Optical manipulation of plasmonic nanoparticles

We investigate numerically the optical forces exerted by an incident light beam on metal nanoparticles (MNP) sustaining the so-called localized surface plasmons (LSP). We first describe how the particle dispersion can be used to tune the respective contribution from extinction and gradient forces. By a suitable adjustment of the illumination conditions, single MNP can be selectively guided, sorted and trapped. The second part of our work investigates the interparticle forces existing within a MNP ensemble. Our results show that MNP located in a conventional optical trap can self-arrange under optical forces according to specific architectures. In particular, at very low distances, they tend to agglomerate into metal clusters leading to very high field concentration in the interstices. PACS 71.45.Gm; 71.36.+c; 87.80.Cc     

Spectral degeneracy breaking of the plasmon resonance of single metal nanoparticles by nanoscale near-field photopolymerization

We report on controlled nanoscale photopolymerization triggered by enhanced near fields of silver nanoparticles excited close to their dipolar plasmon resonance. By anisotropic polymerization, symmetry of the refractive index of the surrounding medium was broken: C infinity v symmetry turned to C2v symmetry. This allowed for spectral degeneracy breaking in particles plasmon resonance whose apparent peak became continuously tunable with the incident polarization. From the spectral peak, we deduced the refractive-index ellipsoid fabricated around the particles. In addition to this control of optical properties of metal nanoparticles, this method opens new routes for nanoscale photochemistry and provides a new way of quantification of the magnitude of near fields of localized surface plasmons.     

The optical properties between an incident wave and the active layer of a bubble-pit AgO<Subscript>x</Subscript>-type super-resolution near-field structure

The deformation and plasmon effects of collective localized surface plasmons between incident light and bubble-pit AgO_x-type super-RENS structure have been studied using finite-difference time-domain (FDTD) method. We find that the polarization, wavelength of incident light, and particle sizes of Ag nanoparticles are sensitive to the plasma resonance. The Ag nanoparticles inside the bubble-pit AgO_x-type super-RENS structure give the additional outer boundaries to the motion of the Ag nanoparticles, and excite more evanescent field which located in the far edge of the bubble from the optical axis of the incident beam. The optical properties between active layer and incident light with polarization direction, different wavelengths, and varied particle sizes of Ag nanoparticles exhibits nonlinear optical behavior in the near field. The far-field signals of different wavelength of incident light confirm the relation between highly localized near-field distributions and enhanced resolution of far-field signals. The subwavelength recording marks smaller than the diffraction limit were distinguishable since the Ag nanoparticles with high localized fields transferred evanescent waves to detectable signals in the far field. PACS 42.79.Vb; 71.15.Rn; 72.15.Rn; 73.22.-f; 73.22.Lp; 78.67.Bf; 73.20.Mf     

金属纳米颗粒LSPR光纤生物传感DDA方法研究

研究了金属纳米颗粒的局部表面等离子体共振(LSPR)行为,并讨论了其在光纤生物传感领域的应用.采用离散偶极近似(DDA)的方法,从理论上分析了金属纳米颗粒的尺寸、形状对其传感灵敏度的影响.计算结果显示,金属纳米颗粒的等离子共振吸收峰同时受到颗粒尺寸和形状的影响,但形状对其传感灵敏度的影响最为明显,计算结果与实验数据能较好地吻合.     

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.     

Manipulation of Nanoparticles Using Dark-Field-Illumination Optical Tweezers with Compensating Spherical Aberration

Based on our previous investigation of optical tweezers with dark field illumination [Chin. Phys. Left. 25（2008）329] nanoparticles at large trap depth are better viewed in wide field and real time for a long time, but with poor forces. Here we present the mismatched tube length to compensate for spherical aberration of an oil-immersion objective in a glass-water interface in an optical tweezers system for manipulating nanoparticles. In this way, the critical power of stable trapping particles is measured at different trap depths. It is found that trap depth is enlarged for trapping nanoparticles and trapping forces are enhanced at large trap depth. According to the measurement, 70-nm particles are manipulated in three dimensions and observed clearly at large appropriate depth. This will expand applications of optical tweezers in a nanometre-scale colloidal system.     

