Detection of chemical interfaces in coherent anti-Stokes Raman scattering microscopy: Dk-CARS. I. Axial interfaces

We develop a full vectorial theoretical investigation of the chemical interface detection in conventional coherent anti-Stokes Raman scattering (CARS) microscopy. In Part I, we focus on the detection of axial interfaces (i.e., parallel to the optical axis) following a recent experimental demonstration of the concept [Phys. Rev. Lett. 104, 213905 (2010)]. By revisiting the Young's double slit experiment, we show that background-free microscopy and spectroscopy is achievable through the angular analysis of the CARS far-field radiation pattern. This differential CARS in k space (Dk-CARS) technique is interesting for fast detection of interfaces between molecularly different media. It may be adapted to other coherent and resonant scattering processes.     

Focus-engineered coherent anti-Stokes Raman scattering microscopy: a numerical investigation

The coherent anti-Stokes Raman scattering (CARS) signal is calculated as a function of focal-field distributions with engineered phase jumps. We show that the focal fields in CARS microscopy can be shaped such that the signal from the bulk is suppressed in the forward detection mode. We present the field distributions that display enhanced sensitivity to vibrationally resonant object interfaces in the lateral dimension. The use of focus-engineered CARS provides a simple means to detect chemical edges against the strong background signals from the bulk.     

Elastic Fields in Immersed Isotropic Solids from Phased Arrays: The Time Harmonic Case

Elastic fields in immersed isotropic solids from phased arrays are investigated theoretically in both far- and near-field regions for a variety of beams. The angular spectrum approach (ASA) is used here as a tool to evaluate both far-field beam patterns and near-fields in immersed solids with planar interfaces. Based on the angular spectra of the arrays in a fluid and the transmission coefficients of longitudinal and shear waves at the fluid / solid interface, the angular spectra in the immersed solid are obtained and used for analyzing far-field beam patterns and for constructing near-fields in the immersed solid. The numerical calculations are optimally implemented. Several examples are presented and analyzed of nonsteered and steered, nonfocused and focused, and longitudinal and transverse beams in copper submerged in water. Emphasis is placed on elastic fields from the oblique incidence of beams steered by arrays. One method of focusing a steered beam is investigated that, in phasing, takes into account refraction at the interface, so that the beam is directed at the desired angle in the solid. Analysis of far-field beam patterns in immersed solids obtained from the ASA is helpful for the design and development of arrays and for the adequate use of existing arrays in the immersion measurements in nondestructive evaluation.     

Electromagnetic field calculations for a sphere illuminated by a higher-order Gaussian beam. II. Far-field scattering

A previously developed theoretical procedure for determination of electromagnetic fields associated with the interaction of a higher-order Gaussian beam with a homogeneous spherical particle is used to investigate the effects of incident beam type on far-field scattering. Far-field scattering patterns are calculated for (0,0), (0,1), and (1,1) mode Hermite-Gaussian beams and for the helix doughnut mode beam. The effects of incident beam type on the angular distribution of far-field scattering, for both on-sphere-center and off-sphere-center focusing, are examined.     

Local delamination on heavily deformed polymer–metal interfaces: evidence from microscopy

In this work the microstructure of interfaces present in heavily bi-axially deformed polymer-coated metal is studied. Cross sections of deformed polymer-coated steel are prepared using several polishing strategies, including the use of focused ion beam, and are imaged using optical microscopy and scanning electron microscopy. We find that the interfaces show significant details right down to the smallest scale observable with the preparation techniques used of about ~10 nm. Local delamination events at these deformed interfaces are observed and are found to be preferentially associated with overhanging parts on the interface. Overhanging parts are frequently observed, but only below a certain length-scale on the interfaces that are otherwise found to be self-affine up to a certain correlation length. The smallest detail includes the tail of the size distribution of the overhanging features. Together this suggests that the physical mechanisms determining the formation of critical features for adhesion operate at sub-grain level as well as at grain level.     

Analytical transmission electron microscopy at organic interfaces

Organic materials are ubiquitous in all aspects of our daily lives. Increasingly there is a need to understand interactions between different organic phases, or between organic and inorganic materials (hybrid interfaces), in order to gain fundamental knowledge about the origin of their structural and functional properties. In order to understand the complex structure–property–processing relationships in (and between) these materials, we need tools that combine high chemical sensitivity with high spatial resolution to allow detailed interfacial characterisation. Analytical transmission electron microscopy (TEM) is a powerful and versatile technique that can fulfil both criteria. However, the application of analytical TEM to organic systems presents some unique challenges, such as low contrast between phases, and electron beam sensitivity. In this review recent analytical TEM approaches to the nanoscale characterisation of two systems will be discussed: the hybrid collagen/mineral interface in bone, and the all-organic donor/acceptor interface in OPV devices.     

