Cloaked near-field scanning optical microscope tip for noninvasive near-field imaging

Near-field imaging is a well-established technique in biomedical measurements, since closer to the detail of interest it is possible to resolve subwavelength details otherwise unresolved by regular lenses. A near-field scanning optical microscope (NSOM) tip may indeed overcome the resolution limits of far-field optics, but its proximity inherently perturbs the measurement. Here, we apply the recent concept of a "cloaked sensor" to an NSOM device in collection mode, showing theoretically how a proper plasmonic cover applied to an NSOM tip may drastically improve its overall measurement capabilities.     

Near-Field Scanning Optical Microscopy Combined with a Tapping Mode Atomic Force Microscope

Tapping mode atomic force microscopy is used to control the tip-sample distance in near field scanning optical microscopy (NSOM), which gives both topographic and near-field images simultaneously. The evanescent waves are scattered by a vibrating silicon-nitride tip in the proximity of sample surfaces and are detected through a microscope objective. This NSOM allows the observation of opaque samples with reflection illumination. A glass grating of 1-μm pitch and an InP grating of 0.5-μm pitch are observed with a lateral resolution of 100 nm.Presented at 1996 International Workshop on Interferometry (IWI ‘96), August 27-29, Saitama, Japan     

Dual mode near-field scanning optical microscopy for near-field imaging of surface plasmon polariton

In this paper, we theoretically and experimentally demonstrate that metal coated apertured probes are efficient near-field probes on surfaces with high reflectivity for the scattering as well as for the collection mode near-field scanning optical microscopy (NSOM). We show that a blunt apertured metal coated tip is very effective in suppressing image dipoles which affect strongly the signals scattered from frequently used sharp metal tips or gold nanoparticle attached probes. By using a simultaneous collection and scattering mode (dual mode) NSOM we measure the near-field images of surface plasmon polariton (SPP) launched from a slit. The collection mode measures propagating SPP along lateral distance in a long scan range with high signal-to-noise ratio, and the scattering mode measures the polarization resolved near-field of SPP. Comparisons of the measured data obtained in the dual mode enable to easily characterize SPP and to separate the measured near-field into the propagating SPP and the directly transmitted light.     

剪切力模式近场扫描光学显微镜的恒幅反馈控制方法研究

剪切力模式近场扫描光学显微镜(Near-field Scanning Optical Microscopy,NSOM) 的音叉探针间距控制系统中,用相位反馈控制和检测剪切力,同时采用比例＋积分（PI）技术实现对音叉探针振幅的反馈控制,使探针振幅在扫描过程中保持为恒定值.用相位信号作为探针与样品间距控制信号,分别在无振幅反馈和有振幅反馈两种情况下,以不同速率扫描得到标准CD_RW光盘光栅的两组图像,并进行了比较分析.实验表明,恒振幅反馈电路的引入有助于提高探针系统的响应速度和灵敏度,改善所得图像的质量及分辨率.     

Subdiffraction‐limited chemical imaging of patterned phthalocyanine films using tip‐enhanced near‐field optical microscopy

A tip‐enhanced near‐field optical microscope, based on a shear‐force atomic force microscope with plasmonic tip coupled to an inverted, confocal optical microscope, has been constructed for nanoscale chemical (Raman) imaging of surfaces. The design and validation of the instrument, along with its application to near‐field Raman mapping of patterned organic thin films (coumarin‐6 and Cu(II) phthalocyanine), are described. Lateral resolution of the instrument is estimated at 50 nm (better than λ/10), which is roughly dictated by the size of the plasmonic tip apex. Additional observations, such as the distance scaling of Raman enhancement and the inelastic scattering background generated by the plasmonic tip, are presented. Copyright © 2016 John Wiley & Sons, Ltd.     

Sub-wavelength imaging of photo-induced refractive index pattern in chalcogenide glass films

Mapping of refractive index patterns with sub-wavelength resolution is achieved using Near-field Scanning Optical Microscopy (NSOM) in reflection mode. Imaging of index pattern is performed on surface gratings photo-imprinted in As₂S₃ films. The NSOM is adapted with a near infrared laser which wavelength (785 nm) is chosen to be within the transparency window of the glass film therefore allowing consistent measure of reflected light. Quantitative measurements of photo-induced index changes can then be obtained from knowledge of the initial film index. Images of gratings with a period of 0.5 micron are easily collected therefore demonstrating sub-wavelength spatial resolution. The technique permits to concurrently obtain a topographic image and index image of the gratings thereby permitting to quantify the extent of photodarkening and photoexpansion simultaneously. It is shown that relief gratings tend to vanish in films aged in air for several months however the index gratings remain.     

