Magnetic domain imaging with a scanning near-field optical microscope using a modified Sagnac interferometer

We report on the combination of a scanning near-field optical microscope and a modified Sagnac interferometer for magnetic-domain imaging in the reflection mode. The Sagnac interferometer is used for detection of the magneto-optical Kerr effect. Since the interferometer is inherently insensitive to polarization changes caused by topography effects, magnetic-domain imaging is not limited to samples with flat surfaces. In this way, it is possible to image magnetic bits written on the tracks of a magneto-optical disc that has a rather pronounced surface profile.     

Imaging of soft and hard materials using a Boersch phase plate in a transmission electron microscope

Using two levels of electron beam lithography, vapor phase deposition techniques, and FIB etching, we have fabricated an electrostatic Boersch phase plate for contrast enhancement of weak phase objects in a transmission electron microscope. The phase plate has suitable dimensions for the imaging of small biological samples without compromising the high-resolution capabilities of the microscope. A micro-structured electrode allows for phase tuning of the unscattered electron beam, which enables the recording of contrast enhanced in-focus images and in-line holograms. We have demonstrated experimentally that our phase plate improves the contrast of carbon nanotubes while maintaining high-resolution imaging performance, which is demonstrated for the case of an AlGaAs heterostructure. The development opens a new way to study interfaces between soft and hard materials.     

A scanning near-field optical microscope having scanning electron tunnelling microscope capability using a single metallic probe tip

A new microscope system that has the combined capabilities of a scanning near-field optical microscope (SNOM) and a scanning tunnelling microscope (STM) is described. This is achieved with the use of a single metallic probe tip. The distance between the probe tip and the sample surface is regulated by keeping the tunnelling current constant. In this mode of operation, information about the optical properties of the sample, such as its refractive index distribution and absorption characteristics, can be disassociated from the information describing its surface structure. Details of the surface structure can be studied at resolutions smaller than the illumination wavelength. The performance of the microscope is evaluated by analysing a grating sample that was made by coating a glass substrate with gold. The results are then compared with the corresponding SNOM and STM images of the grating.     

Photoconductive imaging in a photon scanning tunnelling microscope capable of point-contact current sensing using a conductive fibre probe

A photoconductive photon scanning tunnelling microscope was developed to investigate the point-contact photoconductive properties of condensed matter. In order to detect the current and the optical signal at a local point on a surface, we coated the edge of a bent type fibre probe with indium tin oxide. Thus it was possible to measure both photocurrent and optical property with subwavelength resolution. The performance of the novel microscope was evaluated by analysing an organic thin film of copper phthalocyanine (CuPc), which is known to be an efficient photoconductive material. Photocurrent and current–voltage characteristics were observed at the local point on the CuPc thin films. Furthermore, photoconductive images were obtained with topography and near-field optical imaging using this system. The photoconductive PSTM shows potential in various areas of future optics and electronics.     

A line scanning confocal fluorescent microscope using a CMOS rolling shutter as an adjustable aperture

Traditional confocal microscopy uses a physical aperture barrier to prevent out-of-focus light from reaching the detector. The physical nature of a conventional aperture limits control over the system confocality. We describe a new line scanning confocal microscope that eliminates a need for a physical aperture by employing a software-controllable rolling shutter on a CMOS camera. A confocal image is obtained by synchronizing motion of the rolling shutter and the laser line scanning over a sample. Confocal resolution of this microscope is adjustable in real time and independently established for each fluorescence channel by changing the rolling shutter width. This technology has been implemented in the IN Cell Analyzer 6000 system by GE Healthcare.     

Calibration of the automated z-axis of a microscope using focus functions

Applications in automated microscopy and three-dimensional microscopy require careful calibration of the microscope system. This paper presents methods for calibration of the motorized z -axis (focus or optical axis) of an automated microscope. Apart from the automated microscope the procedures require a CCD camera and a test slide containing a simple bar pattern. The calibration embraces the following characteristics of the z -axis: (a) measuring the motor step size in nanometres; (b) measuring the mechani-cal backlash in the focus mechanism of the microscope and (c) measuring the reproducibility and the stability of the focus position over time. The measurements employ focus functions to determine the z -position of the microscope stage.     

In situ measurements on individual thin carbon nanotubes using nanomanipulators inside a scanning electron microscope

We demonstrate here a novel method for performing in situ mechanical, electrical and electromechanical measurements on individual thin carbon nanotubes (CNTs) by using nanomanipulators inside a scanning electron microscope. The method includes three key steps: picking up an individual thin CNT from a substrate, connecting the CNT to a second probe or an atomic force microscope cantilever for the measurements and placing the CNT onto a holey carbon film on a transmission electron microscope grid for further structure characterization. With the method, Young’s modulus, the breaking strength and the effects of axial strain on electrical transport properties of individual thin CNTs can be studied. As examples, the mechanical, electrical and electromechanical properties of a double-walled CNT (DWCNT) and a single-walled CNT (SWCNT) were measured. We observed a strain-induced metallic-to-semiconducting transition of the DWCNT and a bandgap increase of the SWCNT. More importantly, the electromechanical properties of the SWCNT were correlated to its chirality determined by electron diffraction. The method enables us to relate mechanical, electrical and electromechanical properties of the measured thin CNTs to their atomic structures.     