A novel method for producing electron transparent films of interfaces between cells and biomaterials

Transmission electron microscopy (TEM) investigations of intact interfaces of cells and brittle biomaterials have proven difficult using common TEM preparation techniques. This paper describes a technique to fabricate thin sections for TEM investigation of intact interfaces between human monocytes and sintered hydroxylapatite by the use of focused ion beam (FIB) microscopy. The interfaces were examined using energy filtered TEM.     

Visualization of battery materials and their interfaces/interphases using cryogenic electron microscopy

Future innovations and developments in advanced rechargeable batteries require atomic-scale observation and understanding of the failure mechanisms of secondary batteries. Unfortunately, battery chemistry is highly sensitive to air or moisture and cannot stand electron beam radiation at high dose rates essential for atomic-scale resolution, hence limiting the use of conventional electron or optical microscopes. Recently, cryogenic electron microscopy (cryo-EM) has shown that battery-sensitive materials and interface/interphases can be protected, stabilized, and imaged under cryogenic conditions, facilitating novel insights into key components and phenomena that have a substantial impact on the cell operation. Herein, we highlight the significance and essential role of cryo-EM in characterizing sensitive battery materials (such as Li/Na/K metal anodes, sulfur, lithiated silicon, etc.), key components and interfaces, and summarize the recent contributions and discoveries enabled by cryo-EM. The chemistries and evolving nanostructures at electrode/electrolyte interphase in various electrolytes (both solid and liquid), hosts, artificial interphases, and temperature ranges for lithium-based batteries, and beyond are discussed in detail. Finally, the conclusions and the perspectives on the future direction of cryo-EM in analyzing the battery materials and interfaces are briefly discussed. We believe that the insights and discoveries obtained from this characterizing tool will provide guidelines for developing energy materials with improved electrochemical performance.     

Measurement of the fracture toughness of polymer-non-polymer interfaces

One of the primary factors limiting the development of a better understanding of polymer-non-polymer adhesion is the lack of a good testing method for the measurement of the strength of the interface. In this paper polymer-non-polymer adhesion is evaluated in terms of the fracture toughness of the interface using an asymmetric double cantilever beam testing geometry. The test is applied to the measurement of polystyrene (PS)-glass and PS-silicon (native oxide) interfaces modified by PS-poly(2 vinylpyridine) (PVP) and PS-poly methyl methacrylate (PMMA) diblock copolymers. The importance of mixed mode crack propagation is demonstrated and it is shown that through an appropriate choice of sample geometry, the crack-tip trajectory can be controlled so that the crack is forced to propagate along the interface. The PS-glass test, in particular, is shown to overcome many of the traditional problems of adhesion measurements, such as failure, away from the interface and effects of far-field deformation in the polymer. The interfacial fracture toughness of the PS-glass and PS-silicon interfaces without copolymer modification are approximately the same and weak with values of 1 J m^–2. The addition of the block copolymers results in significant (>40-fold) increases in the interfacial fracture energies. The increase in fracture toughness is dependent on the quantity and degree of organization of the block copolymer at the interface.     

Broadband coherent anti-Stokes Raman spectroscopy with a modeless dye laser

We develop a modeless dye laser for broadband coherent anti-Stokes Raman spectroscopy (CARS) and investigate the operational characteristics of the modeless laser. The energy efficiency of the modeless laser is 6%, and the beam divergence is 0.65 mrad. We construct a compact movable CARS system with the modeless laser and a graphite tube furnace to assess the accuracy of the CARS temperature. It is found that the difference between the averaged CARS temperature and the radiation temperature measured with an optical pyrometer is <2% at a temperature range from 1000 to 2400 K. We also measure the averaged CARS temperature drift owing to the variation of the spectral distribution of the modeless laser, which is <1.5% during 5 h of operation.     

Plasmonic whispering gallery cavities as optical nanoantennas

We study the resonant modes of surface plasmon whispering gallery cavities based on a circular groove in a Au surface. We use spatially, angle-, and polarization-resolved cathodoluminescence spectroscopy to measure the resonant plasmonic local field distribution at deep-subwavelength resolution and determine the far-field radiation distribution for each plasmonic mode. We show mode-selective excitation of the plasmonic modes and resolve the modal angular radiation pattern. The results show that plasmonic whispering gallery resonators can be used as versatile antennas both in receiving and transmitting mode.     

