Near-field spatial imaging of the Ni-like palladium soft-X-ray laser

We report on a series of near-field imaging experiments for the nickel-like 14.7-nm Pd X-ray laser that were conducted with the aim to characterize the two-dimensional source intensity distribution and its position relative to the target surface. The effect of different prepulse amplitudes at variable main pulse-to-prepulse delays is investigated. For larger prepulses (2.8% and 8%) the peak emission is moved farther away from target surface with increasing delay whereas for the weak prepulse (0.5%) the position of peak emission remains rather stable over a delay range of several nanoseconds. A great deal of structure was observed in the near-field image. The size of the smallest structures detected is consistent with the coherence length-obtained from the corresponding far-field data – at the output aperture. The measurements are compared to the simulations performed with the LASNEX and CRETIN codes. The LASNEX code is used to calculate the hydrodynamic evolution of the plasma and provide the temperatures and densities to the CRETIN code, which then does the kinetics calculations to determine the gain.     

Near-field thermoelastic imaging coupled with infrared detection

Near-field thermoelastic imaging is a simple way to investigate the thermal and coupled thermoelastic properties of materials. A few microscopes, deriving from the atomic force microscope, have been used to observe and to quantify the samples observed. But the main problem is the absolute measurement of the temperature, because surface topography and thermal expansion contributions are not easily discernible. In the proposed SThEM (scanning thermoelastic microscope), the tip is excited at the resonance frequency of the cantilever and the sample is periodically heated by the Joule effect. Thus the static contributions (drift, topography) are reduced. Moreover, a radiometric sensor, operating in the far field, has been added in order to quantify the temperature. This multi-acquisition microscope enables one to investigate small objects at the nanoscale with complementary information at the micrometric scale.     

Susceptibilities of nano-particles at the surface of a solid

The self-consistent approach is proposed for calculation of an effective susceptibility of single nano-particle and nano-particle layer at a surface of a solid. This approach is based on exact summation of iteration procedure series, which is applied to solution of self-consistent equation for a local field. The condition when the layer can be considered as dilute one is discussed.     

Near-field optical imaging using force detection with new tip-electrode geometry

We propose a new tip-electrode geometry to detect an (optical) evanescent field using noncontact atomic force microscopy. Using a semi-transparent metal electrode on the prism surface, the force sensitivity due to evanescent field in new tip-electrode geometry was enhanced by a factor of about 1000, comparing with that in old tip-electrode geometry where electrode was located behind the prism. Furthermore, this tip-electrode geometry avoids the electrostatic field caused by the residual charges and contact-electrified charges near the prism surface, which affects the force sensitivity due to evanescent field. We demonstrated the high resolution imaging of the evanescent field on the Au film with 15-nm (λ/33) lateral resolution.     

Near-field calculation based on the T-matrix method with discrete sources

We propose a novel approach to compute the field scattered by a particle in the near-zone, in the framework of the transition matrix formulation. This method is based on the expansion of the total near-field in terms of discrete sources vector spherical wave functions and turns out to be particularly effective to model strongly elongated or flattened particles with high permittivity. The performances are evaluated on gold spheroids with several aspect ratios. This method results very useful to understand the complex behavior of enhanced plasmon fields in resonant metallic nanostructures.     

Near-field optical characterization of interacting and non-interacting gold nanoparticles embedded in a silica thin film

By near-field optics, we characterized the local optical properties of clusters of gold nanoparticles randomly distributed under a 50 nm-thick SiO₂ thin film. A local field enhancement is visible above isolated clusters. A few hundred nanometers away from them, we observed a polarization-dependent pattern with elliptical lobes oriented in the incident polarization direction. A simple simulation shows that the observed near-field images can be represented by the sum of the field of an oscillating dipole and the incident field. When the cluster density is larger, the measured near-field images show numerous bright and dark spots. The position of the bright spots does not necessarily coincide with the gold clusters showing the presence of coupling effects between them.     

Near-field and far-field modeling of scattered surface waves. Application to the apertureless scanning near-field optical microscopy

The detection of surface waves through scanning near-field optical microscopy (SNOM) is a promising technique for thermal measurements at very small scales. Recent studies have shown that electromagnetic waves, in the vicinity of a scattering structure such as an atomic force microscopy (AFM) tip, can be scattered from near to far-field and thus detected. In the present work, a model based on the finite difference time domain (FDTD) method and the near-field to far-field (NFTFF) transformation for electromagnetic waves propagation is presented. This model has been validated by studying the electromagnetic field of a dipole in vacuum and close to a dielectric substrate. Then simulations for a tetrahedral tip close to an interface are presented and discussed.     