Construction and test of a GRIN-based optical objective

Optical fibres with their unique ability to transport light even in a coherent way (fibre bundles) and the possibility to build small volume optical pieces (Graded Index Fibres, GRIN) have a dominant role in the assembly of probes and objectives for microscopy applications requiring noninvasive and flexible operation in small and crowded spaces (in vivo microscopy, endoscopy, inspection). Nowadays, even complex observing procedures like confocal, two-photon and optical coherence tomography can be approached with fibres, making possible in vivo applications and also in place decision and processing. We present here a series of analytical simulations and practical tests made on an experimental GRIN fibre objective light fed through an adaptive optics system aimed to verify the practical possibility to correct a focalized beam of light. We intend this as a first step to the implementation of non-invasive probes making use of forthcoming optical devices (scanners, deformable mirrors) based on MEMS technology.     

Development of a standing-wave fluorescence microscope with high nodal plane flatness

This article reports about the development and application of a standing-wave fluorescence microscope (SWFM) with high nodal plane flatness. As opposed to the uniform excitation field in conventional fluorescence microscopes an SWFM uses a standing-wave pattern of laser light. This pattern consists of alternating planar nodes and antinodes. By shifting it along the axis of the microscope a set of different fluorescent structures can be distinguished. Their axial separation may just be a fraction of a wavelength so that an SWFM allows distinction of structures which would appear axially unresolved in a conventional or confocal fluorescence microscope. An SWFM is most powerful when the axial extension of the specimen is comparable to the wavelength of light. Otherwise several planes are illuminated simultaneously and their separation is hardly feasible. The objective of this work was to develop a new SWFM instrument which allows standing-wave fluorescence microscopy with controlled high nodal plane flatness. Earlier SWFMs did not allow such a controlled flatness, which impeded image interpretation and processing. Another design goal was to build a compact, easy-to-use instrument to foster a more widespread use of this new technique. The instrument developed uses a green-emitting helium–neon laser as the light source, a piezoelectric movable beamsplitter to generate two mutually coherent laser beams of variable relative phase and two single-mode fibres to transmit these beams to the microscope. Each beam is passed on to the specimen by a planoconvex lens and an objective lens. The only reflective surface whose residual curvature could cause wavefront deformations is a dichroic beamsplitter. Nodal plane flatness is controlled via interference fringes by a procedure which is similar to the interferometric test of optical surfaces. The performance of the instrument was tested using dried and fluorescently labelled cardiac muscle cells of rats. The SWFM enabled the distinction of layers of stress fibres whose axial separation was just a fraction of a wavelength. Layers at such a small distance would lie completely within the depth-of-field of a conventional or confocal fluorescence microscope and could therefore not be distinguished by these two methods. To obtain futher information from the SWFM images it would be advantageous to use the images as input-data to image processing algorithms such as conceived by Krishnamurthi et al. (Proc. SPIE, 2655, 1996, 18–25). To minimize specimen-caused nodal plane distortion, the specimen should be embedded in a medium of closely matched refractive index. The proper match of the refractive indices could be checked via the method presented here for the measurement of nodal plane flatness. For this purpose the fluorescent layer of latex beads would simply be replaced by the specimen. A combination of the developed SWFM with a specimen embedded in a medium of matched refractive index and further image processing would exploit the full potential of standing-wave fluorescence microscopy.     

Measurement of potential distribution function on object surface by using an electron microscope in the mirror operation mode

The quantitative theory of image contrast in an electron microscope in the mirror operation mode is given in this paper. This theory permits us to calculate the potential distribution on the object surface from the current density distribution on the microscope screen. The potential distribution results in image formation on the screen. Local electric fields existing on the object surface lead to a perturbation of electron trajectories above the object and to a redistribution of the current density on the screen, causing image contrast. Using the quantitative correlation between these fields and the function of current density distribution on the screen, it is possible to calculate the magnitude of these microfields as well. As illustration, a measured potential distribution on an object surface with spiral structures of adsorbates was analysed. These structures are formed during reaction of CO oxidation on Pt(110). The value of the measured contact potential difference comprised a few hundredths of volt.     

Simple modification of a commercial scanning laser microscope to incorporate dark-field imaging

Dark-field imaging modes have attracted less interest in biological confocal microscopy than the extensive applications of immunofluorescence imaging. There are, however, certain biological and materials science applications where it is necessary to use a confocal imaging mode that is capable of partial or full rejection of light specularly reflected from coverslips, microscope slides or specimen surfaces. In this paper we present a simple modification of a commercial confocal microscope to incorporate dark-field imaging. We discuss theoretical aspects of the resulting dark-field imaging mode and also give experimental examples.     

