Evanescent-wave scattering in near-field optical microscopy

Extended Mie theory is used to investigate the scattering and extinction of evanescent waves by small spherical particles and aggregates of such particles. Metallic, dielectric and metal-coated dielectric particles are taken into consideration. In contrast to plane-wave excitation, p - and s -polarized spectra differ in the case of evanescent waves due to the inherent asymmetry of both polarizations. Furthermore, contributions from higher multipoles are strongly enhanced, compared with plane-wave excitation, and the enhancement factors are polarization dependent. The corresponding changes in the scattering and extinction spectra are most pronounced in cases where higher multipoles exhibit resonances in the spectral range considered. This applies, for example, to morphological resonances of dielectric particles with size parameters > 1. The effect of the surface, where the evanescent wave is generated by total internal reflection, on the scattering and extinction spectra is investigated via numerical field calculations employing the multiple multipole method. In an application to apertureless near-field optical microscopy, the variation of the scattered power is calculated when a silicon particle is scanned across a silver particle in the evanescent field.     

State-selective optical near-field resonant ionization spectroscopy of atoms near a dielectric surface

Spin-sensitive optical near-field microscopy and spectroscopy are proposed based on the study on the conserved quantities in optical near-field interactions of atoms with dielectric surfaces. A two-step photoionization spectra of Cs atoms resolving hyperfine structures are demonstrated near a planar dielectric surface by using evanescent waves. These techniques of state/spin-selective excitation and highly sensitive detection, combined with the techniques of optical pumping, will open up possibilities of space- and polarization-sensitive detection of optical near‐fields using atomic probes. This novel method provides us with a useful technique for the observation of polarization nature of the optical near-field and controlling the spin states of mesoscopic electronic systems.     

TIRF microscopy evanescent field calibration using tilted fluorescent microtubules

Total internal reflection fluorescence microscopy has become a powerful tool to study the dynamics of sub-cellular structures and single molecules near substrate surfaces. However, the penetration depth of the evanescent field, that is, the distance at which the excitation intensity has exponentially decayed to 1/e, is often left undetermined. This presents a limit on the spatial information about the imaged structures. Here, we present a novel method to quantitatively characterize the illumination in total internal reflection fluorescence microscopy using tilted, fluorescently labelled, microtubules. We find that the evanescent field is well described by a single exponential function, with a penetration depth close to theoretically predicted values. The use of in vitro reconstituted microtubules as nanoscale probes results in a minimal perturbation of the evanescent field; excitation light scattering is eliminated and the refractive index of the sample environment is unchanged. The presented method has the potential to provide a generic tool for in situ calibration of the evanescent field.     

主成分分析在光全散射特征波长选择中的应用

为了能在用光全散射法测量颗粒粒径时选择对粒径影响较显著的特征波长进行测量,通过在可见及可见-红外波段内对粒径服从单峰R-R分布颗粒系的消光光谱,一阶微分以及二阶微分消光光谱进行主成分变换,提出一种特征波长选择方法。该方法首先对颗粒系的一阶微分消光光谱进行主成分变换,然后将每个波长下的一阶微分消光谱对主成分贡献率的大小作为特征波长选择的主要依据,并将光谱范围的边界波长也作为特征波长。分别对粒径服从单峰及双峰R-R分布的颗粒系进行数值仿真,并采用标准颗粒的实测数据进行验证。验证结果显示,采用基于主成分分析的波长选择方法计算方便、易于实现,得到的标准颗粒粒径反演误差均小于3%,表明采用提出的波长选择方法能够保证选出的光谱消光值具有较高的信息量。     

Broadside coupling to long-range surface plasmons in metal stripes using prisms, particles, and an atomic force microscope probe

Techniques for broadside coupling to long-range surface plasmon waves propagating along metal stripes are investigated. The baseline technique consists of evanescently coupling an optical input beam originating from a polarization maintaining fiber to the plasmon wave via a right-angle prism positioned above the metal stripe, and providing an optical output some distance away through a mirror arrangement of identical elements. The technique is modeled theoretically using plane waves and implemented to measure the attenuation of the long-range plasmon wave propagating along a metal stripe supported by a thin freestanding dielectric membrane. An alternative technique for providing an output is proposed, whereby a tipless atomic force microscope probe physically contacts the metal stripe to generate out-of-plane scattering and a multimode fiber positioned nearby is used to capture a portion of the scattered light. This technique is easier to implement than the baseline technique, resulting in attenuation measurements of significantly better quality. The goodness of fit of the best fitting linear models to the measurements was significantly improved using this technique (0.93 and 0.99), and the measured attenuations were in very good agreement with the theoretical ones (6.01% and 0.27% error). This simple technique for optical probing and coupling could be applied to other surface plasmon waveguides and possibly to dielectric waveguides with modes having sufficient field strength in their evanescent tail. Output scattering using micron-sized particles located on the metal stripe was also investigated. The stability of the experimental setup was assessed and found to be about 0.01 dB peak to peak over a few minutes at constant temperature using a reference optical signal.     

