非探针近场光学显微镜的成像实现

介绍一种非探针近场光学显微镜，并对它的图像实现过程及处理方法进行了讨论。该装置以阿达玛变换成像理论为基础，用纳米多孔编码板代替光纤探针获得了较高的光学信噪比。成像研究表明，该装置可对近场衰逝波分量进行有效探测并能实现超衍射极限成像，为近场光学探测研究提供了一种新的思路。     

Role of the probe quadrupole moment in apertureless near-field optical microscopy

A theory of an apertureless scanning near-field optical microscope is developed that takes into account the quadrupole moment of the probe in forming the near field and the scattered wave in the far-field zone. It is shown that the quadrupole moment can lead to a change in the conditions for the resonant coupling of the probe tip with the sample surface, as well as to the appearance of additional resonances in the frequency dependence of the response of a near-field microscope. The theory developed is found to be in good agreement with known experimental data in the region of resonant tip—sample coupling.     

A Monatomic Optical Near-Field Microscope with Subnanometer Spatial Resolution

We propose the principle of operation of an optical near-field microscope on the basis of linear steady-state optical dimensional resonances in the system atom-probe–atoms of the sample. It is shown that such a microscope has high sensitivity and a spatial resolution of less than 1 nm and does not set limits on the choice of samples. The distribution of the optical fields in the wave and near zones in the near-field microscope at frequencies close to the optical dimensional resonances arising as a result of dipole-dipole interactions of atoms at small distances has been investigated. The optical scheme of the monatomic near-field microscope is presented.     

Near-field scanning optical microscopy based on surface-enhanced Raman scattering

We describe a near-field optical microscopy technique based on the interaction of a probe molecule with the sample surface (e.g., with a flat metal surface) in the field of external optical radiation and consider the spontaneous Raman scattering characterized, in the presence of a metal surface, by the effective polarizability of the probe molecule, depending on the frequency and the distance to the sample surface. At certain distances from the probe molecule to the surface, the effective polarizability of this molecule (determined with allowance for the polarizing influence of the surface of a semi-infinite medium) at the Stokes frequency sharply increases in comparison to the quantum polarizability of an isolated molecule, which is indicative of the formation of optical near-field resonances. It is shown that the proposed method of near-field optical microscopy is characterized by high sensitivity and high spatial resolution (on the order of 1 Å).     

Resolution enhancement in a reflection mode near-field optical microscope by second-harmonic modulation signals

The motion of the probe tip in a near-field scanning optical microscope, dithered by vibration of a tuning fork, can modulate the reflection signal from the sample surface not only at the fundamental dithering frequency but also at its second harmonic. By lock-in amplification of these modulated signals, enhanced optical images are obtained, even with an uncoated fiber probe. In particular, accurate optical images with higher resolution are obtained when the second-harmonic signal is detected, which results from the parametric modulation of the tip-sample separation at the double frequency of the horizontal dithering motion of the tip. Using a DVD ROM with a track pitch of 0.74 mum as a test sample, we observed that the sharp edges around the pits are clearly resolved with the second-harmonic signals and obtained enhanced resolution of ~70 nm full width at half-maximum.     

Apparent superresolution in near-field optical imaging of periodic gratings

A superresolution phenomenon in images of diffraction gratings obtained with a near-field optical microscope probe positioned at distances of the order of 1>mu;m from the surface was recently reported [Appl. Opt. 33, 876 (1994)]. This experiment was interpreted in terms of evanescent waves observed in the far field. We show that the effect observed in that experiment is a very well known far-field optical phenomenon called the Talbot effect. The existence of self-imaging Talbot planes for gratings with periods of the order of a few micrometers appears promising for many practical applications.     

扫描近场光学显微镜的光耦合偶极子模型

为了研究扫描近场光学显微镜中探针和粗糙样品表面的耦合相互作用,提出了一种光耦合偶极子模型.在该模型中,探针和样品突起都由光极化偶极子表示,在准静态电磁场近似的情况下样品表面的诱导极化效应由影像偶极子表示,应用偶极子辐射理论可以得到系统的自洽场方程.此模型提供了一种直观分析扫描近场光学显微镜中探针和样品相互作用机理的方法.在此基础上,进一步讨论了金属样品的近场成像特点和其特有的局域光学共振现象.数值结果表明：不同于一般的介质样品,金属样品的近场图像与入射光频率直接相关,改变入射光的频率,获得的样品近场图像的形状和对比度都会发生变化.特别是当入射光频率处于样品极化共振范围内时,金属纳米粒子的极化率会出现光极化共振,这样就可以获得样品粒子的最大有效尺寸,为提高系统的分辨率提供了一条重要途径.     

