Au@碳球复合结构中Au颗粒位置调控光吸收

采用时域有限差分（FDTD）法研究Au纳米颗粒@碳球（AuNPs@CS）复合结构的光吸收控制。发现Au纳米颗粒@碳球复合结构中Au颗粒的位置可以控制复合结构光吸收。模型计算中选取两粒Au纳米颗粒以最佳深度（0 nm）嵌入碳球表面。当两粒Au颗粒球心与碳球球心夹角为22.5°和45°时,复合结构光吸收较单一碳球光吸收明显增强;当夹角为315°、270°、180°、90°时,光吸收增量逐渐减小;当夹角为337.5°时,光吸收量低于单一碳球。这一结果主要归因于Au纳米颗粒位置变化可引起表面等离子体光强度和光散射方向的变化。改变碳球表面Au纳米颗粒的数量和位置,可以进一步调节AuNPs@CS复合结构的光吸收。     

Analyses of radiation force and torque on a spherical particle near a substrate illuminated by a focused Laguerre-Gaussian beam

We present the analyses of radiation force and torque on a spherical particle illuminated by Laguerre-Gaussian beam. Since Laguerre-Gaussian beam has angular momentum, the particle can be rotated by illumination of Laguerre-Gaussian beam. Radiation forces and torques for a silver sphere and a silicon sphere near the silver substrate are analyzed. We found that the maximum torque on silicon sphere is about five times higher than the maximum torque on silver sphere and fine control of the focused spot is required to push the particle to the substrate.     

Field Enhancement by a Metallic Sphere on Dielectric Substrates

We calculate the scattered local field around a small gold sphere on dielectric substrates which is illuminated by a collimated monochromatic beam. The numerical results show the field between the sphere and the substrate is much enhanced when the sphere is illuminated by a p-polarized beam. The enhanced area is very localized between the sphere and the substrate and the diameter of the enhanced area on the surface of the substrate is one fifth of the sphere diameter. The field increases as the gap distance decreases or as the refractive index of the substrate increases.This paper was originally presented at the 5th International Conference on NEAR FIELD OPTICS and RELATED TECHNOLOGIES (NFO-5), which was held on December 6–10, 1998 at Coganoi Bay Hotel, Shirahama, Japan, in cooperation with the Japan Society of Applied Physics and Mombusho Grant-in Aid for Scientific Research on Priority Area “Near-field Nano-optics” Project, sponsored by Japan Society for Promotion of Science.     

The effective torque on patchy spheres nearby a flat substrate induced by infinitely thin rods with adsorptive ends

The system studied here is the mixture of infinitely thin rods and a patchy sphere nearby a flat substrate. The adsorption between the rod ends and the patch can induce an effective torque on the sphere, which are studied by the Monte Carlo method. At the proper patch angle, the effective torque makes the patchy surface face away from the substrate, and the effective interaction between the bare surface and the substrate is attractive at the same time. The effective torque strengthens with the approach between the sphere and the substrate, while first strengthens and then weakens with the increase of the included angle between the axis of the patch and the normal of the substrate. The relationships between the effective torque and the adsorptive strength, the patch angle and the diameter of the sphere have also been studied. The effective torque discussed here may play a role in the biological lock-and-key mechanism and be used to adjust the two-dimensional self-assembly of the colloidal particles with the anisotropic surface.     

Absorption of electromagnetic radiation by electrons of a nanosphere

The electrodynamic response of an electron gas on the surface of a nanosphere is investigated. An analytical relationship for the absorption of electromagnetic radiation by the nanosphere is derived. It is demonstrated that the absorption curve at low temperatures has two resonance peaks. The shape, position, and intensity of the peaks are examined. The dependence of the absorption on the radiation frequency exhibits kinks associated with the degeneracy of the electron gas. The number and position of the kinks and the absorption jumps at these kinks are analyzed. Consideration is given to the cases of an isolated sphere and a sphere exchanging electrons with a reservoir.     

Exact solution for the scattering and absorption properties of sphere clusters on a plane surface

A formulation and computational scheme are presented for predicting the scattering and absorption cross-sections, and the scattering matrix elements, of clusters of non-intersecting spheres that are lying on or above an infinite plane surface and exposed to plane-wave radiation. The formulation provides an exact solution to Maxwell's equations and the associated boundary conditions on the spheres and the plane surface, and is applicable for arbitrary refractive indices for the spheres and the surface. A simplified strategy is presented for the calculation of the surface reflection matrix, which transforms the reflected scattered field from one sphere into a regular vector spherical harmonic expansion centered about another sphere. The calculation results are presented for the clusters of one, two, and four polystyrene spheres, with size parameters of one and 10, lying on a silicon substrate, and are compared with the predictions from the normal incidence approximation (NIA) in which the reflectance of the surface is assumed constant at the normal incidence value. The results show that the accuracy of the NIA is highly dependent on the extent of the sphere cluster, the angle of incidence, and the particular quantity (cross-sections, scattering matrix elements) under examination.     