Synthesis and surface functionalization of silica nanoparticles for nanomedicine

There are a wide variety of silica nanoformulations being investigated for biomedical applications. Silica nanoparticles can be produced using a wide variety of synthetic techniques with precise control over their physical and chemical characteristics. Inorganic nanoformulations are often criticized or neglected for their poor tolerance; however, extensive studies into silica nanoparticle biodistributions and toxicology have shown that silica nanoparticles may be well tolerated, and in some case are excreted or are biodegradable. Robust synthetic techniques have allowed silica nanoparticles to be developed for applications such as biomedical imaging contrast agents, ablative therapy sensitizers, and drug delivery vehicles. This review explores the synthetic techniques used to create and modify an assortment of silica nanoformulations, as well as several of the diagnostic and therapeutic applications.     

Acoustic field imaging by light reflection at the free surface of a liquid

In this paper we present a novel technique for acoustic field imaging. This technique is based on reflection of a collimated laser beam at the free surface of a liquid. The reflected beam becomes phase modulated by the acoustic wave as in acoustical holographic systems. We do not use a reference acoustical beam for holographic reconstruction but we observe this phase modulation using dark-field techniques. It gives a measurement of the acoustic field power as a function of the position. The authors have built an optical imaging system and carried out experiments with piezocomposite transducers. The technique presented in this work is able to give fast quantitative information about the performance of individual ceramic rods of the piezocomposite.     

Near-field spectroscopy of surface plasmons in flat gold nanoparticles

We use near-field interference spectroscopy with a broadband femtosecond, white-light probe to study local surface plasmon resonances in flat gold nanoparticles (FGNPs). Depending on nanoparticle dimensions, local near-field extinction spectra exhibit none, one, or two resonances in the range of visible wavelengths (1.6-2.6 eV). The measured spectra can be accurately described in terms of interference between the field emitted by the probe aperture and the field reradiated by driven FGNP surface plasmon oscillations. The measured resonances are in good agreement with those predicted by calculations using discrete dipole approximation. We observe that the amplitudes of these resonances are dependent upon the spatial position of the near-field probe, which indicates the possibility of spatially selective excitation of specific plasmon modes.     

全内反射式衍射光栅近场光学特性

 利用傅里叶模式理论分析了具有高衍射效率的全内反射式衍射光栅在TE 和TM偏振态下的近场光分布特点,讨论了光栅结构参数以及入射角度对光栅内电场增强的影响。结果表明：全内反射光栅内部电场分布对偏振态较敏感,光栅槽深和占宽比对电场增强影响较小,光栅内的峰值电场随光栅周期增大而增大,并且峰值电场随着入射角度的增大而减小。在应用于高功率激光时,降低光栅内部的电场增强可以有效降低损伤风险。     

Near-field probing of photonic crystals

Photonic crystals form an exciting new class of optical materials that can greatly affect optical propagation and light emission. As the relevant length scale is smaller than the wavelength of light, sub-wavelength detection forms an important ingredient to obtain full insight in the physical properties of photonic crystal structures. Spatially resolved near-field measurements allow the observation of phenomena that remain hidden to diffraction-limited far-field investigations. Here, we present near-field investigations in both collection and illumination modes that highlight the power of local studies. We show how propagation losses are unambiguously determined and that light detected in far-field transmission can actually contain contributions from different, sometimes unexpected, local scattering phenomena. Simulations are used to support our findings. Furthermore, it is shown that local coupling of light to a thick three-dimensional photonic crystal is position-dependent and that the spatial distribution of the coupling efficiency itself is frequency-dependent.     

光纤探针型近场光镊光阱力特性研究

基于动量守恒原理,结合麦克斯韦应力张量和三维时域有限差分方法,建立了近场空间内激光光镊对纳米微粒的光阱力计算模型.分析了光纤探针型近场光镊的近场分布以及操作纳米微粒时各轴向光阱力的分布情况,并探讨了光纤探针尖端的捕获尺寸、捕获位置和操作稳定性.结果表明:微粒应处于光纤探针针尖的近场空间内才可实现稳定可靠的纳米操作,不同尺寸的微粒具有不同的捕获效果,且随初始位置的不同微粒的捕获位置亦不同.计算结果为激光近场光镊纳米操作装置的设计和制造提供了理论基础.     

