Vectorial modeling of near-field imaging with uncoated fiber probes: transfer function and resolving power

Using exact 3D vectorial simulations of radiation coupling into uncoated dielectric fiber probes, we calculate amplitude transfer functions for conical single-mode fiber tips at the light wavelength of 633 nm. The coupling efficiency of glass fiber tips is determined in a wide range of spatial frequencies of the incident radiation for opening angles varying from 30 degrees to 120 degrees . The resolution in near-field imaging with these tips is considered for field distributions limited in both direct and spatial-frequency space. The characteristics of the transfer functions describing the relation between probed optical fields and near-field images are analyzed in detail. The importance of utilizing a perfectly sharp tip is also examined.     

Characterization of near-field optical probes

Radiation and collection characteristics of four different near-field optical-fiber probes, namely, three uncoated probes and an aluminum-coated small-aperture probe, are investigated and compared. Their radiation properties are characterized by observation of light-induced topography changes in a photosensitive film illuminated with the probes, and it is confirmed that the radiated optical field is unambiguously confined only for the coated probe. Near-field optical imaging of a standing evanescent-wave pattern is used to compare the detection characteristics of the probes, and it is concluded that, for the imaging of optical-field intensity distributions containing predominantly evanescent-wave components, a sharp uncoated tip is the probe of choice. Complementary results obtained with optical phase-conjugation experiments with the uncoated probes are discussed in relation to the probe characterization.     

近场光镊与AFM探针复合的光阱力分析

本文针对纳米材料的纳米操作,提出了一种复合激光近场光镊与AFM探针进行纳米操作的方法,并基于动量守恒原理,采用三维时域有限差分方法建立了该方案中激光近场对纳米微粒的作用力模型,分析了各轴向光阱力的分布情况,讨论了两探针间距离、针尖材料的电导率、入射平面光场的偏振方向、入射角和波长等参数对近场光阱力的影响.结果表明:位于...     

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.     

Parameter control, characterization, and optimization in the fabrication of optical fiber near-field probes

Tip diameter and transmission efficiency of a visible-wavelength near-field optic probe determine both the lateral spatial resolution and experimental utility of the near-field scanning optical microscope. The commonly used tip fabrication technique, laser-heated pulling of fused-silica optical fiber followed by aperture formation through aluminization, is a complex process governed by a large number of parameters. An extensive study of the pulling parameter space has revealed a time-dependent functionality between the various pulling parameters dominated by a photon-based heating mechanism. The photon-based heat source results in a temperature and viscosity dependence that is a complex function of time and fiber diameter. Changing the taper of the optical probe can affect transmission efficiency by an order of magnitude or more.     

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.     

Numerical simulation of characteristics of near-field microstrip probe having pyramidal shape

A pyramid-type microstrip probe (PTMP) with metal tips is proposed for scanning near-field microscopes to obtain high spatial resolution of a few nanometers and high optical efficiency. Properties of an ordinary PTMP and the PTMP with a single metal tip are investigated by using a rigorous finite-integral technique simulation (MICROWAVE STUDIO package) and analyzing characteristics of working modes of the probe. Numerical simulation has demonstrated that an ordinary PTMP and the PTMT with a single metal tip exhibit large far- and near-transmission coefficients, field enhancement, and high spatial resolution. These high parameters imply that both types of microstrip probe may be utilized for optical and magnetic data storage, nanolithography, and other types of nanotechnology that use light for modification of a thin surface layer.     

Experimental study of near-field light collection efficiency of aperture fiber probe at near-infrared wavelengths

We examined the near-field collection efficiency of near-infrared radiation for an aperture probe. We used InAs quantum dots as ideal point light sources with emission wavelengths ranging from 1.1 to 1.6 μm. We experimentally investigated the wavelength dependence of the collection efficiency and compared the results with computational simulations that modeled the actual probe structure. The observed degradation in the collection efficiency is attributed to the cutoff characteristics of the gold-clad tapered waveguide, which approaches an ideal conductor at near-infrared wavelengths.     