Plasmon spectroscopy of metallic nanoparticles above flat dielectric substrates

We numerically analyze the spectral properties of localized plasmon resonances in metal nanoparticles when these are above a dielectric substrate. This analysis is performed as a function of the various parameters involved in the problem (relative optical properties, particle-substrate separation, angle of incidence, etc.). It can be shown that from the spectral behavior of the resonance in the far field, information about particle near-field interactions can be obtained.     

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.     

Field enhancement by semi-nanocapsule plasmonic antenna at the visible violet wavelength

We investigate, at the visible violet wavelength of 400 nm, the localized field-enhancement properties of an optical antenna consisting of two coupled metallic nanoparticles placed on the silica substrate. Compared to other shapes of optical antennas such as bowtie and coupled elliptical rods, the coupled semi-nanocapsules exhibit a stronger field-intensity enhancement in the gap and relatively weak field intensity at the outer-ends. Furthermore, the intensity enhancement of the semi-nanocapsules antenna can be further enhanced by choosing the suitable direction of illumination. These proposed structures can be used for the design and the applications of an optical antenna at the visible violet wavelength.     

Surface plasmon radiation forces

We report the first experimental observation of momentum transfer from a surface plasmon to a single dielectric sphere. Using a photonic force microscope, we measure the plasmon radiation forces on different polystyrene beads as a function of their distance from the metal surface. We show that the force magnitude at resonance is strongly enhanced compared to a nonresonant illumination. Measurements performed as a function of the probe particle size indicate that optical manipulation by plasmon fields has a strong potential for optical sorting.     

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。     

Anisotropic local-field effects of nanoparticles in plasmonic optics and magneto-optics

A theory of anisotropic optical local-field effects caused by resonantly polarizable small particles in multilayer polarizable media is developed. Considered is the model of a rectangular lattice of ellipsoidal nanoparticles with taking account of “image forces” at an interface in a layered medium. The lattice sums for anisotropic dipolar interactions are found using the Green’s function method in the quasi-point dipole approximation, and the effective polarizabilities of particles in a layer located near an interface are calculated self-consistently. The manifestation of an anisotropic local field of nanoparticles in optical radiation and propagation of evanescent waves responsible for optical near-field effects is investigated. Applications of the obtained results in the polar magneto-optical Kerr effect and reflectance anisotropy spectroscopy in propagating the polarized light along the normal to layers are considered. The resonant features in the spectra due to enhancement of the optical effects under excitation of surface (local) plasmons in nanoparticles of a noble metal are studied.     

Optical trapping of single fluorescent molecules at the detection spots of nanoprobes

We propose a scheme of optical trapping of fluorescent molecules, based on the strongly enhanced optical field due to surface plasmon resonances at laser illuminated metal tips or particles. A semiclassical approach is compared to a quantum-mechanical one. Attractive as well as repulsive forces are possible depending on the wavelength of the optical field. The trapping potential is shown to be strong enough to overcome the Brownian motion in water solution for common optical tweezer light inten-sities. Single molecule resonance Raman spectroscopy probes are particularly well suited for the trap-ping scheme. Finally we propose intracellular probing of the function of biomolecules as an application.     

Coupled optical Tamm states at edges of a photonic crystal enclosed by a composite of core-shell nanoparticles

Coupled optical Tamm states localized at the edges of a photonic crystal enclosed with a nanocomposite are theoretically studied. The nanocomposite consists of nanoparticles with a dielectric core and a metal shell, which are dispersed in a transparent matrix. It is shown that the positions of the spectral peaks are sensitive to the thickness of the outermost photonic crystal layer.     

Optical pulling force on nanoparticle clusters with gain due to Fano-like resonance

We demonstrate that, in a simple linearly-polarized plane wave, the optical pulling forces on nanoparticle clusters with gain can be induced by the Fano-like resonance. The numerical results based on the full-wave calculation show that the optical pulling forces can be attributed to the recoil forces for the nanoparticle clusters composed of dipolar nanoparticles with three different configurations. Interestingly, the recoil forces giving rise to optical pulling forces are exactly dominated by the coupling term between the electric and magnetic dipoles excited in the nanoparticle clusters, while other higher-order terms have a negligible contribution. In addition, the optical pulling force can be tailored by modulating the Fano-like resonance via either the particle size or the gain magnitude, offering an alternative freedom degree for optical manipulations of particle clusters.     

