X-ray tomographic microscopy of fibre-reinforced materials

Aluminium composites containing Al₂O₃ fibres and precipitates of various intermetallic phases are investigated by high-resolution computerized microtomography. Individual fibres 15 μm in diameter and intermetallic phases forming a network with about 15 μm mesh size have been imaged. The capabilities of the method and its further development down 1 μm and less spatial resolution are discussed.     

Scanning tomographic acoustic microscopy using shear waves

We propose to use shear waves instead of longitudinal waves in a novel scanning tomographic acoustic microscope (STAM) in which the specimens are solid. When a specimen with a shear modulus is immersed in the microscope's water bath, mode conversion takes place at the water-solid interface. The shear wave energy is detectable and can be used for image reconstruction. Although wave transmission in most solid specimens is limited to about 20 degrees for longitudinal waves, it is about twice that for shear waves. Also, velocities of shear waves are lower than those of longitudinal waves and hence the wavelengths at the same frequency are smaller. For these and other reasons we can expect that for many specimens the resolution of a shear-wave STAM to be substantially better than that of a longitudinal-wave STAM. We use computer simulation in order to compare the operation of a shear-wave STAM with that of the conventional longitudinal-wave STAM. We have simulated tomographic reconstruction for each. The corresponding critical angles of incidence are computed and tomographic reconstructions of a particular solid specimen is obtained by using the back-and-forth propagation algorithm (BFP). Our simulation results show that shear-wave STAM has better resolution than longitudinal-wave STAM.     

Recent advances in scanning tomographic acoustic microscopy

We present the recent developments of the Scanning Tomographic Acoustic Microscope (STAM). The STAM was proposed as a method to achieve 3D imaging capability for the Scanning Laser Acoustic Microscope (SLAM). With the addition of a quadrature receiver, the complex scattered wave field can now be detected, and consequently the STAM is capable of subsurface holographic and tomographic imaging. The resolution improvement of the STAM can be attributed directly to the detection of the phase information and the image reconstruction technique. The STAM is sensitive to phase errors in the tomographic projections. In particular, the quadrature phase error and the initial phase error in the complex projections are critical to the tomographic reconstruction process. For multiple-angle tomography, high-precision projection registration and alignment become necessary. By obtaining solutions to these implementation problems, we have succeeded in obtaining images superior to the original SLAM images. In addition, quantitative ultrasonic imaging is possible with the STAM, and a method is presented to image the velocity parameter of simple specimens. With these capabilities, the STAM may become a useful tool for high-resolution subsurface nondestructive evaluation.     

Partially coherent lensfree tomographic microscopy [Invited

Optical sectioning of biological specimens provides detailed volumetric information regarding their internal structure. To provide a complementary approach to existing three-dimensional (3D) microscopy modalities, we have recently demonstrated lensfree optical tomography that offers high-throughput imaging within a compact and simple platform. In this approach, in-line holograms of objects at different angles of partially coherent illumination are recorded using a digital sensor-array, which enables computing pixel super-resolved tomographic images of the specimen. This imaging modality, which forms the focus of this review, offers micrometer-scale 3D resolution over large imaging volumes of, for example, 10-15 mm(3), and can be assembled in light weight and compact architectures. Therefore, lensfree optical tomography might be particularly useful for lab-on-a-chip applications as well as for microscopy needs in resource-limited settings.     

Scanning tomographic acoustic microscopy: Development and applications

The scanning tomographic acoustic microscope (STAM) is a device capable of performing subsurface imaging of microscopic specimens. Using ultrasonic energy to interrogate specimens, the STAM nondestructively obtains accurate two- and three-dimensional reconstructions of the internal structures of materials that are opaque to light. Applications include the nondestructive evaluation of integrated circuits and composite materials, characterization of the acoustical properties of substances, and examination of the condition of biological tissues. This article describes the design and development of the STAM, its capabilities, and applications using data obtained from a fully automated and integrated prototype. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 255–262, 1997     

High-resolution optical diffraction microscopy

In an optical diffraction microscopy experiment, one measures the phase and amplitude of the field diffracted by the sample and uses an inversion algorithm to reconstruct its map of permittivity. We show that with an iterative procedure accounting for multiple scattering, it is possible to visualize details smaller than lambda/4 with relatively few illumination and observation angles. The roles of incident evanescent waves and noise are also investigated.     

Open-loop band excitation Kelvin probe force microscopy

A multidimensional scanning probe microscopy approach for quantitative, cross-talk free mapping of surface electrostatic properties is demonstrated. Open-loop band excitation Kelvin probe force microscopy (OL BE KPFM) probes the full response-frequency-potential surface at each pixel at standard imaging rates. The subsequent analysis reconstructs work function, tip-surface capacitance gradient and resonant frequency maps, obviating feedback-related artifacts. OL BE KPFM imaging is demonstrated for several materials systems with topographic, potential and combined contrast. This approach combines the features of both frequency and amplitude KPFM and allows complete decoupling of topographic and voltage contributions to the KPFM signal.     

