Near-field optical properties of silver nanocylinders arranged in a Pascal triangle

The Pascal triangle is a geometric representation of binomial coefficients in triangular form. We utilize this formalism to deterministically arrange silver nanocylinders of different sizes (30, 60, and 90 nm) on a triangle and numerically study their near-field optical properties. We show that near-field intensities at specific points on this triangle depend on the wavelength and angle of incidence. From the wavelength-dependent studies at various junctions of nanocylinders, we obtain maximum near-field intensity at 350 and 380 nm. By varying the angle of incidence of the TM-polarized plane wave, we find systematic variation in the near-field intensity at different junctions of the geometry. Our study will lead to insights in designing controllable electromagnetic hot spots for chip-based plasmonic devices.     

Optical contrast in apertureless microscopy

We report on optical image contrast for a specific apertureless near-field optical microscope. We demonstrate that the main part of the optical image's contrast results from the sample's topography. The coupling mechanism is analyzed, and we show that the microscope can be regarded as an interferometer that sensitively detects near-field components. However, in the basic configuration the reference field of the interferometer is coupled to the topography. Finally, it is demonstrated that, by controlling the phase of the reference field, one can largely decorrelate the optical image from the topography.     

Optimizing the near field around silver tips

Owing to their promise of obtaining optical as well as topographic information in nanometer scale, apertureless near-field scanning optical microscopy (NSOM) and apertureless near-field scanning optical spectroscopy have drawn much attention recently. However, NSOM is still not a mature technique. A proper understanding of and the ability to tune the near field around the tip end is critically important in NSOM instrumentation and in NSOM image interpretation. On the basis of reflection geometry, we systematically studied the effects of a number of parameters pertinent in the application of apertureless NSOM, e.g., polarization, incident angle, wavelength of the incident laser, tip material, and tip length, by using the generalized field propagator technique. Our results show that all the above parameters have a significant influence on near-field enhancement and that care must be taken in the design of the experiment in order to maximize the near field. In addition to apertureless NSOM and spectroscopy, apertureless near-field lithography can benefit from these simulation results.     

Numerical study of the displacement of a three-dimensional Gaussian beam transmitted at total internal reflection. Near-field applications

Longitudinal and transverse shifts of a light beam at total internal reflection was experimentally studied by far-field measurements on the reflected field. We propose to use a scanning tunneling optical microscope (STOM) to study these shifts in transmission, and we present a theoretical model of this proposed experiment to obtain a numerical estimation of these shifts. We study the reflection and the transmission of a three-dimensional polarized incident beam. We verify the validity of our formalism by studying the Goos-Hanchen shift in reflection and by comparing our results with published ones. Then we calculate STOM images of the transmitted field distribution. On the images the well-known Goos-Hanchen shift is easily observed. But we also encounter a smaller shift, perpendicular to the plane of incidence. This transverse shift was also observed in reflection by Imbert and Levy [Nouv. Rev. Opt. 6, 285 (1975)]. We study the variations of the two shifts versus various parameters such as the angle of incidence, the optical index, and the incident polarization. Then we discuss the feasibility of the near-field observation of these shifts.     

光纤探针型近场光镊光强分布特性的数值模拟

光纤探针型近场光镊是近场光学领域中的新型技术,因其可对纳米尺度微粒直接进行捕获和操纵而受到广泛关注,其光纤探针尖端的近场分布特性影响着纳米粒子捕获及操纵的成败探针金属膜外侧电磁场由光波在针尖小孔处衍射而成,根据夫朗和费衍射公式分析了圆形纳米小孔的光波衍射图样,运用时域有限差分法（FDTD）研究了均匀平面波垂直入射于镀膜光纤探针的近场分布,比较了不同锥角、不同出射孔径、不同金属膜厚度及不同入射波长的近场分布情况,并对不同情况下的通光效率进行了分析。通过对各参数的计算与比较,结果表明,当锥角越大、孔径越大、镀合适膜厚并且入射波长越小时,探针尖端的出射光强越大并具有较大的通光效率     

Scanning near-field optical data contrast measurement: a tomographylike near-field reconstruction

A method for the reconstruction of near-field optical signals is proposed to produce a tomographylike map of the field above the nanostructures being studied. The data obtained through lock-in detection are processed employing Chebyshev's polynomial function of the distance between the probe and the sample. The method is first applied to numerically generated near-field evanescent data, with three different decreasing lengths, and then applied to an experimental signal. Therefore the contrast of the signal above nanostructures is discussed to underline the discrepancy between the scanning near-field optical microscopy data and the reconstructions.     