Photonic Jet by a Near‐Unity‐Refractive‐Index Sphere on a Dielectric Substrate with High Index Contrast

Photonic jets are normally generated in transmission mode and are represented as a spatially localized high‐intensity region on the shadow side of a particle‐lens, with a background to medium refractive index contrast of 1.3–1.7 while illuminated by a plane wave. Here, a photonic jet is discovered in the opposing plane wave propagation direction and lies in the area in the upper boundary of a near‐unity refractive index sphere on a high refractive index dielectric substrate. The redistribution of the power flow is inhibited during the reflected wave passing the near‐unity refractive index sphere. This has led to a unique effect on the focus position and shape of the produced photonic jet in reflection mode which can be maximally maintained near the sphere regardless of the modulation of the refractive index for the dielectric substrate material.     

Retarded treatment of substrate-related effects on granular films I. Polarizability of a single sphere

A multipolar treatment of the retarded fields radiated by a metal sphere on a dielectric substrate under the influence of an incident plane wave described. Unlike previous treatments, the present one allows boundary conditions, both on the spherical surface and on the substrate interface, to be matched by simply solving an adequate number of linear equations. The problem is thus reduced to inverting three complex matrices, whose sizes depend on the number of multipoles included in the field forms. The solutions, which represent the object and image multipoles (both in the incidence and in the transmission media) associated with the polarized sphere, are given the form of linear combinations of the multipoles associated with the unperturbed field (incident and reflected waves).     

The mechanism of the attracting acoustic radiation force on a polymer-coated gold sphere in plane progressive waves

Acoustic plane progressive waves incident on a sphere immersed in a nonviscous fluid exert a steady force acting along the direction of wave motion. It is shown here that when an elastic gold sphere is coated with a polymer-type (polyethylene) viscoelastic layer, this force becomes a force of attraction in the long wavelength limit. Kinetic, potential and Reynolds stress energy densities are defined and evaluated with and in the absence of absorption in the layer. Without absorption, the mechanical energy density counteracts the Reynolds stress energy density, which causes a repulsive force. However, in the case of absorption, the attractive force is predicted to be a physical consequence of a mutual contribution of both the mechanical and the Reynolds stress energy densities. This condition provides an impetus for further designing acoustic tweezers operating with plane progressive waves as well as fabricating polymer-coated gold particles for specific biophysical and biomedical applications.     

低损耗薄膜背散仪吸光器的关键技术研究

激光高反射膜片的背散光大小直接影响激光陀螺的精度,低损耗薄膜背散仪是在全积分散射测试理论及立体角积分散射测量原理的基础上提出的一种测量高反射膜片背散光的新方法.在低损耗薄膜背散仪的设计中,吸光器的吸光装置结构及吸光材料的选择直接影响着测量精度.计算表明,在用波长λ=632.8nm、激光功率P=10mW的氦氖激光照射至与...     

Focusing properties of small lenses

The potential use of small substrate lenses for coupling to antennas at millimeter wave frequencies is investigated by analyzing the focusing properties of dielectric spheres with quarter-wave matching layers. The fields and the power density are calculated at various points within the sphere to learn how the focusing deteriorates as the lens is made smaller. The absorption loss of the lens is also calculated. The calculations show that quartz spheres with quarter-wave matching layers, can exhibit good focusing properties down to a radius of about half a free space wavelength. This minimum radius was found to increase almost linearly with the refractive index of the lens. The calculations also indicate that at 94GHz the absorption losses of fused quartz, silicon and gallium arsenide lenses, with radii of one free space wavelength or less, are less than 0.2 dB. As the minimum diameter of a quartz lens is comparable with the spot size in free space, it should be possible to build an imaging lens array in which each lens will act as a separate imaging element.     

Mie theory with spatial dispersion

The classícal Mie theory for the scattering and absorption of electromagnetic radiation by a sphere is extended to the case of a dieelectric sphere which exhibits spatial dispersion. This is achieved by imposing the usual Maxwell boundary conditions as well as Pekar's additional boundary conditions at the surface of the sphere.     

Effects of a finite screening length on the absorption of electromagnetic waves

When an electromagnetic wave impinges on a semiconductor or ionic conductor having a sizeable screening length, it induces diffusion currents in addition to the ohmic currents, which affects the propagation in heterostructures or composite media involving such materials. In the simple geometries and in the low frequency regime studied here, the absorption may be either enhanced or reduced, depending on the parameters, and effects precluded for metals are predicted: extinction of the reflection by a plane wall, complete absorption of an electric multipolar wave by a sphere, disappearance of the scattering by a small sphere, vanishing of both reflection and transmission coefficients for a slab. If the screening length is larger than the skin depth, a slab with intermediate thickness may have a large transparency, and a thick piece of material is expected to be cooled down by the wave near the interface and overheated deeper inside. Received: 29 July 1997 / Revised: 24 November 1997 / Accepted: 20 February 1998     