Differential near-field scanning optical microscopy

We theoretically and experimentally illustrate a new apertured near-field scanning optical microscopy (NSOM) technique, termed differential NSOM (DNSOM). It involves scanning a relatively large (e.g., 0.3-2 mum wide) rectangular aperture (or a detector) in the near-field of an object and recording detected power as a function of the scanning position. The image reconstruction is achieved by taking a two-dimensional derivative of the recorded power map. Unlike conventional apertured NSOM, the size of the rectangular aperture/detector does not determine the resolution in DNSOM; instead, the resolution is practically determined by the sharpness of the corners of the rectangular aperture/detector. Principles of DNSOM can also be extended to other aperture/detector geometries such as triangles and parallelograms.     

Two-color excitation fluorescence microscopy through highly scattering media

We study the performance of two-color excitation (2CE) fluorescence microscopy [Opt. Lett. 24, 1505 (1999)] in turbid media of different densities and anisotropy. Excitation is achieved with two confocal excitation beams of wavelengths lambda(1) and lambda(2), which are separated by an angular displacement theta, where lambda(1) not equal lambda(2), 1/lambda(e) = 1/lambda(1) + 1/lambda(2), and lambda(e) is the single-photon excitation wavelength of the sample. 2CE fluorescence is generated only in regions of the sample where the two excitation beams overlap. The 2CE fluorescence intensity is proportional to the product of the two excitation intensities and could be detected with a large-area photodetector. The requirement of spatiotemporal simultaneity for the two excitation beams makes 2CE fluorescence imaging a promising tool for observing microscopic objects in a highly scattering medium. Optical scattering asymmetrically broadens the excitation point-spread function and toward the side of the focusing lens that leads to the contrast deterioration of the fluorescence image in single- or two-photon (lambda(1) = lambda(2)) excitation. Image degradation is caused by the decrease in the excitation energy density at the geometrical focus and by the increase in background fluorescence from the out-of-focus planes. In a beam configuration with theta not equal 0, 2CE fluorescence imaging is robust against the deleterious effects of scattering on the excitation-beam distribution. Scattering only decreases the available energy density at the geometrical focus and does not increase the background noise. For both isotropic and anisotropic scattering media the performance of 2CE imaging is studied with a Monte Carlo simulation for theta = 0, pi/2, and pi, and at different h/d(s) values where h is the scattering depth and d(s) is the mean-free path of the scattering medium.     

Dual-optical-mode near-field scanning optical microscopy

The simultaneous operation of near-field scanning optical microscopy (NSOM) in reflection and transmission modes is demonstrated. In the transmission mode, a low-noise, large-area silicon photodetector was mounted between the piezoelectric transducer scanning stage and the sample. In the reflection mode, either a photomultiplier tube or two large-area silicon detectors was used for signal collection. The reflection-mode setup consisting of two silicon detectors provides a large numerical aperture of 0.9 as well as symmetrical detection of emitting photons. The dielectric thin films and the light-emitting polymers were used to demonstrate the capability of these two modes of NSOM. A comparison between these two different setups is also presented.     

Mode structure effects in optical resonance excitation

The dependence of the excitation strength on the longitudinal mode structure of a pulsed, resonant excitation field is examined. As an experimental test case, resonance ionization signal fluctuations are studied with a two-step excitation of strontium. Successive ionization signals, the corresponding pulse energies, and the mode structures of the resonant step laser pulses are recorded. The large signal fluctuations, up to 120%, cannot be explained by the modest (1-3%) pulse-energy fluctuations of either the resonant or the photoionizing field. In the case of weak excitation, the fluctuations in the signal correlate strongly with the intensity in a narrow frequency band around the resonance. In this weak-field region, the experimental correlation curves, i.e., the correlation between the signal and the spectral intensity versus the frequency, agree well with calculations based on a simple linear-response model. With the aid of correlation analysis a resonance can be localized with a single-moderesolution. As the resonant field is increased the correlation between the signal and single-mode intensity diminishes and almost disappears at full saturation. However, also in the saturation region, the correlation technique can be applied to localize a resonance with a resolution much better than that determined by the laser linewidth.     

Multiple scattering in optical coherence microscopy

We show that the multiple-scatter rejection provided by optical coherence microscopy (low-coherence interferometry) can be incomplete in optically turbid media and that multiple scattering manifests itself in two distinct ways. Multiple small-angle scattering results in an effective probe field that is stronger than expected from a first-order beam extinction model, but that contains a distorted wave front that enhances the apparent reflectance of small structures relative to those that are larger than the unscattered incident beam. Multiple wide-angle scattering produces a broad diffuse haze that reduces the contrast of subsequent features.     

High-resolution image reconstruction in fluorescence microscopy with patterned excitation

We discuss data treatment strategies in structured illumination microscopy, using simulated and experimental data. In the setup, the illumination pattern is generated by projecting a movable pinhole mask into the sample, and a wide-field fluorescence microscope image is acquired for each pattern position. The structured illumination data obtained from a two-dimensional illumination pattern can be treated by projection strategies such as in video confocal microscopy (sum, maximum, maximum minus minimum, and superconfocal), by a scaled subtraction of the out-of-focus estimate, or by a modified version of the Fourier-space treatment, as is known for data from one-dimensional structured illumination. We investigate the influence of some data processing strategies on unwanted effects such as residual patterning and local deviations from linearity in the reconstructed intensity.     

