Differential Near-Field Scanning Optical Microscopy Using Sensor Arrays

In this paper, we introduce a new aperture-type near-Held scanning optical microscopy (NSOM) imaging concept that relies on specially designed large-area (e.g. >200 nm times 200 nm) aperture geometries having sharp corners. Unlike in conventional NSOM, the spatial resolution of this near-field imaging modality is not determined by the size of the aperture, but rather by the sharpness of the corners of the large aperture. This approach significantly improves the light throughput of the near-field probe and, hence, increases the SNR. Here, we discuss the basic concepts of this near-field microscopy approach and illustrate both theoretically and experimentally how an array of detectors can be utilized to further improve the SNR of the near-field image. The results of this work are particularly relevant for imaging of biological samples at a spatial resolution of < 50 nm with significantly improved image quality.     

Quantitative microscopy

While light microscopy is almost 400 years old, developments of the past decade have offered a variety of new mechanisms for examination of biological and material samples. These developments include exploitation of techniques such as confocal microscopy, scanning near field microscopy, standing wave microscopy, fluorescence lifetime microscopy, and two-photon microscopy. In biology, advances in molecular biology and biochemistry have made it possible to selectively tag (and thus make visible) specific parts of cells, such as actin molecules, or sequences of DNA of 1000 base pairs or longer. In sensor technology, modern charge-coupled device (CCD) cameras are capable of achieving high spatial resolution and high sensitivity measurements of signals in the optical microscope. Modern CCD camera systems are limited by the fundamental quantum fluctuations of photons, which cannot be eliminated by “better” design. Further, proper choice of the sampling density involves not only an understanding of classic linear system theory-the Nyquist theorem-but also the equally stringent requirements of digital measurement theory. Experimental procedures that rely on the CV can be used to evaluate the quality of one's quantitative microscope systems and to identify which components are the “weakest link”. Typical values of relatively straightforward parameters such as size can easily be measured to CVs around 1%     

Enhanced Near-Field Imaging Contrasts of Silver Nanoparticles by Localized Surface Plasmon

Near-field images of Ag nanoparticles are studied using a near-field scanning optical microscopy (NSOM) operating at illumination mode with blue, green, and red probing lights. The obtained far-field intensity contrast between the nanoparticle and background strongly depends on the sizes of nanoparticles and the wavelength of probing light. Experimental NSOM images supported by theoretical 3-D finite-difference time-domain simulation demonstrate that the intensity contrast is enhanced at wavelength close to the localized surface plasmon resonance (LSPR) peak of the nanoparticle. The abilities to distinguish nanoparticles with different LSPR properties on the same substrate can lead to a material-specific NSOM imaging technique.      

Near-field optical studies of semiconductor heterostructures andlaser diodes

Near-field optical microscopy and spectroscopy is emerging as a powerful tool for the investigation of semiconductor structures. Tunable excitation combined with sub-wavelength resolution is providing an unprecedented level of detail on the local optical properties of semiconductor structures. Recent near-field optical studies have addressed issues of laser diode mode profiling, minority carrier transport, near-field photocurrent response of quantum-well structures and laser diodes, imaging of local waveguide properties, and location and studies of dislocations in semiconductor thin films. We present results on the intrinsic resolution limitations of near-field photoconductivity in quantum-well heterostructures and demonstrate that the resolution depends strongly on the amount of evanescent and propagating field components in the semiconductor. Spectroscopic mode-profiling of high-power laser diode emission details the spatial dependence of multiple spectral modes. This paper presents an overview of NSOM techniques for semiconductor systems, its limitations, and present status     

Tutorial on Photoacoustic Microscopy and Computed Tomography

The field of photoacoustic tomography has experienced considerable growth in the past few years. Although several commercially available pure optical imaging modalities, including confocal microscopy, two-photon microscopy, and optical coherence tomography, have been highly successful, none of these technologies can provide penetration beyond ~1 mm into scattering biological tissues, because they are based on ballistic and quasi-ballistic photons. Heretofore, there has been a void in high-resolution optical imaging beyond this penetration limit. Photoacoustic tomography, which combines high ultrasonic resolution and strong optical contrast in a single modality, has broken through this limitation and filled this void. In this paper, the fundamentals of photoacoustics are first introduced. Then, scanning photoacoustic microscopy and reconstruction-based photoacoustic tomography (or photoacoustic computed tomography) are covered.     