Novel photoactive aggregates of rhodamine 6G: dequenched and quasi-stable aggregates induced by a dewetting process

Self-organized rhodamine 6G particles prepared by wetting/dewetting process of an ethanol solution on a hydrophilic glass surface exhibits fluorescence without quenching, showing a sharp linewidth of 2nm with a large redshift, which indicates an existence of dye aggregates, similar to J-aggregates, inside the particle. Polarized evanescent field excitation showed that the dye molecule's transition moment along the pi-conjugation was oriented unidirectionally within particles and parallel to the substrate surface. This deduced dye orientation showed correlation between adjacent, but separated, particles and pointed roughly 45 degrees off the dewetting direction. In contrast, the particles of another pi-conjugated NK1420 dye, J-aggregates of which grows easily from an oversaturated solution, showed dye orientation along the dewetting direction preferably, still indicating the effect of self-organization, however based on a different mechanism. An annealing procedure revealed that both aggregates are in quasi-stable states, which is consistent with the rapidness of the dewetting process that may lead to crystallization in nonequilibrium.     

Dielectrophoretic motions of multiple particles and their analogy with the magnetophoretic counterparts

To investigate the dielectrophoretic motions of multiple particles, we have performed the so-called direct numerical simulations on two-dimensional flows involving inertialess dielectric particles of two to five suspended in a viscous fluid under a uniform external electric field and then compared the results with those of the corresponding magnetophoretic counterparts. For the simulations, the electric field (or the force acting on each particle) is described by the numerical solution of the Maxwell equation (or Gauss??s law), where the smoothed representation technique is employed to tackle the jump of electric conductivity across the particle-fluid interface. The flow field, on the other hand, is described by the solution of the continuity and momentum equations, where one-stage smoothed profile method is employed to satisfy the no-slip condition at the interface. In all the simulations, the particles are initially equi-spaced on a circle with an origin at the center while a uniform electric field is externally imposed. Results show that all particles move with repelling or attracting one another depending on the locally nonuniform electric field formed due to the presence of multiple particles. Consequently, they become clustered largely into two groups, then revolve in the clockwise or counterclockwise direction, and finally get aligned in a line with the field direction. One exception is their initial configuration which is symmetric with respect to the axis perpendicular to the electric-field direction, where all the particles move eternally far away from one another with keeping the symmetry. In addition, it is found that the two-dimensional relative motions of dielectric particles under a uniform external electric field are qualitatively in fairly exact agreement with those of paramagnetic particles suspended in a nonmagnetic fluid under a uniform external magnetic field.     

Implementing Profile Permittivity Reconstruction Technique for Dielectric Tube in a Rectangular Waveguide

In this article a new approach for accurate prediction of scattering parameters of lossy dielectric materials is suggested. It is based on reconstructing complex permittivity profile of the test cell placed within rectangular waveguide and numerical determination of proper scattering parameters using algorithm Kao or its modification. Both propagating and evanescent modes excited in the test cell consisting of the dielectric tube filled with lossy materials are investigated to validate resonance and non-resonance phenomena. Comparison with earlier reported results shows a good agreement. The advantage of this method is the simplicity of its realization and accuracy in prediction of scattering parameters measured in practice.     

Modeling Clutter from Bosnian and Puerto Rican Rough Ground Surfaces for GPR Subsurface Sensing Applications Using the SDFMM

The Steepest Descent Fast Multipole Method (SDFMM) is used to analyze the distorting effect of random rough ground surfaces on scattered and transmitted electromagnetic waves. Two well-measured loamy soils: Bosnian and Puerto Rican clay loam are investigated, each with a variety of surface roughness. This study is important in understanding the effects of different soil properties and is meant to be an a priori phase of investigating scattering from buried targets under the rough ground. In this work, we investigated the scattering from rough soil ground without buried objects. The SDFMM is an integral equation-based fast algorithm that is well suited for two-dimensional penetrable rough surfaces (3-D scattering) in the frequency domain. The scattered and transmitted near electric field of an incident Gaussian beam are calculated at different locations above and below the mean plane of the dielectric rough interface. The receiver locations above are chosen to simulate GPR measurement protocols. The obtained numerical results show that the scattered field undergoes more distortion than the transmitted field from both soil types. Moreover, the transmitted fields into the higher dielectric constant Puerto Rican soil experience more distortion than those transmitted into Bosnian soil.     