Thermal infrared near-field spectroscopy

Despite the seminal contributions of Kirchhoff and Planck describing far-field thermal emission, fundamentally distinct spectral characteristics of the electromagnetic thermal near-field have been predicted. However, due to their evanescent nature their direct experimental characterization has remained elusive. Combining scattering scanning near-field optical microscopy with Fourier-transform spectroscopy using a heated atomic force microscope tip as both a local thermal source and scattering probe, we spectroscopically characterize the thermal near-field in the mid-infrared. We observe the spectrally distinct and orders of magnitude enhanced resonant spectral near-field energy density associated with vibrational, phonon, and phonon-polariton modes. We describe this behavior and the associated distinct on- and off-resonance nanoscale field localization with model calculations of the near-field electromagnetic local density of states. Our results provide a basis for intrinsic and extrinsic resonant manipulation of optical forces, control of nanoscale radiative heat transfer with optical antennas, and use of this new technique of thermal infrared near-field spectroscopy for broadband chemical nanospectroscopy.     

Near-field dynamics of optical Yagi-Uda nanoantennas

We present near-field measurements of optical Yagi-Uda nanoantennas that are used in receiving mode. The eigenmode imaging of amplitude and phase by apertureless scanning near-field optical microscopy allows us to investigate the dynamics of the local out-of-plane electric field components and to visualize the temporal evolution of this time-harmonic reception process. The antenna directionality manifests itself by the dependence of the local field enhancement at the feed element on the illumination direction. Simulations taking into account the substrate confirm our observation of the directionality. Our work demonstrates the possibility to characterize multielement nanoantennas by electromagnetic antenna near-field scanners.     

Local near-field enhancement of random Sb-SiN films

Local near-field enhancement of random Sb-SiN films has been studied. Specimens consisting of a random Sb-SiN film and an optical recording layer were prepared and exposed to a focused laser beam. Laser-induced ablation occurred on the recording layer adjacent to random Sb-SiN film much faster and at much lower power than on a single recording layer. These results indicate that an optical field can be enhanced by random Sb-SiN films. The enhanced field was subsequently investigated by scanning near-field optical microscopy, and the pictures revealed that the enhanced field was localized.     

Optical noise induced by Gaussian sources in stokes parameter measurements

A novel formalism for determining the source-induced noise in Stokes parameter measurements is derived for sources with Gaussian statistics. The formalism is based on a concise expression for the autocovariance functions of the Stokes parameters in terms of the second-order correlation properties of the optical field. At the output of an optical system, source-induced noise can result not only from the intensity fluctuations of the source but also from phase or polarization fluctuations. To describe the effect of the system, another formalism for the propagation of the second-order correlation properties of the optical field is derived. We apply the formalisms to analyze source-induced noise at the output of a birefringent medium, and in coherence-multiplexing networks.     

Optical near-field Raman imaging with subdiffraction resolution

We report optical near-field Raman imaging with subdiffraction resolution (approximately 120 nm) without field enhancement effects. Chemical discrimination on tetracyanoquinodimethane organic thin films showing localized salt complexes is accomplished by detailed Raman maps. Acquisition times that are much shorter than previously reported are due to the high Raman efficiency of the materials and to careful collection and detection of the optical signals in our near-field Raman spectrometer.     

Adiabatic nanofocusing scattering-type optical nanoscopy of individual gold nanoparticles

We explore imaging of local electromagnetic fields in the vicinity of metallic nanoparticles using a grating-coupled scattering-type near-field scanning optical microscope. In this microscope, propagating surface plasmon polariton wavepackets are launched onto smooth gold tapers where they are adiabatically focused toward the nanometer-sized taper apex. We report two-dimensional raster-scanned optical images showing pronounced near-field contrast and demonstrating sub-30 nm resolution imaging of localized surface plasmon polariton fields of spherical and elliptical nanoparticles. By comparison to three-dimensional finite-difference time domain simulations, we conclude that virtually background-free near-field imaging is achieved. The microscope combines deep subwavelength resolution, high local field intensities and a straightforward imaging contrast, making it interesting for a variety of applications in linear and nonlinear nanospectroscopy.     

Near-field second-harmonic generation

Near-field microscopy of second-harmonic generation combines high surface sensitivity of the nonlinear optical process and high spatial resolution of the near-field optics. It enables investigation of nonlinear optical phenomena at the nanoscale and provides an opportunity to develop a highly sensitive optical technique for materials characterization. In this paper we overview apertured and apertureless near-field approaches for local studies of second-harmonic generation. Near-field second-harmonic generation at metal surfaces and nanostructures and related electromagnetic field enhancement and confinement effects are considered. The latter demonstrates the feasibility of achieving nanoscale light sources using second-harmonic generation. Applications of second-harmonic near-field microscopy for characterization of magnetic and ferroelectric materials are also discussed. Nonlinear nano-optics can lead to the development of novel photonic devices on the sub-wavelength-scale as well as new tools for imaging and local optical studies of materials, chemical and biological species.     