Near-Field Scanning Nanophotonic Microscopy—Breaking the Diffraction Limit Using Integrated Nano Light-Emitting Probe Tip

We introduce a novel scanning ldquonanophotonicrdquo microscope through monolithic integration of a nanoscale LED (Nano-LED) on a silicon cantilever. We review two recent trends of incorporating miniature light sources on the scanning probes for near-field scanning optical microscopy: one is to attach fluorephores at the tip to define a small light source, while the other is to integrate an LED and a nanometer aperture into scanning probes, based on silicon microfabrication techniques. The creation of Nano-LED combines the advantages of previous two approaches: no external sources are required and the reduction of the light source size directly leads to resolution improvement. Two types of Nano-LEDs have been successfully demonstrated utilizing nanofabrication and microelectromechanical systems technologies: 1) formation of thin silicon dioxide light-emitting layer between heavily doped p + and n+ silicon layers created by a focused ion beam and 2) electrostatic trapping and excitation of CdSe/ZnS core-shell nanoparticles in a nanogap. We employed these probes into a standard near-field scanning and excitation setup. The probe successfully measured optical as well as topographic images of chromium test patterns with imaging resolutions of 400 and 50 nm, respectively. In addition, the directional resolution dependence of the acquired images suggests the size and shape of the light source. To our knowledge, these results are probably the first successful near-field images directly measured by such tip-embedded light sources. With the potential emission capability from near UV to IR and additional mass producibility, the nanophotonic microscope presents exciting opportunities in near-field optics, integrated circuit technology, nanomanufacturing and molecular imaging, and sensing in biomedicine.     

Optical coherence tomography for ophthalmic imaging: new techniquedelivers micron-scale resolution

The authors have developed a new technique for micron scale resolution cross-sectional imaging of ocular and other biological tissue, called optical coherence tomography (OCT). OCT is similar to B-scan ultrasonic imaging, except that image contrast relies on differences in optical rather than acoustic backscattering characteristics of tissue. In contrast to ultrasound and nonlinear optical gating techniques, low-coherence interferometry is used to resolve the position of reflective or optical backscattering sites within a sample. Two-dimensional tomographic images of a thin, optical slice of tissue may be obtained with 10 μm longitudinal and lateral resolution. Optical heterodyne detection and the application of noise-reduction techniques originally developed for optical communication achieve sensitivity to reflected light as small as 10^-10 of the incident optical power. OCT is non-contact, non-invasive, and has superior resolution to conventional clinical ultrasound. Unlike scanning laser ophthalmoscopy and scanning laser tomography, the optical sectioning capability of OCT is not limited by the pupil aperture and ocular aberrations. OCT may be implemented in a compact, low-cost, fiber-optic based interferometer that is easily coupled to existing ophthalmic instrumentation. Here, the authors demonstrate high-speed in vivo OCT imaging in both the anterior and posterior eye, and highlight the system's potential usefulness for the early diagnosis and quantitative monitoring of a variety of ocular diseases and treatments     

Light propagation in tissue during fluorescence spectroscopy withsingle-fiber probes

Fluorescence spectroscopy systems designed for clinical use commonly employ fiberoptic probes to deliver excitation light to a tissue site and collect remitted fluorescence. Although a wide variety of probes have been implemented, there is little known about the influence of probe design on light propagation and the origin of detected signals. In this study, we examined the effect of optical fiber diameter, probe-tissue spacing and numerical aperture on light propagation during fluorescence spectroscopy with a single-fiber probe. A Monte Carlo model was used to simulate light transport in tissue. Two distinct sets of excitation-emission wavelength pairs were studied (337/450 nm and 400/630 nm). Simulation results indicated that increasing fiber diameter or fiber-tissue spacing increased the mean excitation-emission photon pair pathlength and produced a transition from high selectivity for superficial fluorophores to a more homogeneous probing with depth. Increasing numerical aperture caused an increase in signal intensity, but axial emission profiles and pathlengths were not significantly affected for numerical aperture values less than 0.8. Tissue optics mechanisms and implications for probe design are discussed. This study indicates that single-fiber probe parameters can strongly affect fluorescence detection and should be considered in the design of optical diagnostic devices     

