存在褶皱和杂质;SEM表征结果表明对于不同制备流程的GE/AgNPs复合结构上的Ag纳米颗粒基本呈球形。基本符合Ostwald熟化理论,通过对退火温度和时间的控制能获得平均粒径在40~60 nm的银颗粒,且分布较均匀。此外,在不同退火顺序中,石墨烯的加入对银纳米颗粒的扩散形成扩散势垒,从而出现较大的不规则的颗粒。(3) 基底稳定性试验和仿真分析。通过基底本身的Raman mapping测试,分析了石墨烯拉曼特征峰峰值和半高宽的变化,得知基底对石墨烯本身的拉曼增强效果主要来源于银纳米颗粒间的电磁场增强。同时采用浓度为10-6 mol·L-1的罗丹明6G (R6G)水溶液作为探针分子,对转移了石墨烯的GE/AgNPs复合基底和未转移石墨烯的Ag纳米颗粒基底进行了SERS稳定性实验。结果表明GE/AgNPs复合基底在1~33 d内衰减较缓慢,30 d后仍能探测到拉曼信号约为原来信号的35.1%~40.6%;而纯Ag基底上随着Ag纳米颗粒在空气中迅速氧化,基底的SERS性能显著下降,在30 d后只有原来信号的5.9%~11.3%。此外,通过实验得到覆盖了石墨烯之后的增强因子约为6.05×105。最后采用时域有限差分算法（FDTD）计算了复合结构的电磁场分布和理论增强因子,其理论增强因子可以达到5.7×105。实验和仿真结果的差异,主要是源于石墨烯的化学增强作用。     

Substrate-Modulated Electromagnetic Resonances in Colloidal Cu2O Nanospheres

Dielectric nanoparticles are expected to complement or even replace plasmonic nanoparticles in many optical and optoelectronic applications, because they exhibit small absorption losses and support strong electric and magnetic resonances simultaneously. Dielectric nanoparticles need to be deposited on various substrates in many applications. Understanding the substrate effect on the electromagnetic resonances of dielectric nanoparticles is of great importance for engineering their resonance properties and designing optical devices. In this study, moderate-refractive-index cuprous oxide nanospheres with uniform sizes and shapes are synthesized. The scattering spectra and images of the nanospheres deposited on three types of substrates are analyzed experimentally and theoretically. When supported on indium tin oxide–coated glass slides and Si wafers, the color of the nanospheres varies from blue, cyan, green, yellow, orange and red, covering almost the entire visible region. When deposited on gold films, the electromagnetic resonances of the nanospheres redshift intensively and a new effective magnetic resonance mode appears. The enhanced Raman scattering reveals that large electromagnetic field enhancements are produced in the gap region between the nanosphere and the substrate. The results shed light on the manipulation of the electromagnetic responses of dielectric nanoparticles and the design of dielectric metamaterials in the presence of various substrates.     

Magnetron and pulsed laser deposition of silver and gold nanoparticles and discontinuous films and their optical properties

Ag and Au nanoparticles are obtained by magnetron sputtering and pulsed laser deposition under different conditions, and the features of their absorption spectra associated with plasmon resonances are investigated. Optimal deposition conditions for obtaining small (5?C10 nm) silver nanoparticles with a high density of surface distribution include an increased argon pressure (2.5 × 10^?2 Torr) and a low discharge voltage (100 V). Gold nanoparticle arrays obtained by pulsed laser deposition at a temperature of 200°C in vacuum are more uniformly distributed on the substrates than those deposited at room temperature in argon. It is shown that the maximum of the plasmon absorption shifts toward shorter wavelengths with a decrease in the equivalent thickness of metal films and depends not only on this thickness but also on the type of substrate, which is responsible for the morphology of nanoparticle arrays.     

Immunoassays Based on Surface-Enhanced Fluorescence using Gap-Plasmon-Tunable Ag Bilayer Nanoparticle Films

A novel gap-plasmon-tunable Ag bilayer nanoparticle film for immunoassays is demonstrated. Different from a traditional Ag monolayer nanoparticle film, a desired number of polyelectrolyte (PEL) layers are deposited on the nanoparticles before the self-assembly of a second Ag nanoparticle layer. Interestingly, by controlling the number of the PEL interlayers, the gap plasmon between the two Ag nanoparticle layers can be tuned across the visible spectral range. The ability of the presented Ag bilayer nanoparticle films in fluorescence enhancement has been examined experimentally. A maximal enhancement of around 15.4 fold was achieved when 7 layers of polyelectrolyte were used. When this optimal Ag bilayer nanoparticle film was applied to fluorescence immunoassay, a performance with approximately 3.3-fold enhancement was obtained compared with that performed on a traditional glass substrate. The experimental results suggest that the presented gap-plasmon tunable Ag bilayer nanoparticle films have great potential in fluorescence-based immunoassays. The method of the bilayer-film construction presented here also provides new insights into the rational design of the plasmonic substrates.     

Thin-film-based sensitivity enhancement for total internal reflection fluorescence live-cell imaging

We investigated experimentally the evanescent field enhancement based on dielectric thin films in total internal reflection microscopy. The sample employed two layers of Al2O3 and SiO2 deposited on an SF10 glass substrate. Field intensity enhancement measured by fluorescent excitation of microbeads relative to that of a control sample without dielectric films was polarization dependent, determined as 4.2 and 2.4 for TE and TM polarizations, respectively, and was in good agreement with numerical results. The thin-film-based field enhancement was also applied to live-cell imaging of quantum dots, which confirmed the sensitivity enhancement qualitatively.     

Single pulse near field study on a Co(3 nm)/Cu(6 nm)/Co(20 nm) multilayer structure by using a femtosecond laser

Single pulse near field study on a Co(3 nm)/Cu(6 nm)/Co(20 nm) multilayer structure was experimentally investigated with a laser pulse width of 200 fs at a wavelength of 775 nm. For the near field experiments, we have used polystyrene colloidal particles of 700 nm diameter deposited by spin coating on top of the multilayer structure, as well on top of Co (50 nm) and Cu (50 nm) thin films. The diameter and the morphologies of the holes were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). We have estimated the fluence thresholds values for the near field and discuss their values in respect with the enhancement factor of the intensity of the electromagnetic field due to the use of the colloidal particles. We compare the depths and the widths of the holes obtained at the same peak laser fluence for the Co thin film (50 nm), Cu thin film (50 nm) and Co(3 nm)/Cu(6 nm)/Co(20 nm) multilayer structure. Depending on the laser fluence, the ablation depth can reach the first, the second, or the third layer. Theoretical estimations of the intensity enhancement were done using the finite-difference time-domain (FDTD) by using the RSoft software. This type of a selective distribution of the ablation depth, in the near field regime, of a planar metal/dielectric interface can open new perspective in the excitation of propagating surface plasmons.