A simple method to disentangle nanoparticle optical properties by darkfield microspectroscopy

We present a darkfield optical microspectroscopy technique devoted to the disentangled measurement of the absorption and scattering cross sections of nanoparticle (NP) samples with variable concentration. The robustness of the method, including the needed instrumental calibrations, is examined in detail by analyzing and quantifying the major sources of statistic and systematic errors. As an exemplary case, results are presented on a gold NP colloid. The technique takes advantage of a simple inverted microscope, coupled with a spectrograph and equipped with a darkfield condenser and a variable numerical aperture objective to obtain spectra either in darkfield or brightfield optical configurations. By adopting the Lambert–Beer (LB) equation modeling, we were able to disentangle and measure with a single setup the absorption, scattering, and extinction coefficients of the same sample by combining three spectra, obtained by opportunely varying the objective numerical aperture. Typical plasmonic resonances were recognized at approximately 520 and 750 nm. Optical coefficients were measured as a function of particle number density (0.04–3.94 µm^?3, corresponding to 40 µM–4 mM nominal Au concentration) and good linearity was verified up to ～1.5 µm^?3 (～1 mM Au). Moreover, extinction and scattering cross sections were quantified and the validity of the LB approximation was reviewed. Besides its applications to plasmonic NPs, this method may be appropriate for any colloid, provided there exists a characteristic spectral feature in the ultraviolet‐visible‐near infrared range. This technique may be exploited to localize NPs in biological samples. Microsc. Res. Tech. 77:886–895, 2014. © 2014 Wiley Periodicals, Inc.     

A Nondestructive Technique for Local Measurement of Dielectric Loss in Substrates of Integrated Circuits at Microwaves

A device for nondestructive measurements of dielectric loss in substrates of integrated circuits and film materials in a local 2-mm-diameter zone has been developed. The main element of the device is a coaxial resonator with a measurement hole at its end and a clamp for dielectric substrates in the form of a cylindrical waveguide that is evanescent for measurement frequencies. In such a system, emission of electromagnetic waves from the measurement hole is absent, although the phenomenon of emission is characteristic of a coaxial resonator with a hole to which a dielectric is applied. The absence of radiation loss simplifies the determination of the loss tangent for dielectric substrates. A formula for calculations and a measurement technique in the range of 10 GHz are presented.     

An evanescent-field optical microscope

The classic diffraction limit of resolution in optical microscopy (～γ/2) can be overcome by detecting the diffracted field of a submicrometre-size probe in its near field. The present stage of this so-called scanning near-field optical microscopy (SNOM) is reviewed. An evanescent-field optical microscope (EFOM) is presented in which the near-field regime is provided by the exponentially decaying evanescent field caused by total internal reflection at a refractive-index transition. A sample placed in this field causes a spatial variation of the evanescent field which is characteristic for the dielectric and topographic properties of the sample. The evanescent field is frustrated by a dielectric probe and thus converted into a radiative field. In our case the probe consists either of an etched optical fibre or of a highly sharpened diamond tip. The probe is scanned over the sample surface with nanometre precision using a piezo-electric positioner. The distance between probe and sample is controlled by a feedback on the detected optical signal. The resolution of the microscope is determined by both the gradient of the evanescent field and the sharpness of the tip. Details of the experimental set-up are discussed. The coupling of the evanescent field to the submicrometre probe as a function of probe-sample distance, angle of incidence and polarization has been characterized quantitatively. The observed coupling is generally in agreement with presented theoretical calculations. Microscopy has been performed on a regular latex sphere structure, which clearly demonstrates the capacity of the evanescent-field optical microscope for nanometre-scale optical imaging. Resolution is typically 100 nm laterally and 10 nm vertically. The technique is promising for biological applications, especially if combined with optical spectroscopy.     

Non-regularly shaped plasmon resonant nanoparticle as localized light source for near-field microscopy

We study numerically two-dimensional nanoparticles with a non-regular shape and demonstrate that these particles can support many more plasmon resonances than a particle with a regular shape (e.g. an ellipse). The electric field distributions associated with these different resonances are investigated in detail in the context of near-field microscopy. Depending on the particle shape, extremely strong and localized near-fields, with intensity larger than 10⁵ that of the illumination wave, can be generated. We also discuss the spectral dependence of these near-fields and show that different spatial distributions are observed, depending which plasmon resonance is excited in the particle.     

Absorption and scattering correction in fluorescence confocal microscopy

In three-dimensional (3-D) fluorescence images produced by a confocal scanning laser microscope (CSLM), the contribution of the deeper layers is attenuated due to absorption and scattering of both the excitation and the fluorescence light. Because of these effects a quantitative analysis of the images is not always possible without restoration. Both scattering and absorption are governed by an exponential decay law. Using only one (space-dependent) extinction coefficient, the total attenuation process can be described. Given the extinction coefficient we calculate within a non-uniform object the relative intensity of the excitation light at its deeper layers. We also give a method to estimate the extinction coefficients which are required to restore 3-D images. An implementation of such a restoration filter is discussed and an example of a successful restoration is given.     