Infrared Scanning Near-Field Optical Microscopy Below the Diffraction Limit

Infrared scanning near-field optical microscopy (IR-SNOM) is an extremely powerful analytical instrument since it combines IR spectroscopy's high chemical specificity with SNOM's high spatial resolution. In order to do this in the infrared, specialty chalcogenide glass fibers were fabricated and their ends tapered to generate SNOM probes. The fiber tips were installed in a modified near-field microscope and both inorganic and biological samples illuminated with the tunable output from a free-electron laser located at Vanderbilt University. Both topographical and IR spectral images were simultaneously recorded with a resolution of ~ 50 and ~ 100 nm, respectively. Unique spectroscopic features were identified in all samples, with spectral images exhibiting resolutions of up to lambda/60, or at least 30 times better than the diffraction limited lens-based microscopes. We believe that IR-SNOM can provide a very powerful insight into some of the most important biomedical research topics.     

The spatial resolution of near-field optical microscope onchromosomes and cell traces

The investigation of biological samples in molecular medicine and biology by near-field optical microscopy is subject to nonconstant experimental conditions, such as humidity and elasticity. Contrary to far-field microscopy, the obtainable spatial resolution in scanning near-field optical microscopy (SNOM) greatly depends on the specific experimental conditions. The experimental determination of the modulation transfer function (MTF), therefore, uses regular solid-state structures. This paper introduces a method for the approximate in situ determination of the MTF using, as an example, SNOM transmission measurements of metaphase humane chromosomes and cell traces. The method has its origins in the linear system transfer theory. In order to eliminate effects of nonconstant optical near-field conditions, the transfer function is determined from the properties of the light source and the measured intensity function at the edge of a chromosome or cell trace, which depends on the transmission of the probe     

Characterization and properties of anatase TiO2 film prepared via colloidal sol method under low temperature

Nanocrystalline titanium dioxide particles and films with anatase structures have been prepared via solvothermal method under low temperature. The products were characterized by X-ray diffraction and transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The optical properties were characterized in the ultraviolet-visible region by optical absorption measurement. The relationship between the optical band gaps and the structures was studied.     

OPGW光缆生产中的光纤特性变化

光纤截止波长、模场直径、色散、数值孔径等参数取决于光纤折射率分布和几何尺寸,在光缆生产过程中这些参数基本保持不变.但由于光纤受力后会产生光弹性效应,导致折射率变化,所以理论上光纤性能不是恒定不变.如果光纤衰减、强度以及老化等问题与光缆生产工艺及光缆材料有关,在光缆生产过程中,会有残余应力存留于光纤中,在高温、高湿、低温...     

Observation of artificial nano-domains in ferroelectric thin films using nonlinear dielectric imaging and piezo imaging

Abstract

We have developed scanning nonlinear dielectric microscopy (SNDM) that is the first successful purely electrical method for observing polarization distributions of ferroelectric materials. Now the resolution of SNDM has been improved to sub-nanometer.

On the other hand, the piezoelectric response imaging (piezo-imaging) using scanning force microscopy (SFM) is well known as the method for observing the polarization distributions. In this study, we compare the resolution of SNDM with that of the piezo-imaging and confirm that the resolution of SNDM is much higher than that of piezo-imaging. Then, as the fundamental study to apply the SNDM system to a ferroelectric reading-recording system, we switched and observed the ferroelectric domains using the SNDM system.     

Line-Field Optical Coherence Tomography Using Frequency-Sweeping Source

In this paper, we propose a line-field optical coherence tomography (OCT) with frequency-sweeping source, which uses imaging optics to form a line focus on the sample and obtains cross-sectional OCT images of biological tissues without any mechanical scanning. Therefore, real-time imaging becomes possible even when the sweeping rate of the source is not high. Moreover, a full 3-D volume image can be acquired by simply incorporating 1-D mechanical scanning. Our line-field OCT using frequency-sweeping source has a sensitivity of ~88 dB, an axial resolution of 8.3 mum, and an acquisition speed of 45 fps.     