Numerical analysis of the spectral response of an NSOM measurement

Near-field Scanning Optical Microscopy (NSOM) is a powerful tool for investigating optical field with resolution greater than the diffraction limit. In this work, we study the spectral response that would be obtained from an aperture NSOM system using numerical calculations. The sample used in this study is a bowtie nanoaperture that has been shown to produce concentrated and enhanced field. The near- and far-field distributions from a bowtie aperture are also calculated and compared with what would be obtainable from a NSOM system. The results demonstrate that it will be very difficult to resolve the true spectral content of the near-field using aperture NSOM. On the other hand, the far-field response may be used as a guide to the near-field spectrum.     

A resonance tracking method for stable operation of a near-field scanning optical microscope in liquid environment

We report on the novel design of the near-field scanning optical microscope (NSOM) which operates in liquid environment. A resonance tracking digital scanning method is applied to compensate the resonance shift due to the evaporation of the liquid in the atmospheric pressure. By this method, stable operation of NSOM system is demonstrated by showing topographic images of the metallic grating embedded in liquid environment.     

Reflection-scanning near-field optical microscopy and spectroscopy of opaque samples

Opaque samples are imaged by Scanning Nearfield Optical Microscopy (SNOM) in reflection mode: A quartz glass fiber tip is used both to illuminate the sample and to collect light locally reflected from or emitted by the surface. The collected light is coupled out by a 2×2 fiber coupler and fed into a grating spectrometer for spectral analysis at each sampled point. The tip-sample distance is controlled by a shear-force feedback system. The simultaneous measurement of topography and optical signals allows an assessment of imaging artifacts, notably topography-induced intensity changes. It is demonstrated that an optical reflectance contrast not induced by topographic interference can be found on suitable samples. Local spectral analysis is shown in images of a photoluminescent layer.     

Evaluation of nanoscale roughness measurements on a plasma treated SU-8 polymer surface by atomic force microscopy

A comparison between roughness data obtained with an atomic force microscope (AFM) on different surfaces requires reliable roughness parameters. In order to specify the appropriate parameters for nanoscale roughness measurements, we compared the root mean square (rms) roughness and the relative surface area (sdr) as function of varying scan size, speed and pixel size. By using oxygen plasma (24 kJ) treated SU-8 with an average rms roughness of 2.6 ± 0.5 nm as reference surface, the repeatability of the method was evaluated for dynamic (tapping) and contact mode. The evaluation of AFM images indicated a decrease of the effective tip radius after a few measurements. This degradation of the tip lowers the resolution of the image and can affect roughness measurements.     

近场扫描光学显微镜中一种新的切向力检测系统

在近场扫描光学显微镜（ＮＳＯＭ）［１］中,近场距离控制一般采用切向力控制法。检测切向力有两种方法：光学检测法和非光学检测法。目前普遍采用非光学检测法,基本上是采用压电陶瓷管控制探针和样品的距离。本文提出一种新的切向力检测系统,利用双压电片实现近场距离控制。实验结果表明,检测灵敏度大大提高,扫描力显微（ＳＦＭ）像的分辨率可达纳米量级。     

Tapping mode atomic force microscope combined with reflection scanning near-field optical microscope (AF/RSNOM)

A new kind of scanning probe microscope is introduced in this paper, which is a combination of atomic force microscope and reflection scanning near field optical microscopy (AF/RSNOM) with equi-amplitude tapping mode. The principle and recent experiment result of AF/RSNOM are reported. Besides convenient operation, the bi-functional probe tip of AF/RSNOM brings an even illumination for every sampling position. Experiment result and analysis show that the signal to noise ratio (SNR) of AF/RSNOM optical image is much better than that of other RSNOM without tapping working mode.     

Nanoscopic study of second-harmonic generation in organic crystals with collection-mode near-field scanning optical microscopy

Collection-mode near-field scanning optical microscopy (NSOM) is used to map nanoscopic second-harmonic generation (SHG) in N -(4-nitrophenyl)- (L) -prolinol crystals. A spatial resolution of 98 nm is achieved. Near-field polarization-dependent SHG measurement is performed, and a local effective SHG susceptibility of 224+/-18 pm/V is obtained.     