Surface waves and atomic force microscope probe-particle near-field coupling: discrete dipole approximation with surface interaction

Evanescent waves on a surface form due to the collective motion of charges within the medium. They do not carry any energy away from the surface and decay exponentially as a function of the distance. However, if there is any object within the evanescent field, electromagnetic energy within the medium is tunneled away and either absorbed or scattered. In this case, the absorption is localized, and potentially it can be used for selective diagnosis or nanopatterning applications. On the other hand, scattering of evanescent waves can be employed for characterization of nanoscale structures and particles on the surface. In this paper we present a numerical methodology to study the physics of such absorption and scattering mechanisms. We developed a MATLAB implementation of discrete dipole approximation with surface interaction (DDA-SI) in combination with evanescent wave illumination to investigate the near-field coupling between particles on the surface and a probe. This method can be used to explore the effects of a number of physical, geometrical, and material properties for problems involving nanostructures on or in the proximity of a substrate under arbitrary illumination.     

Lambda/4 resonance of an optical monopole antenna probed by single molecule fluorescence

We present a resonant optical nanoantenna positioned at the end of a metal-coated glass fiber near-field probe. Antenna resonances, excitation conditions, and field localization are directly probed in the near field by single fluorescent molecules and compared to finite integration technique simulations. It is shown that the antenna is equivalent to its radio frequency analogue, the monopole antenna. For the right antenna length and local excitation conditions, antenna resonances occur that lead to an enhanced localized field near the antenna apex. Direct mapping of this field with single fluorescent molecules reveals a spatial localization of 25 nm, demonstrating the importance of such antennas for nanometer resolution optical microscopy.     

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     

Deducing structural variations of the apex of probes used in near-field optical microscopy through simultaneous measurement of shear force and evanescent intensity

We propose a simple method employing the simultaneous detection of evanescent intensity and shear force to deduce variations in the near-field optical morphology of the apex of the probes used in near-field microscopy. Fabrication of our probes involves sharpening by chemical etching, metal coating, and removal of metal from the apex. We show that through the simultaneous measurement of shear force and evanescent intensity, it is possible to detect variations in the optical morphology of the very apex of the probes during near-field imaging by a scanning near-field optical microscope.     

Evanescent coupling in magneto-optical and phase-change disk systems based on the solid immersion lens

Results of numerical computations pertaining to evanescent wave coupling for near-field magneto-optical and phase-change disks based on the concept of the solid immersion lens are presented. We investigated the relation between the coupling efficiency and the width of the air gap in terms of the throughput of the recording process and the resolution of the readout signal. The simulations show a drastic decrease with a widening air gap of the coupling efficiency by means of evanescent waves into the recording medium. In magneto-optical readout, loss of the signal may be attributed to the reduction of magneto-optical interaction, the rise of reflectance, and the variation of the relative phase between the two components of polarization. In the phase-change readout the reduced reflectivity contrast between crystalline and amorphous marks is the cause of signal reduction.     

Scanning evanescent fields using a pointlike light source and a nanomechanical DNA gear

The characterization of three-dimensional inhomogeneous illumination fields is a challenge in modern microscopy. Here we use a four-arm DNA junction as a nanomechanical translation stage to move a single fluorescent quantum dot through an exponentially decaying evanescent field. Recording the emission of the quantum dot within the evanescent field as well as under homogeneous illumination allows one to directly obtain the intensity distribution of the excitation field without additional deconvolution. Our method will allow the characterization of a broad range of illumination fields and to study near-field effects between small optical probes.     

Direct measurement of standing evanescent waves with a photon-scanning tunneling microscope

We present a detailed analysis of a standing evanescent wave that is caused by total internal reflection of an Ar-ion laser beam on a glass prism and investigate the coupling to a subwavelength dielectric tip of a photon-scanning tunneling microscope that is raster scanned at a close distance over the prism surface. The intensity of the evanescent field is spatially modulated with a period of 239.2 nm. It decays exponentially with a constant of 103.9 nm with increasing distance from the prism surface. Precise measurements of the standing evanescent wave can be used to calibrate the scanner and permit one to determine the spatial resolution and the coupling efficiency of the tip.     

