On the theory of near-field magneto-optical microscopy via plasmonic modes of nanowire

The results of a theory of near-field magneto-optical microscopy with a linear probe scanning the near-surface region of a sample are presented. A cylindrical nanowire supporting surface plasmons is considered as a model of the probe. The polar magneto-optical Kerr effect is studied in scattering of light by near-surface nanowire and magnetic nanodomain. The problem is solved self-consistently in the dipolar interactions of the nanowire with the sample surface and in magnetization-linear approximation of magneto-optics. The near-field magneto-optical response depending on the distance between probe and domain is obtained, and the microscopy resolving power is estimated.     

一种有源微波近场成像系统

针对早期乳腺癌检测等近场目标的探测成像,设计了一种基于2.45GHz连续波信号的微波近场成像实验系统,主要由硬件电路和成像算法组成.系统电路用小体积、高性能和低成本的射频微波集成芯片实现,大大缩小了系统的体积并降低了成本,电路由天线模块、收发模块和数据采集控制模块构成.成像算法采用有限元微波共焦算法,完成对被测区域日标的探测成像.     

Biological imaging with a near-field optical setup

Noncontact scanning near-field optical microscope (SNOM) systems can be used to optically resolve samples in atmospheric conditions at theoretical resolutions comparable to those of transmission electron microscope and atomic force microscope systems. SNOM systems are also increasingly used to image biological samples. In this study we custom built a SNOM system with the aim of further demonstrating the potential applications of near-field optical examination of biological material. In this study we were able to image both fixed whole-cell samples in air and liquid environments and live whole-cell samples in liquids. The images acquired were of a relatively low resolution, but this work has shown that SNOM systems can be used to monitor the dynamics of living cells at subnanometric resolutions in the z axis and for fluorescent imaging of whole cells in a liquid medium.     

Interference effect in apertureless near-field fluorescence imaging

Apertureless scanning near-field optical microscopy has been used to image fluorescent latex spheres with a resolution of a few tens of nanometers and good signal-to-noise ratio. The near-field fluorescence images reveal optical interference with several highly contrasted fringes located around the spheres. The origin of the interference is discussed in detail, and models are used to explain their formation. Spatial coherence is also discussed.     

Optical near-field Raman imaging with subdiffraction resolution

We report optical near-field Raman imaging with subdiffraction resolution (approximately 120 nm) without field enhancement effects. Chemical discrimination on tetracyanoquinodimethane organic thin films showing localized salt complexes is accomplished by detailed Raman maps. Acquisition times that are much shorter than previously reported are due to the high Raman efficiency of the materials and to careful collection and detection of the optical signals in our near-field Raman spectrometer.     

Fluorescence lifetime imaging with near-field scanning optical microscopy

Near-field scanning optical microscopy (NSOM) is a high-resolution scanning probe technique capable of obtaining simultaneous optical and topographic images with spatial resolution of tens of nanometers. We have integrated time-correlated single-photon counting and NSOM to obtain images of fluorescence lifetimes with high spatial resolution. The technique can be used to measure either full fluorescence lifetime decays at individual spots with a spatial resolution of <100 nm or NSOM fluorescence images using fluorescence lifetime as a contrast mechanism. For imaging, a pulsed Ti:sapphire laser was used for sample excitation and fluorescent photons were time correlated and sorted into two time delay bins. The intensity in these bins can be used to estimate the fluorescence lifetime at each pixel in the image. The technique is demonstrated on thin films of poly(9,9'-dioctylfluorene) (PDOF). The fluorescence of PDOF is the results of both inter- and intrapolymer emitting species that can be easily distinguished in the time domain. Fluorescence lifetime imaging with near-field scanning optical microscopy demonstrates how photochemical degradation of the polymer leads to a quenching of short-delay intrachain emission and an increase in the long-delay photons associated with interpolymer emitting species. The images also show how intra- and interpolymer species are uniformly distributed in the films.     

Radar cross section measurements using near-field radar imaging

In this paper a technique to obtain the far-field scattering signature of bodies, using near-field measurements, is proposed. The method is based on near-field radar imaging techniques. The backscattered field data are collected in a controlled environment over a large frequency band and aspect angle using a near-field antenna. A focused radar image of the body is generated. Probe correction to compensate for the radiation pattern of the interrogating antenna is conducted during the two-dimensional imaging of the object. The contribution from each scattering center to the total backscattered far-field is obtained from the radar image. The proposed technique is applied to obtain the far-field radar cross section (RCS) for an object from near-field measurements conducted in an anechoic chamber at the University of Pretoria, South Africa     

Direct near-field optical imaging of higher order plasmonic resonances

We map in real space and by purely optical means near-field optical information of localized surface plasmon polariton (LSPP) resonances excited in nanoscopic particles. We demonstrate that careful polarization control enables apertureless scanning near-field optical microscopy (aSNOM) to image dipolar and quadrupolar LSPPs of the bare sample with high fidelity in both amplitude and phase. This establishes a routine method for in situ optical microscopy of plasmonic and other resonant structures under ambient conditions.     

Topography characterization of a deep grating using near-field imaging

Using near-field optical microscopy at the wavelength of 633 nm, we image light intensity distributions at several distances above an approximately 2-microm-deep and a 1-microm-period glass grating illuminated from below under the condition of total internal reflection. The intensity distributions are numerically modeled, and an inversion procedure based on a least-squares-fit optimization is employed to extract the grating geometry from the optical images.     