Review In Situ high-temperature optical microscopy

High-temperature optical microscopy is an essential in situ characterisation and monitoring technique with wide applications in different areas of materials science. The devices used include commercial available instruments, known as heating microscopes, and custom-made devices, usually called high-temperature processing microscopes or thermo-optical instruments. The different areas of applications of high-temperature optical microscopy are discussed on the basis of practical examples drawn from the literature. Besides the classical use of the technique to study the melting and softening behaviour of glass, slags, ashes and other silicate and ceramic materials, this review covers alternative applications, in particular the use of heating microscopes as optical dilatometers to investigate the sintering kinetics of powder compacts. In this regard, the advantages of the technique over conventional dilatometry are emphasised. A variety of custom-made devices is described, developed to investigate particular problems, such as delamination and curling of laminate composites during densification, cosintering of multilayer metal-ceramic and ceramic-ceramic systems, and wetting behaviour of liquid phases on rigid substrates. As a particular example of such a custom-made equipment, a novel, multi-purpose high-temperature processing microscope is described, and its application potential, which is well beyond that of commercial devices, is outlined. This instrument is unique in that it combines both vertical and horizontal sample observation capability, as well as the possibility to investigate samples of relatively large sizes (65 mm³), i.e. about 10 times larger than those suitable for commercial heating microscopes.     

Subnanometric stabilization of plasmon-enhanced optical microscopy

We have demonstrated subnanometric stabilization of tip-enhanced optical microscopy under ambient condition. Time-dependent thermal drift of a plasmonic metallic tip was optically sensed at subnanometer scale, and was compensated in real-time. In addition, mechanically induced displacement of the tip, which usually occurs when the amount of tip-applied force varies, was also compensated in situ. The stabilization of tip-enhanced optical microscopy enables us to perform long-time and robust measurement without any degradation of optical signal, resulting in true nanometric optical imaging with high reproducibility and high precision. The technique presented is applicable for AFM-based nanoindentation with subnanometric precision.     

过焦扫描光学显微测量技术综述

随着半导体产业的发展和器件性能的不断提升,半导体器件的特征尺寸越来越小,器件结构越来越复杂,对检测仪器的性能提出了更高的要求。首先介绍过焦扫描光学显微法（Through-focus Scanning Optical Microscope,TSOM）的测量装置及测量原理,该方法可实现三维几何参数的无损测量,因其具有精度高、速度快、成本低等优点,可以满足在线测量的需求;然后从TSOM图构建和待测参数提取两个方面对TSOM方法的研究进展进行了梳理和归纳;最后对TSOM方法未来的研究重点和发展方向进行了展望。该方法有望为我国半导体制造产业提供新的检测手段,为优化和提升我国半导体制造工艺提供重要的技术支撑。     

Nanoshells for in vivo imaging using two-photon excitation microscopy

Gold nanoshells have been intensively investigated and applied to various biomedical fields because of their flexible optical tunability and biological compatibility. They hold great potential to serve as luminescent contrast agents excitable with near-infrared (NIR) lasers. In this paper, we describe the development of nanoshells with a peak of plasmon resonance at 800 nm and their subsequent use for in vivo blood vessel imaging using two-photon excitation microscopy at an excitation wavelength of 750 nm. We were able to image single nanoshell particles in blood vessels and generate optical contrast for blood vessel structure using luminescent signals. These results confirm the feasibility of engineering nanoshells with controlled optical properties for single-particle-based in vivo imaging.     

Multiphoton fluorescence excitation in continuous-wave infrared optical traps

The multiphoton fluorescence excitation observed in acontinuous-wave (cw) single-beam gradient force optical trap(optical tweezers) is reported for latex beads labeled with ayellow-green fluorescent dye (BODIPY). The fluorescenceemission spectra of the yellow-green beads trapped and excited by thesame 1064-nm laser light are identical to the spectra excited by the365-nm UV light. The influence of the numerical aperture of theobjective on the slope of the log-log power-dependence has beendemonstrated for BODIPY-Oil solution under cw and pulsed-laserconditions. The possibility that three-photon excitation processoccurs is discussed within the context of a dog-bone saturationmodel. Other possibilities for the observed fluorescence excitationhave been discussed.     

Ultrafast optical pulse digitization with unary spectrally encoded cross-phase modulation

A method to digitize the intensity of ultrashort laser pulses for high-speed optical signal processing is described. This digitization was based on the spectral broadening of a weak probe (carrier) pulse by a more intense pump (signal) pulse through the nonlinear optical process of cross-phase modulation (XPM). The signal pulse intensity was varied to generate different spectral widths that can be encoded into digital form. Using a 50-ps time-divided multiplexing pulse train with a waveguide splitter, combiner, and an array of fibers with variable lengths, a unary XPM encoding approach is demonstrated. The spectral encoding scheme can be used to achieve a 5-GHz sampling rate at a 16-level accuracy.