Hydrophobic optical elements for near-field optical analysis (NOA) in liquid environment--a preliminary study.

Near-field Scanning Optical Microscopy (NSOM) in liquid environment is expected to allow time resolved morphological mappings on cellular surfaces on the nanoscale level. Near-field Optical Analysis (NOA) via NSOM exploits the energy transfer from the tip of an optical element (tip diameter > or = 20nm), oscillating within the range of the characteristic length of the energy transfer ( approximately 10nm) in the near-field of the surface to be analysed. In NOA, a molecular assembly is monitored by visible light with a resolution far below the wavelength of visible light. Actually, NOA is successfully applied in mapping local optical contrasts, for instance in photonic crystals with dielectric periodicities on the nanoscale. NSOM could in principle be performed in two different modes: tapping mode, with tip-oscillations perpendicular, or shear force mode, with tip-oscillations parallel to the substrate. Both basic modes have specific advantages and disadvantages. In biological systems (e.g. in cell cultures), where scanning in liquids is prevalent, elongated optical elements non-invasively operated in the shear force modus could have some specific advantages when compared to contact modus systems. While tapping mode NSOM provides satisfactory nanoscale images even on solid surfaces covered with millimetres of liquids, the performance of shear force mode NSOM is presently largely confined to operations on dry samples. This is due to the inability of conventional shear force mode NSOM systems to provide sharp topographic images of sample surfaces substantially covered with liquids. By equipping a conventional NSOM system with hydrophobic optical elements, shear force mode based topographic images could be obtained on biological samples in dry as well as in aqueous environment, and with resolutions on the nanoscale level.     

Recent progress of nano-technology with NSOM

Recent progress of nano-technology with near-field scanning optical microscope (NSOM) is surveyed in this article. We focus mainly on NSOM, nano-scale spectroscopy with NSOM, probe technology of NSOM, and study of nano-structured metallic surface with NSOM. First, we follow developments of aperture NSOM and apertureless NSOM, and then address progress of NSOM-combined spectroscopy which is so sufficiently advanced with apertureless NSOM technology to provide chemical information on length scales of a few nanometers. Recent achievement of nano-scale Raman and IR spectroscopy will be introduced. Finally, research on nano-optic elements using surface plasmon polariton with NSOM is introduced as an example of NSOM applications to nano-structured metallic surfaces.     

Near-field microscopy with a scanning nitrogen-vacancy color center in a diamond nanocrystal: A brief review

We review our recent developments of near-field scanning optical microscopy (NSOM) that uses an active tip made of a single fluorescent nanodiamond (ND) grafted onto the apex of a substrate fiber tip. The ND hosting a limited number of nitrogen-vacancy (NV) color centers, such a tip is a scanning quantum source of light. The method for preparing the ND-based tips and their basic properties are summarized. Then we discuss theoretically the concept of spatial resolution that is achievable in this special NSOM configuration and find it to be only limited by the scan height over the imaged system, in contrast with the standard aperture-tip NSOM whose resolution depends critically on both the scan height and aperture diameter. Finally, we describe a scheme we have introduced recently for high-resolution imaging of nanoplasmonic structures with ND-based tips that is capable of approaching the ultimate resolution anticipated by theory.     

微波模拟近场光学显微镜实验

介绍了近场光学显微镜的基本原理，并利用微波代替可见光模拟了近场光学显微镜实验．     

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.     

10 nm Spatial Resolution Fluorescence Imaging of Single Molecules by Near-Field Scanning Optical Microscopy Using a Tiny Aperture Probe

We demonstrate high-resolution fluorescence imaging of single molecules using near-field scanning optical microscopy (NSOM) with a tiny aperture probe for two different wavelengths in visible range in the illumination mode of operation. The spatial resolutions obtained at both excitation wavelengths were almost the same and the highest resolution realized was about 10 nm. To discuss the achievable resolution in aperture NSOM, we also employed a computer simulation by the finite-difference time-domain method for various aperture sizes and wavelengths. The resolution of 10 nm is predicted to be contributed by the single peak of localized near-field light around the rim of the aperture.     

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.     

Field enhancement analysis of an apertureless near field scanning optical microscope probe with finite element method

Plasmonic field enhancement in a fully coated dielectric near field scanning optical microscope (NSOM)probe under radial polarization illumination is analyzed using an axially symmetric three-dimensional (3D)finite element method (FEM) model. The enhancement factor strongly depends on the illumination spot size, taper angle of the probe, and the metal film thickness. The tolerance of the alignment angle is investigated. Probe designs with different metal coatings and their enhancement performance are studied as well. The nanometric spot size at the tip apex and high field enhancement of the apertureless NSOM probe have important potential application in semiconductor metrology.     

Effect of anisotropic strain on the crosshatch electrical activity in relaxed GeSi films

The physical origin of the crosshatch electrical activity in relaxed GeSi films was studied using a near-field scanning optical microscope (NSOM). The contrast and patterns in the near-field photocurrent images depend on the polarization direction of the NSOM light. These results rule out composition nonuniformity, junction depth variation, and scanning artifacts as dominant sources of the contrast. Numerical calculations show that local changes in band structure due to strain fields of the misfit dislocations are responsible for the experimental observations.     