Photocatalytic deposition of a gold nanoparticle onto the top of a SiN cantilever tip

We propose a new technique for depositing a gold nanoparticle onto the tip of a dielectric support. We employed the photocatalytic effect of titanium dioxide for the deposition. When the titanium dioxide immersed in a solution including gold ions is subject to optical exposure, the excited electrons in the conduction band reduce gold ions into gold metal. Illumination by an evanescent wave generated with a total reflection configuration limits the deposition region to the very tip. In experiments we obtained 100–300 nm gold particles on SiN cantilever tips for atomic force microscopes. The contrast of evanescent interference fringes measured by a near-field scanning optical microscope with this gold nanoparticle probe has proved to be higher than that with a non-deposited SiN probe by a factor of 1.5.     

A spectroscopic investigation of the shape dependency of gold nanoparticles grown on roughened surfaces

We present an investigation of the optical excitation of surface plasmons on Au films deposited on roughened surfaces by using a glancing angle deposition technique. By adjusting the deposition parameters of calcium fluoride and Au thin films, the spectral position of the surface plasmon resonances can be shifted through the green and into the near infrared region. In particular, we find that a rougher surface with obliquely deposited Au produces distinct spheroid-shaped nanoparticles (NPs). This results in stronger resonances with narrower linewidths, whereas smoother films result in broad red-shifted absorption. Imaging with an atomic force microscope and a scanning electron microscope provides information of NP geometry which are used as inputs for theoretical simulations of the observed spectra. The consequence of geometry distributions and inter-particle interactions are discussed. The ability to control the shape, therefore the optical response, of Au NPs over an arbitrarily large active area is of paramount importance in nano-science, especially in biological sensing applications and surface enhanced Raman scattering.     

消光法测量乳剂颗粒的研究

为研究消光法颗粒粒径测量中浓度和双层球形颗粒问题,根据Lambert-Beer定律,对不同浓度下的乳剂和PBA/PMMA双层球颗粒乳剂采用多波长消光法进行测量分析。实验结果表明:当颗粒物浓度超出某一临界浓度范围后,由消光法测量的粒径结果将受浓度变化的剧烈影响;对于核壳结构的双层球形颗粒,如果两层结构的折射率相差不大,采用其中一种折射率通过消光法来测量,其结果仍比较准确,误差不大。     

Resonance enhanced dynamic light scattering

We present a novel light scattering setup that enables probing of dynamics near solid surfaces. An evanescent wave generated by a surface plasmon resonance in a metal layer is the incident light field in the dynamic light scattering experiment. The combination of surface plasmon resonance spectroscopy and dynamic light scattering leads to a spatiotemporal resolution extending a few hundred nanometers from the surface and from microseconds to seconds. The comparison with evanescent wave dynamic light scattering identifies the advantages of the presented technique, e.g., surface monitoring, use of metal surfaces, and biorelevant systems. For both evanescent wave geometries, we define the scattering wave vector necessary for the analysis of the experimental relaxation functions.     

The computation of particle size-shape distributions in transmission microscopy

A method has been developed, and a computer program written, to correct measurements of particle length and width for the effects of section thickness and swelling in the preparation of sectioned samples for transmission microscopy. The method is applied to the system of silver particles in gelatin, and the resulting calculated extinction spectra are compared with those observed and calculated without correction. Improved correlations show that the method is useful.     

Apertureless near-field optical microscopy of metallic nanoparticles

Image formation in apertureless near-field optical microscopes employing evanescent-wave excitation is studied quantitatively as a function of the polarisation and the wavelength of the excitation. Aggregate Mie theory is used to describe the probe-sample interactions self-consistently, including retardation. Only p-polarised excitation yields images, which closely reproduce the sample, and the contrast is much higher in this case than for s-polarised waves. Particular attention is paid to the case of imaging of metallic nanoparticles, for which local and nonlocal versions of aggregate Mie theory are compared. Nonlocality arises from the excitation of longitudinal bulk plasmons at the particle surface. It is shown that this effect is essential in the imaging of such particles and implies comparatively rapid convergence, in contrast to the local theory. The converged images calculated within the nonlocal theory resemble the results of the local theory, when, arbitrarily, within the latter only dipole-dipole interactions are taken into account. Significant qualitative and quantitative differences, however, are shown to exist. Signal and contrast enhancements due to resonant excitation of surface plasmon polaritons are studied quantitatively using the results of the converged nonlocal theory.