Propagation and diffraction of locally excited surface plasmons.

With the use of optical near-field techniques, it is now possible to excite or observe surface plasmons with high lateral resolution. A theoretical study is presented of surface plasmon excitation by near-field optical probes and the influence of well-defined structures on surface plasmon propagation and surface plasmon detection in the far field. The generation and the diffraction of the surface plasmon is calculated by using a theoretical scheme founded upon a first-order perturbation expansion of the Rayleigh-Fano method. A very good agreement is obtained between numerical and experimental results. The theoretical tools used should prove a useful guideline for future experiments of nanooptics with surface plasmons.     

Effect of mesoscopic semiconductor perturbations on the electromagnetic field distribution in near-field optical microscopy

The effect of mesoscopic semiconductor perturbations in the volume of a dielectric sample on the electromagnetic field intensity distribution was theoretically studied for a scanning near-field optical microscope operating in the collecting mode for the surface structure investigation. A system of integral equations is constructed which takes into account for the first time the possibility that inhomogeneities are present simultaneously on the surface and in the bulk of the sample. It is shown that the near-field distortions introduced by the above perturbations significantly depend both on the location of a perturbation and on the properties of free charge carriers in this region. This effect must always be taken into account in near-field optical microscopy.     

Optical singularities and power flux in the near-field region of planar evanescent-field superlenses

We rigorously analyze the optical singularities and power flux in the near-field region of the novel superlenses reported in [Science317, 927 (2007)] For this purpose, we derive near-field expressions and a general criterion to classify the optical singularities in the vacuum, which are valid when the (s- or p-polarized) electromagnetic fields are generated by any planar field distribution with Cartesian or azimuthal symmetry. Such general results are particularized to the superlenses [Science317, 927 (2007)], for which we identify a sequence of optical vortices and saddles that arise from evanescent-field interference. While the saddles are always located around the focal region, the vortex locations depend on the source field. The features of the topological connection between vortices and saddles are also discussed.     

Lasing frequencies and thresholds of the dipole supermodes in an active microdisk concentrically coupled with a passive microring

The lasing spectra and threshold values of material gain for the dipole-type supermodes of an active microdisk concentrically coupled with an external passive microring are investigated. TE polarized modes are treated accurately using the linear electromagnetic formalism of the 2-D lasing eigenvalue problem (LEP) with exact boundary and radiation conditions. The influence of the microring on the lasing frequencies and thresholds is studied numerically, demonstrating threshold reduction opportunities. This is explained through the analysis of the mode near-field patterns and the degree of their overlap with the active region, as suggested by the optical theorem applied to the LEP solutions.     

反射式近场光学显微镜样品近场光分布特性

建立了一种反射式近场光学显微镜中样品近场光分布特性的模型,应用矢量衍射理论,得到了系统的场方程。在弱波动条件下,采用微扰法对场方程进行了求解,能方便地得到样品表面的各阶近场光反射和透射模复振幅表达式。计算结果表明,一级场分布要比零级场小一个量级,各阶近场信号的强弱完全由面形函数的傅里叶变换决定。通过与零级结果的比较,证明了计算结果的正确性。提供了一种计算样品表面近场分布简便方法,对反射式近场光学显微镜中调制检测技术具有指导意义。     

Surface plasmon near-field resonance characteristics of silver shell nanocylinders arranged in triangular geometry

The optical near-field surface plasmon effects of a triangular system of silver nanoshell cylinders are numerically studied using the two-dimensional finite difference time domain method. The dependence of interparticle distance, shell thickness of the cylinder, dielectric constant of shell core as well as embedding medium, and orientation of the optical source plane on the plasmonic resonances of the nanocylinder shells is studied. The plasmonic resonances are found to have strong dependence on the interparticle distance. As the size of the particle is increased, the field intensity peak shows a redshift. The resonance condition varies with the dielectric constant of the environment as well as the core. In addition, the orientation of the incident source plane has a significant role in the near-field intensity distribution. Since the near-field intensity has the same trend as that of the scattering cross section, the results can be used in the design of various applications like sensing, antennas, and waveguides.     

纳米光学天线研究现状及进展

基于spp共振效应的纳米天线结构能有效收集光的能量,并将它限制在亚波长尺度,其巨大的局域场增强效应为纳米光子学大范围的应用提供了广阔的前景,例如,纳米尺度内光信息的处理和传播、近场显微镜超分辨率成像、高效太阳能电池、超高密度近场光存储、纳米光刻、量子单光予源、高效率纳米电磁波集中器、高灵敏度生化传感器等。文章在介绍纳米天线的基本构造和工作原理的基础上,综述了其在多个领域的应用。