反射式无孔径近场Raman研究(英文)

近场扫描光学显微技术与Raman光谱技术的结合能够在纳米尺度下提供化学 /结构信息 ,这对很多应用都是至关重要的 ,比如硅器件 ,纳米器件 ,量子点及生物样品单分子研究。本文报导了采用无孔径探针的近场Raman研究。我们的系统有两大特征 :1 近场Raman的增强是通过金属探针上的银镀层实现的 ,无需样品准备 ;2 系统在反射模式下工作 ,适用于任何样品。这两点对实际应用是至关重要的。我们首次在实际硅器件上用 1秒积分时间获得了 1维近场Raman映射和 2维近场Raman图象。我们首次展示了由于积分时间短 ,该技术可用于成象用途。因此 ,这是近场扫描Raman研究中的一次巨大进步。此外 ,我们系统中采用的金属探针可同时用于AFM及电学特性成象 ,比如电阻 ,电容 ,这些是器件应用中的重要参数。     

Negative refraction and imaging properties of circular photonic crystals

The electromagnetic wave propagation in a two-dimensional (2D) circular photonic crystal (CPC) is investigated. The CPC structure is composed of air holes in the dielectric background material. The finite element method is used to study the optical and propagating properties of the CPC slab. Numerical simulations show that negative refraction and near-field imaging can appear in a 2D CPC slab. We also find that the high-symmetry CPC slab possesses an optics axis along the vertical direction intersecting the symmetric center. As a result, the CPC slab can exhibit an excellent imaging performance when a source is placed on the optical axis.     

Near-field characteristics of the parametric loudspeaker using ultrasonic transducers

A parametric speaker is a device for generating and focusing highly directional sound beams. It is essentially a by-product that comes with the nonlinearity of ultrasound. It is noteworthy that this directional beam was controlled and utilized mostly for far-field applications in the past. We empirically study the directivity and attenuation characteristics of the parametric loudspeaker in the near-field where we desire to use it. Physical parameters for experiments are imported from a theoretical model based on the far-field approximation. The findings are that increases in aperture size and modulation frequency cause higher directivity, but have more than twice the beamwidth of the far-field approximation. The attenuation also does not obey the inverse-square law which describes far-field spreading from acoustic sources. The results conclusively explain a series of formation and attenuation of the virtual sound sources and define limitations of use in the near-field.     

Near-field optics of a dielectric surface

The problem of classical optics related to the electromagnetic-field distribution near the surface of a dielectric medium with allowance made for its discrete structure is solved. The effect of a probe on the near-field optical observations is taken into account, and the condition is obtained under which this effect can be neglected. It is shown that the near-field observations can be used to get information not only on the surface structure but also on the effective fields in the near-surface region. A method for optically controlling the probe position over the surface is proposed.     

Observation of band gap and surface defects of ZnO nanoparticles synthesized via hydrothermal route at different reaction temperature

The effects of reaction temperature on the average particle size, surface defects and band gap of ZnO nanoparticles have been systematically investigated. The hydrothermal method was employed to synthesize ZnO nanostructures. The nanostructures of the resultant ZnO were studied by means of X-ray diffraction, Transmission Electron Microscopy, Ultraviolet-visible absorption, Raman, Fourier transform Infra-red and Photoluminescence spectroscopy. With increase in the reaction temperature, the peak position of the ultraviolet emission shifts slightly towards the red wavelength and the crystal quality was improved. The prepared ZnO nanoparticles have residual intermediate compound on the surface in the form of an acetate group, which acts as defect centers for the emission of yellow emission. Spectra analyses show that the visible emission depends strongly on the preparation conditions.     

近场X射线位相衬度成像及位相恢复算法

理论分析了X射线衍射位相成像和近场位相恢复算法。定义了一个最佳成像距离，给出了基于最佳成像距离和特征空间频率的衍射场光强分布新的表达形式。基于模拟位相物体给出了数值模拟结果。由此得出了探测距离和多色辐照对相衬图像和位相恢复结果的影响。本文给出的结果将对同轴x射线相衬成像实验具有一定的指导作用。     

Near-field radiative heat transfer between two plane surfaces with one having a dielectric coating

The effect of a dielectric coating on the near-field radiative heat transfer between two plane surfaces is numerically studied in the framework of the fluctuational electrodynamics. The dielectric coating is assumed to be a SiC or SiO₂ film, which is on top of the emitter. The results show that the near-field radiative flux between the plane surfaces can be either diminished or enhanced by the dielectric coating, depending on the thermal radiative properties of the emitter and the receiver. Furthermore, the dielectric coating effect on the near-field radiative flux can be very different from that on the far-field radiative flux. Detailed analysis on the variations of the TE- and TM-wave components of the radiative flux by adding the dielectric coating is provided, along with the physical mechanisms that account for these changes. Dielectric coatings such as SiC and SiO₂ films are widely seen in microelectronic structures and nanofabrication devices. The results obtained in this work should be valuable for further study and nanotechnological applications of near-field radiative heat transfer.