Super-Achromatic Rapid Scanning Microendoscope for Ultrahigh-Resolution OCT Imaging

Microendoscope is a critical technology to enable high-resolution imaging of internal luminal organs with optical coherence tomography. This paper reports the development of an achromatic compound microlens and a rapid scanning microendoscope based on the microlens that offers an ultrahigh transverse resolution of 4 mum (and an axial resolution of 2.2 mum when using a low-coherence light source with a broad spectrum bandwidth of 150 nm). The rapid scanning endoscope is capable of ultrahigh-resolution (UHR) optical coherence tomography (OCT) imaging in real time at an imaging speed of about 1220 lateral scans/s, with the image quality comparable to a slow bench-top UHR-OCT system (~100 scans/s). The superior performance of a scanning endoscope made of a compound microlens over an endoscope made of a conventional GRIN (gradient index) lens in imaging biological tissues has also been demonstrated.     

Optical raster-scanning displays based on surface-micromachinedpolysilicon mirrors

We demonstrate high-resolution, raster-scanning display systems based on pairs of orthogonally scanning, surface-micromachined mirrors. The first mirror of the raster-scanning pair determines the line-scan rate of the display and is driven at its resonant frequency which is on the order of 4.7 kHz. The second mirror, driven at a frequency below its resonance and scanning orthogonally to the first mirror, determines the image refresh rate. Both mirrors have a maximum optical scanning angle of 15°. Single-chip and two-chip scanners are demonstrated. The resolution of the single-chip display, based on average pixel size, is 102×119 pixels. The curvature of the mirror surfaces are characterized and optically compensated to achieve this resolution     

Scanning tunneling microscopy of DNA

The systematic removal or destruction of DNA by the scanning tunneling microscope (STM) stems from the low conductivity of the molecule. Imaging DNA at exceedingly low currents and high voltages prevents damaging contact in humid conditions. The STM images of DNA on mica are evidence that the STM could be used to study genomic structure on a nanometer scale. Thus, the STM may join the scanning force microscope as a powerful imaging tool for DNA studies because: 1) Such resolution is adequate for observing the curvature of DNA and DNA-protein complexes, and also the gross structure of DNA-protein complexes. 2) Both microscopes may be operated in solution to examine DNA molecules in physiological conditions. However, Lindsay et al. (1990) have encountered the above mentioned difficulties using the STM. 3) While the STM is sensitive to the electronic states of adsorbed molecules, the scanning force microscope can detect chemical parameters such as adhesion. Information in addition to topography enhances one's ability to interpret images. Undoubtedly, the scanning tunneling microscope is a very important tool with which to study conductive surfaces. For poorly conducting molecules adsorbed on substrates, hybrid microscopes with which to examine the surface by alterative methods may neatly facilitate systematic investigations     

ESD辐射场与带孔缝金属腔体耦合的数值研究

采用FDTD法研究了ESD电流注入细导线所产生的辐射场在其近场区域与带孔缝金属腔体的耦合问题 ,建立了耦合数值模型。计算分析表明 ,在金属腔体内的孔缝周围区域存在较强的耦合场 ,腔体内被激励起以TE10 1模、TE2 0 1模和TE112 模为主的谐振场 ;对面积相等的方孔、矩形孔和孔阵 ,通过长边与导线垂直的矩形孔和方孔的耦合场比较强 ,而通过长边与导线平行的矩形孔和孔阵的耦合场比较弱     

Deep learning autofluorescence-harmonic microscopy

Laser scanning microscopy has inherent tradeoffs between imaging speed,field of view(FOV),and spatial resolution due to the limitations of sophisticated mechani...     

High-resolution study of water trees grown in silver nitratesolution

Water trees were grown in polyethylene submerged in a silver nitrate solution. Treed regions were then examined with optical microscopy, scanning electron microscopy, transmission electron microscopy (TEM) and energy dispersive spectroscopy. These water trees provided the basis for improved TEM resolution due to the formation of electron dense silver-rich particles in the treed region. The results show water trees to be composed of individual and discrete entities and do not support the theory of interconnecting channels. The possible presence of interconnecting `chemical' pathways can not be ruled out. The decomposition of the silver nitrate to metallic silver particles suggests the necessity for increased electron density in the treed region