Probe-beam diffraction in a pulsed top-hat beam thermal lens with a mode-mismatched configuration

The Fresnel diffraction integral is used directly to describe the thermal lens (TL) effect with a mode-mismatched collinear configuration. The TL amplitudes obtained with Gaussian, Airy, and top-hat beam excitations are computed and compared. Numerical results for beam geometries optimized for both near- and far-field detection schemes are presented, and the analytical results developed by Bialkowski and Chartier [Appl. Opt. 36, 6711 (1997)] for a Gaussian beam TL effect are summarized in simplified form. Both the numerical and the analytical results demonstrate that, under a beam geometry optimized for either near- or far-field detection, the Gaussian beam TL experiment has approximately the same maximum signal amplitude as does the photothermal-interference scheme. A comparison between the optimum near- and far-field detection beam geometries indicates that a practical mode-mismatched TL instrument should be based on the far-field detection geometry. The computation results further demonstrate that the optimum beam geometry and the TL amplitude depend largely on the excitation-beam profile. The top-hat beam TL experiment is approximately twice as sensitive as the Gaussian beam TL scheme.     

Theoretical analysis of tuning coherent laser array for several applications

Theoretical analysis for the far-field diffraction of the coherent laser array (CLA) is presented. Based on the theoretical model of the propagation of CLA, the conditions and restrictions for coherent beam combination, two-dimensional steering, and dark-hollow beam generation are theoretically described in detail. With properly organized phase distributions and tilt control, the peak location of the far-field pattern of the CLA could shift two-dimensionally in a large scale of steering angle. With additional amplitude modulation, the far-field pattern could have special shapes. The simulated results agree with the theoretical analysis. It is a feasible way to realize all these applications by a CLA with well-arranged phase distributions and/or additional amplitude modulation.     

Refractive effects in coherent anti-Stokes Raman scattering microscopy

Coherent anti-Stokes Raman scattering (CARS) microscopy with high sensitivity and high three- dimensional resolution has been developed for the vibrational imaging of chemical species. Due to the coherent nature of the CARS emission, it has been reported that the detection of epi-CARS and forward-CARS (F-CARS) signals depends on the size and shape of the sample. We investigate theoretically and experimentally the effects on the CARS signal of refractive index mismatches between the sample and its surroundings. Backward-CARS and F-CARS signals are measured for different polystyrene bead diameters embedded in different refractive index solvents. We show that index mismatches result in a backward-reflected F-CARS signal that generally dominates the experimentally backward-detected signal. Simulations based on geometrical and wave optics comparing forward- and backward-detected signals for polystyrene beads embedded in different index solvents confirm our findings. Furthermore, we demonstrate that the maxima of forward- and backward-detected signals are generated at different positions along the optical axis in the sample if refractive index mismatches are present between the sample and its surroundings.     

Electron microscopy: probing the atomic structure and chemistry of grain boundaries, interfaces and defects

At the heart of ‘ dynamic embrittlement’ phenomena is the stress-induced segregation of microscopic quantities of embrittling impurities to fracture surfaces. Grain boundaries and interfaces are often the natural weak links in a material. The structural and chemical information of such internal interfaces can be probed on an atomic scale using transmission electron microscopy. High-resolution electron microscopy can be used to determine the atomic coordinates of crystalline interfaces. Electron-energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy can be performed on any boundary or defect and can provide chemical information such as composition and bonding. The spatial resolution of EDS is limited by low collection efficiency to around 10 Å. The more efficient signal collection for EELS allows almost atomic resolution for light elements. EELS fine structure offers a fingerprint of the local boundary arrangements, and also insight into the bonding (and possible reactivity) of boundaries and other defects. Overall, electron microscopy can be used to identify the atomistic characteristics of those interfaces susceptible to dynamic embrittlement. The structural and electronic information obtained can be used as a starting point for semi-empirical and ab initio simulations.     

Grating mosaic based on image processing of far-field diffraction intensity patterns in two wavelengths

A practical grating mosaic method is proposed based on quantitative image processing of three far-field diffraction intensity patterns in two wavelengths. This method aims at making a perfect mosaic of two planar gratings that can substitute for a single and larger grating without introducing wavefront aberration at any wavelength. The zeroth-order and first-order far-field patterns of one wavelength are analyzed for separating and eliminating the angular mosaic errors. The first-order far-field patterns of two wavelengths are applied for separation of the lateral and longitudinal phase errors. Then the three patterns are considered together to enlarge the target range of coarse adjustment required for further fine adjustment in longitudinal position. Experimentally, angular and positional detection sensitivities of less than 6 microrad and 14 nm were achieved, respectively, and the periodicity in positional adjustment was checked, which departed less than 1.8% from the theoretical period. The performance of the perfect mosaic grating was diagnosed with the far-field diffraction intensity pattern in a third wavelength, and the necessity for a perfect mosaic was verified.     