Tip-based nanoscale selective growth of discrete silicon nanowires by near-field laser illumination

Growth of discrete silicon nanowires is reported with nanoscale location selectivity by employing near-field laser illumination. An uncoated dielectric atomic force microscope (AFM) tip provides a nanometer-scale light source that is sufficiently localized to induce nucleation and subsequent growth of a single nanowire under its optical near-field. Far-field laser-induced heating is additionally supplied to the substrate to both relieve the required near-field light budget and also assist stable epitaxial growth. Specific catalysts are selected for the nanowire growth by non-contact mode AFM imaging. Through real-time monitoring of the deflection of the AFM cantilever during the growth process, the gap between the tip and the sample and hence truly near-field illumination are maintained throughout the growth process. The study shows that tip-based near-field laser illumination could be an effective tool for the direct integration of semiconductor nanowires.     

Imaging contrast under aperture tip–nanoantenna array interaction

Periodic arrays of paired and single gold nanorods were imaged in the near field using reflection and transmission modes of a near-field scanning optical microscope at various wavelengths and polarizations of light in the visible range. The paired nanorods act like nanoantenna, and an array of them was initially designed as a negative-index material for the near infrared. Reverse contrast in reflection and transmission images is observed under illumination from the small aperture of a metal-coated fiber probe. By changing the relative orientation of the rods to the polarization, the reverse contrast switches to the normal contrast of near-field imaging. Coupling between the aperture and the nanorod array makes the contrast higher. Transmission through the aperture is enhanced if the aperture probe is positioned between the nanorods. The average near-field transmission exhibits an opposite sign of anisotropy relative to the far-field case. Aperture probes with larger diameters always show normal imaging contrast. The results demonstrate that the broad angular spectra of small-aperture sources play a crucial role in near-field interactions with nanorod arrays. The results also show that angular redistributions of these spectra after transmission or reflection from the nanorod array are likely due to excitation of localized and propagating plasmons.     

SETUP AND APPLICATION OF SCANNING NEAR-FIELD OPTICAL MICROSCOPY

The research on the setup and application of scanning near-field optical microscopy (SNOM) performed in our laboratory is reviewed in this report. We have constructed a versatile low temperature scanning near-field optical microscope with the capability of near-field imaging and spectroscopy, operating at liquid nitrogen temperature. A special designed coaxial double lens was used to introduce the illumination beam through a 200μm fiber; the detected optical signal was transmitted via a fiber tip to an avalanche photon detector. The performance test shows the stability of the new design. The shear force image and optical image of a standard sample are shown. A system of SNOM working at room temperature and atmosphere was used to characterize semiconductors and bio-molecular samples. It revealed the unique features of semiconductor microdisks in the near-field that is significantly different from that of far-field. The effects of different geographic microstructures on the near-field light distribution of InGaP, GaN, and InGaN multi-quantum-well microdisk were observed.     

Imaging with reflection near-field optical microscope: contributions of middle and far fields

Within the framework of a microscopic formalism, we present a theoretical analysis of the imaging properties of scanning near-field optical microscopy in reflection. The detected optical signal is assumed to be proportional to the intensity of the self-consistent field at the site of the probe dipole. Influence of different components (near, middle and far fields) of the field propagator on the total field intensity is numerically studied for two arrangements of object dipoles and for various probe-surface distances. We demonstrate that for relatively large distances the middle-field components dominate in the total field and ensure the subwavelength resolution. Imaging properties of the reflection near-field microscopy observed experimentally are qualitatively explained with the help of our numerical simulations.     

基于半导体纳米线/锥形微光纤探针的被动式近场光学扫描成像

本文结合近场扫描结构和纳米线-微光纤耦合技术,提出了一种基于硫化镉纳米线/锥形微光纤探针结构的被动近场光学扫描成像系统.该系统采用被动式纳米探针,保留了纳米探针对样品表面反射光的强约束优势.其理论收集效率为4.65‰,相比于传统的金属镀膜近场探针收集效率提高了一个数量级,可有效地提高扫描探针对样品形貌信息的检测能力;而后通过硫化镉纳米线与微光纤之间高效的倏逝场耦合,将检测的光强信号传输到远场进行光电探测,最终实现对目标样品形貌的分析成像,其样品宽度测量误差在7.28%以内.该系统不需要外部激发光路,利用显微镜自身光源进行远场照明,被动扫描探针仅作为样品表面反射光的被动收集系统.本文基于半导体纳米线/锥形微光纤探针的被动式近场光学扫描成像方案,可有效地降低探针的制备难度和目标光场的检测难度,简化扫描成像的结构,为近场光学扫描显微系统之后的发展提供新的思路.     