Measuring local optical properties: near-field polarimetry of photonic block copolymer morphology

Ultrahigh molecular weight polystyrene-b-polyisoprene block copolymers (BCs), noted for their photonic behavior, were imaged using transmission near-field scanning optical microscopy (NSOM) and NSOM polarimetry. Our improved scheme for polarization modulation (PM) polarimetry, which accounts for optical anisotropies of the NSOM aperture probe, enables mapping of the local diattenuation and birefringence (with separately aligned diattenuating and fast axes) in these specimens with subdiffraction limited resolution. PM-NSOM micrographs illuminate the mesoscopic optical nature of these BC specimens by resolving individual microphase domains and defect structures.     

High-Contrast Imaging of Nano-Channels Using Reflection Near-Field Scanning Optical Microscope Enhanced by Optical Interference

We demonstrated a contrast enhancement in a near-field scanning optical microscope (NSOM) by optical interference with an aperture probe in reflection (illumination-collection) mode operation. We observed a NiO film deposited on a sapphire substrate and clearly visualized 2-nm-deep nano-channel structures on the surface of the film. The reflection NSOM enhanced by optical interference is quite a promising instrument for high-resolution optical detection and estimation of low-contrast nanostructures.     

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.     

The Influence of Defects on the Propagation Light within Polymeric Optical Waveguides Studied by Polarized Near-Field Scanning Optical Microscopy

We have developed a polarization-preserving near-field scanning optical microscopy (NSOM) optical fiber probe and with it observed the influence of defects and weak stresses on a propagation light within polymeric optical waveguides. To characterize the influence, we intentionally printed an indentation in the vicinity of the waveguide and then evaluated the resulting influence using polarized guide-collection-mode NSOM images taken around the indentation. When transverse magnetic polarized light enters a waveguide, the light intensity becomes greater on the near side of the indentation than on the far side, as measured by a linearly polarized component perpendicular to the direction of light propagation. The most probable cause of this phenomenon is microdefects generated by the printing of the indentation. The polarized NSOM technique is useful in searching for small defects or stresses within integrated photonic devices.     

Fabrication of high-quality fiber micro-probe for near-field optical microscope by laser-assisted melting and pulling method

The fiber micro-probe for near-field scanning optical microscope (NSOM) was fabricated using end-fixed laser-assisted melting and pulling method. The scanning electron microscope (SEM) observation showed that the fiber probes obtained by this method possessed small diameter of tip and large conic angle. The reproducibility came close to 70%. The effects of various fabrication parameters on probes were investigated, including laser power, pulling force, and the diameter of melting zone. The optimal processing conditions were derived based on the experiments and theoretical analysis. This research provides an alternative and advantageous method for fabrication of high-quality fiber probes.     

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。     

Near-field microscopy with a single-photon point-like emitter: Resolution versus the aperture tip?

We discuss theoretically the concept of spatial resolution in near-field scanning optical microscopy (NSOM) in light of a recent work [Opt. Express 17 (2009) 19969] which reported on the achievement of active tips made of a single ultrasmall fluorescent nanodiamond grafted onto the apex of a substrate tip and on their validation in NSOM imaging. Since fluorescent nanodiamonds tend to decrease steadily in size, we assimilate a nanodiamond-based tip to a point-like single photon source and compare its ultimate resolution with that offered by standard metal-coated aperture NSOM tips. We demonstrate both classically and quantum mechanically that NSOM based on a point-like tip has a resolving power that is only limited by the scan height over the imaged system whereas the aperture-tip resolution depends critically on both the scan height and aperture diameter. This is a consequence of the complex distribution of the electromagnetic field around the aperture that tends to artificially duplicate the imaged objects. We show that the point-like tip does not suffer from this “squint” and that it rapidly approaches its ultimate resolution in the near-field as soon as its scan height falls below the distance between the two nano-objects to be resolved.     

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     

Investigation of plasmonics resonance infrared bowtie metal antenna

An approximate resonance wavelength equation that varies with metal antenna structure size is developed to design a bowtie gold metal antenna working at near-infrared (IR) wavelength. Bowtie antenna structures with resonance wavelength of 1.06 μm, 1.55 μm and 10.6 μm are designed based on this equation. A finite-difference time domain (FDTD) algorithm with total field scattered field (TFSF) source simulation shows the resonance wavelength of the designed structures being precisely in agreement with the expected wavelengths from the equation. Planar integration of the metal bowtie antennas is discussed as well. Gold nanohole bowtie antenna arrays are fabricated and the near-field optical transmission properties of the nanohole array are investigated with a near-field scanning optical microscope (NSOM). Our experimental results verify the near-field optical transmission performance and further demonstrate that they are in agreement with the theoretical calculation results. The high enhancement efficiency and integration of the metal bowtie antennas open the possibility of a wide application in IR optoelectronics detection and imaging.     

Near-field optical characterization of GaN and InxGa1−xN/GaN heterostructures grown on freestanding GaN substrates

Using near-field scanning optical microscopy (NSOM), we report the spatial distribution of photoluminescence (PL) intensity in III-nitride-based semiconductor layers grown on GaN substrates. Undoped GaN, In_0.11Ga_0.89N, and In_0.13Ga_0.87N/GaN multi-quantum wells (MQWs) were grown by metal organic chemical vapor deposition (MOCVD) on freestanding GaN substrates. Micro-Raman spectroscopy has been used to evaluate the crystalline properties of the GaN homoepitaxial layers. The variation of the PL intensity from the NSOM imaging indicates that the external PL efficiency fluctuates from 20% to 40% in the 200 nm InGaN single layer on freestanding GaN, whereas it fluctuates from 20% to 60% in InGaN/GaN MQWs. In the NSOM-PL images, bright island-like features are observed. After deconvolution with the spatial resolution of the NSOM, the size of these features is estimated to be in the range of 150–250 nm.     

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.