Spatial coherence of synchrotron radiation

The spatial coherence properties of a monochromatic component of synchrotron radiation from an insertion device in the Fraunhofer limit are analyzed in the general case when the coherence distance is comparable with the beam width, expressing them by simple products and convolutions of Fourier transforms and autocorrelations on the single-electron field amplitude and the electron-beam position and angular distributions. In particular, the Gaussian approximation is discussed, in which case the far-field amplitude satisfies the Schell condition (its statistical properties can be described by a coherence factor depending only on the difference of the reciprocal space coordinates), and this discussion leads to simple estimates of the coherence widths. The coherence widths deviate from the Van Cittert-Zernike values when one or more of the phase space widths of the electron beam are close to (or smaller than) the diffraction limit.     

Scattering of the transverse magnetic modes from an abruptly ended strongly asymmetrical slab waveguide by an accelerated integral equation technique.

We study the problem of the scattering of the first TM guided mode from an abruptly ended strongly asymmetrical slab waveguide by an improved iteration technique, which is based on the integral equation method with "accelerating" parameters. We demonstrate that the values of these parameters are related to the variational principle, and we save approximately 1-2 iterations compared with the case in which these parameters are not employed. The tangential electric-field distribution on the terminal plane, the reflection coefficient of the first TM guided mode, and the far-field radiation pattern are computed. Furthermore, a simple technique based on the Aitken extrapolation procedure is employed for faster computation of the higher-order solutions of the reflection coefficient. Numerical results are presented for several cases of abruptly ended waveguides, including systems with variational profile, while special attention is given to the far-field radiation pattern rotation and its explanation.     

Use of analyte-modulated modal power distribution in multimode optical fibers for simultaneous single-wavelength evanescent-wave refractometry and spectrometry

A new method is described for the simultaneous determination of absorbance and refractive index of a sample medium. The method is based on measurement of the analyte-modulated modal power distribution (MPD) in a multimode waveguide. In turn, the MPD is quantified by the far-field spatial pattern and intensity of light, i.e., the Fraunhofer diffraction pattern (registered on a CCD camera), that emerges from a multimode optical fiber. Operationally, light that is sent down the fiber interacts with the surrounding analyte-containing medium by means of the evanescent wave at the fiber boundary. The light flux in the propagating beam and the internal reflection angles within the fiber are both affected by optical absorption connected with the analyte and by the refractive index of the analyte-containing medium. In turn, these angles are reflected in the angular divergence of the beam as it leaves the fiber. As a result, the Fraunhofer diffraction pattern of that beam yields two parameters that can, together, be used to deduce refractive index and absorbance. This MPD based detection offers important advantages over traditional evanescent-wave detection strategies which rely on recording only the total transmitted optical power or its lost fraction. First, simultaneous determination of sample refractive index and absorbance is possible at a single probe wavelength. Second, the sensitivity of refractometric and absorption measurements can be controlled simply, either by adjusting the distance between the end face of the fiber and the CCD detector or by monitoring selected modal groups at the fiber output. As a demonstration of these capabilities, several weakly absorbing solutions were examined, with refractive indices in the range from 1.3330 to 1.4553 and with absorption coefficients in the range 0-16 cm-1. The new detection strategy is likely to be important in applications in which sample coloration varies and when it is necessary to compensate for variations in the refractive index of a sample.     

Plasmon spectroscopy and imaging of individual gold nanodecahedra: a combined optical microscopy, cathodoluminescence, and electron energy-loss spectroscopy study

Imaging localized plasmon modes in noble-metal nanoparticles is of fundamental importance for applications such as ultrasensitive molecular detection. Here, we demonstrate the combined use of optical dark-field microscopy (DFM), cathodoluminescence (CL), and electron energy-loss spectroscopy (EELS) to study localized surface plasmons on individual gold nanodecahedra. By exciting surface plasmons with either external light or an electron beam, we experimentally resolve a prominent dipole-active plasmon band in the far-field radiation acquired via DFM and CL, whereas EELS reveals an additional plasmon mode associated with a weak dipole moment. We present measured spectra and intensity maps of plasmon modes in individual nanodecahedra in excellent agreement with boundary-element method simulations, including the effect of the substrate. A simple tight-binding model is formulated to successfully explain the rich plasmon structure in these particles encompasing bright and dark modes, which we predict to be fully observable in less lossy silver decahedra. Our work provides useful insight into the complex nature of plasmon resonances in nanoparticles with pentagonal symmetry.