光子扫描隧道显微镜探测场的数值模拟计算

对光子扫描隧道显微镜（ＰＳＴＭ）的显微成象机理。扫描探测模型做了较为系统的分析和论述。针对具体的探测模型，利用微机编程计算ＰＳＴＭ探测光场的空间分布，并对场分布随扫描控制参量的变化规律进行系统的研究，得到了与实际探测结果一致的分布规律，最终为ＰＳＴＭ的显微成象技术提供了更为具体的理论指导。     

Measurement of the enhanced evanescent fields of integrated waveguides for optical near-field sensing

The sensitivity of an integrated optical sensing device can be enhanced by coating it with a high refractive index layer, while both incoupled intensity and spatial resolution are maintained. The potential for enhanced sensing is demonstrated using titanium indiffused waveguiding structures in LiNbO(3) coated with a TiO(2) film. To the best of our knowledge, it could be measured for the first time that the outcoupled intensity at the surface was enhanced by a factor of 12-15 while keeping the penetration depth of the evanescent field constant of the order of only a few tens of nanometers. The evanescent fields of the guided modes were measured and characterized with a scanning near-field optical microscope and are in accordance with the numerical simulations.     

Near-Field Photothermal Heating with a Plasmonic Nanofocusing Probe

Noble metal nanostructures support plasmon resonances—collective oscillation of charge carriers at optical frequencies—and serve as effective tools to create bright light sources at the nanoscale. These sources are useful in broad application areas including, super-resolution imaging and spectroscopy, nanolithography, and near-field optomechanical transducers. The feasibility of these applications relies on efficient conversion of free-space propagating light to plasmons. Recently, we demonstrated a hybrid nanofocusing scheme for efficient coupling of light to plasmons at the apex of a scanning probe. In the approach, free-space light is coupled to propagating surface plasmon polaritons (SPPs) on the tapered shaft of the scanning probe. The SPPs propagate adiabatically towards the probe tip where they are coupled to localized plasmons (LSPs). The nanofocusing scheme was explored in a near-field scanning optical microscope for super-resolution imaging, near-field transduction of nanomechanical vibrations, and local detection of ultrasound. Owing to the strong concentration of light at the probe, significant heating of the tip and a sample positioned in the optical near-field is expected. This paper investigates the local heating produced by the plasmonic nanofocusing probe under steady-state conditions using the tip-enhanced Raman scattering approach. In addition, a finite element model is explored to study the coupling of free propagating light to LSPs, and to estimate the temperature rise expected in a halfspace heated by absorption of the LSPs. This study has implications for exploring the plasmonic nanofocusing probe in heat-assisted nanofabrication and fundamental studies of nanoscale heat transport in materials.     

波数空间中活塞辐射体近场声压分布的计算及测试

在近场范围内,声源在任何平面上的声场都可以通过平面传播波和倏逝波的叠加来得到。以圆形活塞为例,在波数空间中利用角谱法求得其辐射声场的传播波和倏逝波的分布。借助数值分析方法,对圆形活塞声源声轴线上的声压进行计算,得到相应的近场声压分布曲线。由角谱法计算得到的近远场分界点与点源组合法所得结果一致。对不同声源半径时,不同辐射频率下的声压分布曲线进行比较,得出了近场声压的变化规律。阶梯圆盘在辐射理论上应等于对应圆形活塞的辐射。对设计的阶梯圆盘的轴向声压进行了测试,实验测试结果与理论计算结果基本吻合。     

Three-dimensional optical-transfer-function analysis of fiber-optical two-photon fluorescence microscopy

The three-dimensional optical transfer function is derived for analyzing the imaging performance in fiber-optical two-photon fluorescence microscopy. Two types of fiber-optical geometry are considered: The first involves a single-mode fiber for delivering a laser beam for illumination, and the second is based on the use of a single-mode fiber coupler for both illumination delivery and signal collection. It is found that in the former case the transverse and axial cutoff spatial frequencies of the three-dimensional optical transfer function are the same as those in conventional two-photon fluorescence microscopy without the use of a pinhole.However, the transverse and axial cutoff spatial frequencies in the latter case are 1.7 times as large as those in the former case. Accordingly, this feature leads to an enhanced optical sectioning effect when a fiber coupler is used, which is consistent with our recent experimental observation.