基于半导体纳米线/锥形微光纤探针的被动式近场光学扫描成像

本文结合近场扫描结构和纳米线-微光纤耦合技术,提出了一种基于硫化镉纳米线/锥形微光纤探针结构的被动近场光学扫描成像系统.该系统采用被动式纳米探针,保留了纳米探针对样品表面反射光的强约束优势.其理论收集效率为4.65‰,相比于传统的金属镀膜近场探针收集效率提高了一个数量级,可有效地提高扫描探针对样品形貌信息的检测能力;而后通过硫化镉纳米线与微光纤之间高效的倏逝场耦合,将检测的光强信号传输到远场进行光电探测,最终实现对目标样品形貌的分析成像,其样品宽度测量误差在7.28%以内.该系统不需要外部激发光路,利用显微镜自身光源进行远场照明,被动扫描探针仅作为样品表面反射光的被动收集系统.本文基于半导体纳米线/锥形微光纤探针的被动式近场光学扫描成像方案,可有效地降低探针的制备难度和目标光场的检测难度,简化扫描成像的结构,为近场光学扫描显微系统之后的发展提供新的思路.     

FABRICATION AND APPLICATION OF NEAR-FIELD OPTICAL FIBRE PROBE

In this paper, the fabrication of a large cone angle near-field optical fibre probe, using the two-step chemical etching method and bent probe, is introduced, and the controlling parameters of the coated Cr－Al film at the probe tip are presented. The scanning electron microscopy images display that the tip diameter of the uncoated large cone angle fibre probe obtained is less than 50nm, the cone angle over 90°, and the diameter of light aperture at the coated probe tip is less than 100nm. The measured results of the optical transmission efficiency for various probe tips show that the uncoated straight optical fibre probe, film-coated straight probe and film-coated bent probe are 3×10^-1, 2×10^-3, and 1×10^-4 times that of the flat fibre probe, respectively. In addition, the force images and near-field optical images of a standard sample are acquired using a large cone angle and film-coated bent probe.     

Polarization-dependent contrast of near-field and far-field-propagating signal intensities in C-mode scanning near-field optical microscopy/photon scanning tunneling microscopy

A recent paper by Nayaet al. (Opt. Commun.124 (1996) 9) presented high-resolution imaging results obtained in the sub-100-nm range with a collection-mode near-field optical microscope. The images exhibit apparent polarization dependence. A simple modeling and calculation based on the experiment, using a semi-microscopic and perturbative approach, showed that the far-field-propagating signal intensity converted from the near-field can qualitatively explain the polarization dependence of the experiment if the taper angle of the probe tip is taken into account.     

Beyond the Rayleigh criterion: grating assisted far-field optical diffraction tomography

We propose an optical imaging system, in which both illumination and collection are done in far field, that presents a power of resolution better than one-tenth of the wavelength. This is achieved by depositing the sample on a periodically nanostructured substrate illuminated under various angles of incidence. The superresolution is due to the high spatial frequencies of the field illuminating the sample and to the use of an inversion algorithm for reconstructing the map of relative permittivity from the diffracted far field. Thus, we are able to obtain wide-field images with near-field resolution without scanning a probe in the vicinity of the sample.     

Near-field imaging of pyramid-like nanoparticles at a surface

An exact analytical solution of the self-consistent equation for the local field is used to calculate the near-field optical images of pyramid-like nano-objects placed at a surface of a solid. The diagram method developed previously for near-field image formation is generalized in order to describe layered objects, which are treated as many-body systems. The near-field optical images of triangular and square pyramids are calculated for the illumination configuration as well as those of triangular and square prisms. It is found that the near-field images of nanoparticles having the dielectric constant close to that of the substrate change rapidly and in a complicated manner with the probe–sample distance.     

动态化学腐蚀法制备大锥角近场光纤探针

基于液压传递原理,设计出一种用动态化学腐蚀法制备大锥角近场光纤探针的装置.实验结果表明,在传统的化学腐蚀法制备光纤探针的过程中,通过控制腐蚀液液面的上升速度,可以有效地控制探针针尖的形状以及锥角的大小.在此基础上,还论述了分步控制光纤与腐蚀液液面的相对位移的方法在大锥角光纤探针的制备中所具有的独特优越性.利用所制备的大锥角近场光纤探针在扫描近场光学显微镜上对直径为200nm的小球进行探测,其力学像证实该探针具有较高的形貌分辨率(约为50nm). 关键词： 动态化学腐蚀法 大锥角 近场光纤探针     

Development of Scanning Near-Field Optical Microscope Working under Cryogenic Temperature and Strong Magnetic Field

We report on the development of a cantilever-based scanning near-field optical microscope (SNOM) working in an extreme environment, at cryogenic temperature around 10 K and under strong magnetic field up to 7 T. We designed a new optical system based on an infinite conjugate microscope, which extracts the near-field signal from a small aperture through a narrow chamber into free space as collimated light. Using this system, we successfully measured near-field and topographical images of a metal-hole sample simultaneously. Combining the local optical accessing technique with the external control of the electronic state, this SNOM system will be a powerful tool to study optical properties of semiconductor nanostructures.