Detection of contrast objects inside biological media by near-field microwave diagnostics

Theoretical and experimental investigations demonstrate the prospects of near-field microwave location of malignant tumors in biological tissue. The theoretical analysis is performed in terms of the theory of near-field location developed for layered structures. The experimental verification of the theory is accomplished by sounding an aqueous medium with a controllable permittivity. The tumor contrast is calculated based on the ideas of the dielectric properties of healthy and affected human tissues. The reason for and the degree of background contrasts as applied to near-field measurements are studied experimentally. An optimal measuring scheme for minimizing the masking effect of background contrasts is proposed. The 2D images of a tumor-simulating contrast object immersed in the aqueous medium at different depths are obtained. The prospects of near-field diagnostics for subsurface temperature measurement are discussed.     

Theoretical Analyses on the Resolution of Collection Mode Scanning Near-Field Optical Microscopy

Numerical simulations have been carried out in the framework of waveguide theory to model collection mode scanning near-field optical microscopy (SNOM). The theoretical model includes the optical fiber end and describes the metal coated aperture on the probe tip. The developed formalism goes beyond the existing Bethe-Bouwkamp theories for electromagnetic transmission of subwavelength apertures. The finite coating and optical fiber end are now taken into account. The new features enable us to simulate the near-field probes that are widely used in the collection mode SNOM. The emphases of the numerical analyses have been mainly on the resolution mechanism of the microscopy. Influence on the resolution from important parameters of the probe tips, such as the size of the apertures and the probe-sample distance, is extensively studied. The resolution dependence has been analyzed in the light of the near-field coupling efficiency of the probe tip. An optimum tip size has been found which is balanced between the significant signal transmission and the resolution of the device.     

Simulation of near-field images of a semiconductor surface with different carrier distributions

A method for determining the near-field image of a semiconductor surface with inhomogenous distribution of carriers has been proposed. This method is based on the linear response theory. As a result, the solution to the self-consistent Lippmann-Schwinger equation is expressed in terms of the effective susceptibility. A calculation is carried out for a semiconductor surface with inhomogenous distribution of electrons under the surface. It is shown that polarizations of both the probe field and the field at the detector in the far zone significantly affect the near-field distribution. The results obtained suggest that the orientation of aggregates (defects) under the semiconductor surface can be determined. The approach developed is universal and can be used for simulation in the scanning near-field microscopy for different types of objects under the surface.     

新型扫描近场微波显微术

简要回顾了近场显微术,特别是扫描近场微波显微术的发展历史;简要介绍了一种新型扫描近场微波显微镜的结构、工作原理以及相应的微波近场显微波近场显微理论;介绍了它在超导材料、铁电材料、高密度存储技术研究中的应用。     

Determination of the sheet resistance of semiconductor films via near-field microwave microscopy

A method is proposed for determining the sheet resistance of a semiconductor film on a dielectric substrate with the help of a near-field (NF) microwave microscope. The method is based on the theory of NF probing of the object with an arbitrary 1D permittivity profile. The theoretical model parameters are found from calibration measurements performed with the use of a universal set of reference samples. The test structures are GaN films grown on an Al₂O₃ substrate. A comparison with dc measurements indicates that the method error is approximately 20%.     

基于线性成像系统的光学超分辨显微术回顾

随着纳米科学技术的发展,如何打破光学衍射极限,将光学显微术的分辨本领推进到纳米尺度,已经成为光学领域的一个核心议题.在此背景下,过去的三十年间,发展了多种超分辨光学显微技术,并在生物、材料、化学领域取得了一系列令人瞩目的应用.本文以衍射理论为线索,回顾各类基于线性成像系统的超分辨光学显微技术;对以固浸物镜、结构光照明、扫描近场光学显微术、完美透镜以及超振荡透镜为代表的超分辨光学显微技术进行综述,讨论各种技术的原理,对其特点、应用与局限加以总结,并对该领域的未来发展予以展望.     

The cladded parabolic-index profile waveguide: analysis and application to stripe-geometry lasers

A two-dimensional real waveguide having a parabolic distribution of dielectric permittivity in the core region, flanked by cladding layers of constant refractive index is proposed as a model for a class of stripe-geometry injection lasers. The model is analysed by a series-solution method and results are given in terms of plots of normalized propagation constant, near-field filling factor, and far-field half-angle, all versus normalized frequency. These results are compared with those for the alternative models of (a) a homogeneous core waveguide, and (b) a medium whose dielectric permittivity varies quadratically with position throughout the whole of space.Apart from the series-solution method which was employed, a survey is included of other techniques available for analysing this waveguide model. These include perturbation theory, variational analysis, an exact solution of the scalar wave equation, and the WKB approximation.     

Near-field magneto-optics and microscopy in resonant scattering of light from a linear nanoprobe

A theory is presented for the polar near-field magneto-optical Kerr effect in scattering of light from a linear nanoprobe. In the framework of Green’s function technique, a solution is obtained for the problem of near-field magneto-optics and apertureless scanning microscopy of lateral magnetic inhomogeneities (domains) with nanometer scale. The probe in the form of a nanowire and the sample with a near-surface magnetic nanolayer are considered to support surface plasmons. Electromagnetic coupling between a nanow-ire and a sample surface (polarizability of the complex “probe plus image charges”) is taken into account self-consistently. Magneto-induced polarization of an ultrathin near-surface layer is treated within linear approximation in magnetization which is perpendicular to the layer. The polarization, spectral and angular characteristics of light scattering modulated by magnetization and resonantly enhanced by surface plasmons are examined. Dependence of the near-field magneto-optical response on the probe-domain distance along the sample surface is obtained. The resolution power of scanning near-field microscopy is estimated and the factors to influence it are pointed out.     