Near-field scanning optical microscopy imaging of multimode interference

By use of a near-field scanning optical microscope (NSOM) in collection mode, the intensity distribution along a 2 x 2 multimode interference coupler was directly imaged as a function of wavelength. Although calculations can predict the general trend of wavelength dependence and the approximate positions of multiple images in the coupler, the accuracy is poor because of uncertainties in the waveguide width. We show that direct imaging using a NSOM bypasses calculational uncertainties and proves to be a powerful technique for studying these waveguide devices.     

近场光学显微镜中利用剪切力原理进行样品/探针间距测控的非光学方法

扫描近场光学显微镜（ＳｃａｎｎｉｎｇＮｅａｒ－ｆｉｅｌｄＯｐｔｉｃａｌＭｉｃｒｏｓｃｏｐｅ：ＳＮＯＭ）自８０年代中期以来获得了迅速的发展并得到了具有纳米（亚纳米）尺度分辨率的光学图像。作为ＳＮＯＭ的关键技术之一，样品与探针间距的控制尤为重要，而实现起来比较困难。人们提出了多种方法，其中基于剪切力原理的控制方法最常用而且实验证明是可靠的。本文综述了扫描近场光学显微镜发展以来所出现的基于剪切力原理进行样品／探针间距的探测的一些主要的非光学方法。包括利用ＰＭＴ直接探测样品辐射，利用音叉、电容、阻抗以及压电效应的探没方法等，并对各种方法的优缺点进行了分析比较。     

Scanning near-field optical microscopy

Scanning Near-field Optical Microscopy (SNOM) allows the investigation of optical properties on subwavelength scales. During the past few years, more and more attention has been given to this technique that shows enormous potential for imaging, sensing and modification at near-molecular resolution. This article describes the technique and reviews recent progress in the field.     

Theoretical analysis of optical force density distribution inside subwavelength-diameter optical fibers

We investigate the microscopic optical force density distributions respectively inside a subwavelength-diameter(SD)fiber with flat endface and inside one with oblique endface by using a finite-difference time-domain(FDTD) method.Optical force density distributions at the fiber endfaces can now be readily available. The complete knowledge of optical force density distributions not only reveal features regarding the microscopic near-field optomechanical interaction, but also provide straightforward explanations for the sideway deflections and other mechanical motions. Our results can provide a useful reference for better understanding the mechanical influence when light transports in a microscale or nanoscale structure and for developing future highly-sensitive optomechanical devices.     

Cloaking apertureless near-field scanning optical microscopy tips

We numerically demonstrate that properly designed plasmonic covers can be used to enhance the performance of near-field scanning optical microscopy (NSOM) systems based on the employment of apertureless metallic tip probes. The covering material, exhibiting a near-zero value of the real permittivity at the working frequency, is designed in such a way to dramatically reduce the undesired scattering due to the strongly plasmonic behavior of the tip. Though the light scattering by the tip end is necessary for the correct operation of NSOMs, the additional scattering due to the whole probe affects the signal-to-noise ratio and thus the resolution of the acquired image. By covering the whole probe but not the very tip, we show that unwanted scattering can be effectively reduced. A realistic setup, working at mid-IR frequencies and employing silicon carbide covers, has been designed and simulated to confirm the effectiveness of the proposed approach.     

Dissipation Energy in Tapping-Mode Atomic Force Microscopes Caused by Liquid Bridge

The dissipation of energy during the process of contact and separation between a tip and a sample is very important for understanding the phase images in the tapping mode of atomic force microscopes(AFMs). In this study, a method is presented to measure the dissipated energy between a tip and a sample. The experimental results are found to be in good agreement with the theoretical model, which indicates that the method is reliable.Also, this study confirms that liquid bridges are mainly produced by extrusion modes in the tapping mode of AFMs.     

Scanning Near-Field Optical Microscopy and Microspectroscopy of Green Fluorescent Protein in Intact Escherichia coli Bacteria

Scanning near-field optical microscopy (SNOM) yields high-resolution topographic and optical information and constitutes an important new technique for visualizing biological systems. By coupling a spectrograph to a near-field microscope, we have been able to perform microspectroscopic measurements with a spatial resolution greatly exceeding that of the conventional optical microscope. Here we present SNOM images of Escherichia coli bacteria expressing a mutant green fluorescent protein (GFP), an important reporter molecule in cell, developmental, and molecular biology. Near-field emission spectra confirm that the fluorescence detected by SNOM arises from bacterially expressed GFP molecules.