Broadband metamaterial absorber on a single-layer ultrathin substrate

In this article, a broadband metamaterial microwave absorber on a low-cost FR-4 Epoxy substrate is proposed. The unit cell of the absorber consists of a staircase shape metallic patch placed on the top of the metal-backed ultrathin dielectric substrate having a thickness of 1.9 mm (0.07 λ₀). The absorption of more than 90% is achieved with this proposed low profile single-layer microwave absorber throughout the operation band from 8.86 to 15.5 GHz. The performance is analyzed for different values of incident angle, polarization angle, substrate height, and dielectric constant. The surface current and the power loss density at the top and bottom planes at the two absorption peaks of 9.46 and 13.90 GHz are also analyzed to elaborate the absorption mechanism of the structure. Experimental result closely follows the simulated one. The broadband characteristics of the design with relative absorption bandwidth (RAB) of 54.51% at both TE and TM polarizations of incident wave for a wide incident angles makes it versatile for applications in the X and Ku bands of microwave frequencies. The proposed work is very compact (unit cell size: 0.22 λ₀) with ultrathin substrate height (0.07 λ₀) and giving RAB performance of 54.51% comparable with that of others. Thus with this single-layer low-cost substrate material a broadband absorber is achieved.     

Electrodynamic response of a nanosphere placed in a magnetic field

Absorption of electromagnetic radiation by 2D electrons at the surface of a quantum sphere placed in a weak magnetic field is studied. It is shown that at low temperatures, the absorption curve exhibits four resonance peaks observed in the Faraday geometry (photon wave vector parallel to the magnetic field) and six peaks in the Voigt geometry (photon wave vector perpendicular to the magnetic field). In the particular case of the Voigt geometry, where the photon polarization vector is parallel to the magnetic field, the absorption curve exhibits only two resonance peaks. The shape, position, and intensity of the peaks are examined. It is shown that at temperatures close to zero, steps of two types appear in the absorption curve. One type of steps is associated with crossing of the μ-?ω level by the electron energy levels, while steps of the other type arise when electron energy levels cross the chemical potential μ.     

Sound scattering from two concentric fluid spheres (L)

The solution to the problem of plane wave and point source scattering by two concentric fluid spheres is derived. The effect of differences in sound speed, density, and absorption coefficient is taken into account. The scattered field is then found in the limit as the outer sphere becomes an infinitely thin shell and compared to the solution for a single fluid sphere for verification. A simulation is then performed using the concentric fluid sphere solution as an approximation to the human head and compared to the solution of a single fluid sphere with the properties of either bone or water. The solutions were found to be similar outside of the spheres but differ significantly inside the spheres.     

Absorption coefficient of luminescent bodies in the emission band

Einstein's theory associating the coefficients of absorption, induced emission and spontaneous emission cannot be applied to luminescent systems, because the radiative transitions occur with the absorption or emission of phonons. It may be generalised to include luminescent systems by the use of a hollow sphere whose internal walls are covered with a substance irradiated by a Wood lamp, placed at the centre of the sphere. From this we determine the absorption coefficient of the luminescent substance in the emission band.     

Infrared absorption by surface phonons and surface plasmons in small crystals

A continuum model is developed which describes the infrared absorption of interacting spheres very much smaller than the wavelength of light. A mode with uniform polarization inside the sphere is assumed responsible for the absorption. The frequency of this mode in a small isolated sphere can be greatly shifted from the bulk value due to the surface polarization charge. If the particles are not isolated, this frequency is also influenced by the dipolar interaction of neighboring particles. We find that it is possible to define an average dielectric function for a layer of small interacting, absorbing spheres which has the Lorentz form for either phonon or plasmon excitations. If both plasmon and phonon excitations are possible in the same particle, the absorption coefficient of the coupled system displays two resonances and an antiresonance.     

高斯光束中吸收双层球形微粒的横向光俘获

为了研究吸收双层球形微粒的横向光俘获,基于几何光学模型提出了双层带吸收球形微粒的光俘获模型,对TEM₀₀模式高斯光束照射下外层有光吸收的双层电介质球形微粒受到的横向光俘获力进行了数值模拟,取得了光俘获力特性的一系列结果.结果显示,双层球形微粒的外层吸收系数对包括稳态俘获位置,峰值强度,稳态俘获的刚度等光俘获特性有很大影响.此外,内外径的比率对吸收双层球形微粒的光俘获特性也有调制性的影响.在一定条件下,带吸收的双层球形微粒可以被俘获在光轴上,也可能被俘获在中心在光轴上的圆环上. 关键词： 光俘获 几何光学模型 高斯光束 吸收双层球     

Integrating sphere effective optical path length calibration by gas absorption spectroscopy

A method of integrating sphere effective optical path length (EOPL) evaluation using tunable diode laser absorption spectroscopy for gas detection was demonstrated. Oxygen was used as a sample gas for an 8.38 cm diameter integrating sphere calibration; 393.7 ± 1.3 cm EOPL was obtained from the wavelength modulation spectroscopy with second harmonic calibration by measuring oxygen P11 line at 764 nm, which is in agreement with that of 393 cm by using direct absorption spectroscopy calibration. The EOPL calibration accuracy of this method can reach 0.33 %. It has been justified that the EOPL of an integrating sphere is independent of the incident light intensity.