Nonlinear chemical imaging microscopy: near-field third harmonic generation imaging of human red blood cells

Third harmonic generation (THG) imaging using a near-field scanning optical microscope (NSOM) is demonstrated for the first time. A femtosecond, tunable near-infrared laser was used to generate both nonresonant and resonantly enhanced third harmonic radiation in human red blood cells. We show that resonantly enhanced THG is a chemically specific bulk probe in NSOM imaging by tuning the excitation source onto and off of resonance with the Soret transition of oxyhemoglobin. Additionally, we provide evidence that tightly focused, nonresonant, far-field THG imaging experiments do not produce contrast that is truly surface specific.     

Absorption coefficient imaging by near-field scanning optical microscopy in bacteria

We present a method for obtaining a position-dependent absorption coefficient from near-field scanning optical transmission microscopy. We show that the optical transmission intensity can be combined with the topography, resulting into an absorption coefficient that simplifies the analysis of different materials within a sample. The method is tested with the dye rhodamine 6G, and we show some analysis in biological samples such as bacteria KIebsiella pneumoniae and Pseudomonas aeruginosa. The calculated absorption coefficient images show important details of the bacteria, in particular for P. aeruginosa, in which membrane vesicles are clearly seen.     

Effect of near-field coupling on far-field inelastic scattering imaging of gold nanoparticles

The optical near-field enhancement induced by coupling between noble nanoparticles and the substrate has been studied by a far-field imaging method. The longitudinal mode of the incident laser is revealed to contribute to the coupling. The far-field images of individual gold nanoparticles exhibit a peanut-shaped pattern; these were constructed by the intensity of inelastically scattered light. The coupling between gold nanoparticles and the silicon substrate leads to the patterned image. By tuning the separation between the gold nanoparticles and substrate using SiO(2) layers of different thickness, the coupling efficiency decreases with the thickness of the SiO(2) layer.     

Design of magneto-optical system for imaging of magnetic flux created during a conductor-superconductor phase transition

According to the Kibble-Zurek model, flux lines are spontaneously created during a fast conductor-superconductor phase transition. The model predicts both the spatial density and the correlations of the flux array. We present the design of a magneto-optical system with a projected single-flux-line resolution. Such a system can allow detailed measurements of the distribution of flux created spontaneously during a conductor-superconductor phase transition.     

High spatial resolution imaging with near-field scanning optical microscopy in liquids.

The mechanism of tuning fork-based shear-force near-field scanning optical microscopy is investigated to determine optimal experimental conditions for imaging soft samples immersed in liquid. High feedback sensitivity and stability are obtained when only the fiber probe, i.e., excluding the tuning fork prongs, is immersed in solution, which also avoids electrical shorting in conductive (i.e., buffer) solutions. Images of MEH-PPV were obtained with comparable spatial resolution in both air and water. High optical resolution (approximately160 nm fwhm) was observed.     

Fabrication and characterization of fluorescent rare-earth-doped glass-particle-based tips for near-field optical imaging applications

Fluorescent rare-earth-doped glass particles glued to the end of an atomic force microscope tip have been used to perform scanning near-field optical measurements on nanostructured samples. The fixation procedure of the fluorescent fragment at the end of the tip is described in detail. The procedure consists of depositing a thin adhesive layer on the tip. Then a tip approach is performed on a fragment that remains stuck near the tip extremity. To displace the particle and position it at the very end of the tip, a nanomanipulation is achieved by use of a second tip mounted on piezoelectric scanners. Afterward, the particle size is reduced by focused ion beam milling. These particles exhibit a strong green luminescence where excited in the near infrared by an upconversion mechanism. Images obtained near a metallic edge show a lateral resolution in the 180-200-nm range. Images we obtained by measuring the light scattered by 250-nm holes show a resolution well below 100 nm. This phenomenon can be explained by a local excitation of the particle and by the nonlinear nature of the excitation.     

Defect imaging and characterization in composite structures using near-field microwave nondestructive testing techniques

The use of composites as building materials in different industries, aircrafts, cars, ships, buildings, bridges, etc., is rapidly growing. Composite structures become weaker than expected due to the presence of an inhomogeneity or a disbond within their layers. The presence of these defects can cause catastrophic results unless their presence is detected and their effects are assessed. Microwave nondestructive imaging techniques have several unique attributes which make them attractive for inspecting composite structures. Near-field microwave imaging is based on transmitting a wave into a dielectric structure, which is located in the near-field of a sensor, and using a signal proportional to the magnitude or phase of the transmitted or reflected wave to create a two or three dimensional image of the structure under investigation. To analyze the features and properties of an experimental image, it is important to form and optimize the image theoretically before application. In this paper, we will present the image formation model and illustrate its potential on optimizing the detection capability of subsurface inclusions in multi-layered composites.     

Microwave near-field imaging with open-ended waveguide—Comparison with other techniques of nondestructive testing

The results of an experimental study of microwave imaging with an openended waveguide at 30 GHz for NDE of dielectric materials are presented. Using a hybrid measurement method, the near-field point spread function is investigated. Improvement of sensitivity and resolution via deconvolution also is discussed. A comparison of results obtained with more prominent NDE methods as ultrasonics, X-rays, and thermal waves demonstrates the performance of the technique.     

Inversion in magneto-optical effects

Magnetically induced birefringence and dichroism in certain colloidal suspensions exhibit inversion in sign of the effect. The paper discusses various mechanisms which may give rise to such effects. Methods are suggested to trace origin of such inversions. Certain theoretical and experimental results are also discussed.     

Comparative characteristics of magneto-optical materials

Experimental results of measurements for Russian magneto-optical glasses MOG101, MOG105, MOG04, and MOG10 are presented, including the coefficient of electronic nonlinearity, energy of thermal defocusing, and power resistance. For comparison, the same parameters are shown for the optical glass K8 (an analog of the well-known BK7 glass), the fused silica QU-1, and the laser phosphate glass GLS-22P. The obtained data allow consumers to choose the proper magneto-optical materials for the device operation mode.