琼脂糖凝胶/纳米金复合结构的制备及其表面增强拉曼性质

以琼脂糖凝胶为模板,将预先制备好的胶体金颗粒负载在琼脂糖凝胶的网状结构中,制备了琼脂糖凝胶/纳米金复合膜结构,采用透射电子显微镜、扫描电子显微镜、紫外-可见-近红外光谱仪对复合膜的结构和光学性质进行了表征,实验数据表明纳米金颗粒均匀的分散在琼脂糖凝胶膜上,并且呈现出优异的光学吸收特性。基于琼脂糖凝胶的溶胀收缩特性和纳米金颗粒可调的表面等离子体共振吸收特性,将琼脂糖凝胶/纳米金复合膜作为表面增强拉曼(SERS)基底材料,研究了其对拉曼信号分子尼罗蓝A(NBA)的SERS检测效果。结果表明,琼脂糖凝胶的多孔网状结构为纳米金颗粒的富集提供了良好的载体,随着琼脂糖凝胶在空气中失水收缩,纳米金颗粒间距离逐渐缩短,产生动态的热点效应,对拉曼信号分子具有良好的增强效应。     

Formation of silver nanoparticles in photothermorefractive glasses during electron irradiation

The formation of silver nanoparticles in photothermorefractive glasses during electron irradiation and subsequent heat treatment is experimentally studied. The optical density spectra of samples at various stages of heat treatment are compared. The formation of metallic nanoparticles is shown to occur during both electron irradiation and subsequent heat treatment. Nanoparticles are localized in layers parallel to the surface. At the initial stage of nanoparticle formation, the key role is played by the appearance of a negative bulk charge in the near-surface layer in glass, which results in the field-assisted migration of positive metal ions and a characteristic layered distribution of metallic nanoparticles in the glass volume.     

Enhancement of photoluminescence of the CdSe/CdS quantum dots on quartz substrates in the presence of silver nanoparticles

Multilayer systems consisting of layers of hybrid quantum dots are fabricated. The quantum dots with the CdSe/CdS core/shell structure are chemically synthesized and deposited on the surface of quartz glass that contains ion-synthesized silver nanoparticles in the near-surface region. Silver nanoparticles exhibit optical absorption owing to the localized surface plasmon resonance. Variations in the photoluminescence intensity of the layer related to an increase in the distance from the quartz surface with metal nanoparticles are studied. An increase in the photoluminescence intensity is observed under excitation in the spectral region of the plasmon absorption of silver nanoparticles. An optimal distance between the layers is determined to maximize the enhancement of the photoluminescence of quantum dots in the presence of the near field of metal nanoparticles.     

Surface-plasmon-enhanced optical forces in silver nanoaggregates

We use extended Mie theory to investigate optical forces induced by and acting on small silver nanoparticle aggregates excited at surface plasmon resonance. It is shown that single molecules can be trapped at junctions between closely spaced nanoparticles, which are simultaneously pulled together by optical forces. These effects could significantly influence surface-enhanced Raman scattering and related spectroscopies under normal experimental conditions and contribute to single-molecule sensitivity.     

Influence of the incorporation of metals on the optical properties of MCM-41

We studied the optical properties of mesoporous aluminosilica incorporated with metal nanoparticles, which can be used to efficiently enhance catalytic activity for CO oxidation. Analysis of the PL spectra indicated that the incorporation of Al greatly enhanced the generation of oxygen related defects. When Au nanoparticles were deposited onto Al-MCM-41, both the PL and PLE spectra suggested that the metal nanoparticles were strongly attracted by the electrostatic force induced by the charged oxygen defects. Electron spin resonance measurement provided further evidence, registering a spike in the intensity following incorporation of the metal particles, as the strong interaction between the F+ center and the metal particles induced electron transfer to the trapped O₂ molecule. These discoveries shed further light on the perplexing mechanism by which the Si/Al ratio and metal particles can give rise to dramatic enhancement of the catalytic activity for CO oxidation.