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.     

Resolution of Internal Total ReflectionScanning Near-field Optical Microscopy

In this paper, the probe-sample interaction equation based on Mie's scattering theory is derived, and the resolution of scanning near field optical microscopy is calculated numerically. The results show that the offset of far-field component to near-field component in total field plays an important role in the resolution and the size of samples also has influence on resolution.     

Resolution of Internal Total Reflection Scanning Near-field Optical Microscopy

In this paper, the probe-sample interaction equation based on Mie′s scattering theory is derived, and the resolution of scanning near field optical microscopy is calculated numerically. The results show that the offset of far-field component to near-field component in total field plays an important role in the resolution and the size of samples also has influence on resolution.     

Near-Field Fluorescence and Topography Characterization of a Single Nanometre Fluorophore by Apertureless Tip-Enhanced Scanning Near-Field Microscopy

Tip-enhanced near-field fluorescence and topography characterization of a single nanometre fluorophore is conducted by using an apertureless scanning near-field microscopy system. A fluorophore with size 80hm is mapped with a spatial resolution of 10hm. The corresponding near-field fluorescence data shows significant signal enhancement due to the apertureless tip-enhanced effect. With the nanometre spatial resolution capability and nanometre local tip-enhanced effect, the apertureless tip-enhanced scanning near-field microscopy may be further used to characterize a single molecule by realizing the local near-field spectrum assignment corresponding to topography at nanometre scale.     

Dispersion effect in optical microscopy systems with a supersphere solid immersion lens

This paper studies the dispersion effect of the supersphere solid immersion lens (SIL) on a near field optical microscopy system by using the vector diffraction theory. Results show that when a real non-monochromatic beam illuminates a supersphere SIL microscopy, the dispersion effect of the SIL has an important influence on the image quality. As the wavelength bandwidth of the non-monochromatic beam increases, the size of the focused spot increases and its intensity decreases in near-field microscopy systems with a supersphere SIL.     

Near-field and far-field modeling of scattered surface waves. Application to the apertureless scanning near-field optical microscopy

The detection of surface waves through scanning near-field optical microscopy (SNOM) is a promising technique for thermal measurements at very small scales. Recent studies have shown that electromagnetic waves, in the vicinity of a scattering structure such as an atomic force microscopy (AFM) tip, can be scattered from near to far-field and thus detected. In the present work, a model based on the finite difference time domain (FDTD) method and the near-field to far-field (NFTFF) transformation for electromagnetic waves propagation is presented. This model has been validated by studying the electromagnetic field of a dipole in vacuum and close to a dielectric substrate. Then simulations for a tetrahedral tip close to an interface are presented and discussed.     

Boundary-value problems in near-field optical microscopy and optical size resonances

We have solved a boundary-value problem for a ball probe interacting with a flat dielectric surface in an external optical radiation field. This interaction gives rise to the optical size resonance at frequencies significantly different from the natural frequencies of two-level atoms both in the medium and in the probe with allowance for the local field corrections. These resonances depend significantly on the distance from the probe center to the surface, on the ball probe size, on the concentration of two-level atoms in the probe and in the medium, on the spectral line width, and on the atomic inversion. The field strengths inside and outside the ball probe and a semiinfinite dielectric medium have been calculated in the near-field and wave zones. It is shown that the proposed electrodynamic theory of optical near-field microscopy agrees with the results of experimental measurements.     

Simplified triangular ground plane cloak by oblique multilayer dielectrics

Based on the effective medium theory, the triangular ground plane cloak can be realized by thin layered systems. Two solutions of parameter setting of the layered cloak are suggested to demonstrate the invisibility performance of a hybrid incoming wave. The hybrid parameters are derived from the equivalent of both anisotropies of permittivity and permeability to the alternating layers. The performance of the designed layered cloak is validated by both TM and TE wave simulations with near-field distributions and average scattering power outflows on an observation semicircle. From the simulation results, the layered cloak with both hybrid parameters and improved hybrid parameters can reflect the incoming TM/TE waves in a specular direction, and the latter behaves with a better overall invisibility performance.     

Scanning plasmon near-field microscopy on dye clusters

A scanning plasmon near-field microscope has been used to image dye clusters on a silver surface. This demonstrates the potential of optical near-field microscopy for the investigation of surfaces and non-conducting adsorbate layers. Images recorded in parallel with the conventional scanning tunneling microscope show the underlying conductive substrate.     

Surface-plasmon vortices in nanostructured metallic films

Light scattering by a small protrusion on a metal surface is analyzed within the framework of perturbation theory. Upon normal incidence of a linearly polarized monochromatic wave, slight deviations of the protrusion’s shape from a circularly symmetric one lead to the formation of optical vortices in the near-field region due to resonant excitation of circular surface plasmons. This agrees with the results of scanning near-field optical microscopy experiments revealing distinct spiral patterns in the in-plane near-field intensity distribution for metallized nanostructured polymer substrates.     

Phase interpretation for polarization-dependent near-field images of high-density gratings

It has been described that the near-field images of a high-density grating at the half self-imaging distance could be different for TE and TM polarization states. We propose that the phases of the diffraction orders play an important role in such polarization dependence. The view is verified through the coincidence of the numerical result of finite-difference time-domain method and the reconstructed results from the rigorous coupled-wave analysis. Field distributions of TE and TM polarizations are given numerically for a grating with period d = 2.3λ, which are verified through experiments with the scanning near-field optical microscopy technique. The concept of phase interpretation not only explains the polarization dependence at the half self-imaging distance of gratings with a physical view, but also, it could be widely used to describe the near-field diffraction of a variety of periodic diffractive optical elements whose feature size comparable to the wavelength.