Nanosecond time-resolved investigations using the in situ of dynamic transmission electron microscope (DTEM)

Most biological processes, chemical reactions and materials dynamics occur at rates much faster than can be captured with standard video rate acquisition methods in transmission electron microscopes (TEM). Thus, there is a need to increase the temporal resolution in order to capture and understand salient features of these rapid materials processes. This paper details the development of a high-time resolution dynamic transmission electron microscope (DTEM) that captures dynamics in materials with nanosecond time resolution. The current DTEM performance, having a spatial resolution <10nm for single-shot imaging using 15ns electron pulses, will be discussed in the context of experimental investigations in solid state reactions of NiAl reactive multilayer films, the study of martensitic transformations in nanocrystalline Ti and the catalytic growth of Si nanowires. In addition, this paper will address the technical issues involved with high current, electron pulse operation and the near-term improvements to the electron optics, which will greatly improve the signal and spatial resolutions, and to the laser system, which will allow tailored specimen and photocathode drive conditions.     

Effects of strain gradients on strain measurements using geometrical phase analysis in the transmission electron microscope

An analysis of the effects of lens aberrations on the phase contrast images used for strain measurement by geometric phase analysis (GPA) in the TEM shows that errors may result when strain gradients are present and when [formula: see text] at the reciprocal lattice spacing used for the analysis. This conclusion is demonstrated experimentally using a model specimen consisting of a strained Si layer grown epitaxially on a Si-Ge alloy substrate. Image simulations for this model specimen show that strain gradients can introduce oscillations in strain maps when the defocus is not optimized for GPA. This work also makes it possible to identify other potential sources of error in GPA.     

A method for characterizing longitudinal chromatic aberration of microscope objectives using a confocal optical system

We describe a novel method of characterizing the longitudinal chromatic aberration of microscope objectives by recording a series of axial responses as a function of wavelength as a plane reflector is scanned through the focal region of a confocal microscope. Measurements are presented for a variety of objectives with differing degrees of correction. The use of the chromatic focal shift to measure surface profiles is also discussed.     

Re-evaluation of differential phase contrast (DPC) in a scanning laser microscope using a split detector as an alternative to differential interference contrast (DIC) optics

In this paper, differential phase imaging (DPC) with transmitted light is implemented by adding a suitable detection system to a standard commercially available scanning confocal microscope. DPC, a long‐established method in scanning optical microscopy, depends on detecting the intensity difference between opposite halves or quadrants of a split photodiode detector placed in an aperture plane. Here, DPC is compared with scanned differential interference contrast (DIC) using a variety of biological specimens and objective lenses of high numerical aperture. While DPC and DIC images are generally similar, DPC seems to have a greater depth of field. DPC has several advantages over DIC. These include low cost (no polarizing or strain‐free optics are required), absence of a double scanning spot, electronically variable direction of shading and the ability to image specimens in plastic dishes where birefringence prevents the use of DIC. DPC is also here found to need 20 times less laser power at the specimen than DIC.     

A study of the human hair structure with a Zernike phase contrast X-ray microscope

We have observed the internal structure of human hair shafts with a transmission Zernike phase contrast hard X‐ray microscope. Due to the high spatial resolution and the high contrast of the microscope, we could image scales, macrofibrils, medulla and melanin without staining. The structure of a black hair shaft is compared with that of a white one.     

The analysis of quartz c-axis fabrics using a modified optical microscope

A new fully automated microfabric analyzer (MiFA) is described that can be used for the fast collection of high‐resolution spatial c‐axis orientation data from a set of digital polarized light images. At the onset of an analysis the user is presented with an axial‐distribution diagram (AVA –‘Achsenverteilungsanalyse’) of a thin section. It is then a simple matter to build‐up c‐axis pole figures from selected areas of interest. The c‐axis inclination and colatitudes at any pixel site is immediately available to create bulk fabric diagrams or to select measurements in individual areas. The system supports both the interactive selection of c‐axis measurement sites and grid array selection. A verification process allows the operator to exclude dubious measurements due to impurities, grain boundaries or bubbles. We present a comparison of bulk and individual c‐axis MiFA measurements to pole figures measured with an X‐ray texture goniometer and to data collected from a scanning electron microscope furnished with electron backscatter diffraction (EBSD) facility. A second sample, an experimentally deformed quartzite, illustrates that crystal orientations can be precisely linked to any location within an individual grain.     

Local phase measurements of light in a one-dimensional photonic crystal

For the first time the local optical phase evolution in and around a small, one-dimensional photonic crystal has been visualized with a heterodyne interferometric photon scanning tunnelling microscope. The measurements show an exponential decay of the optical intensity inside the crystal, which consists of a periodic array of subwavelength air rods fabricated in a conventional ridge waveguide. In addition it is found that the introduction of the air rods has a counterintuitive effect on the phase development inside the structure. The heterodyne detection scheme allows the detection of low-intensity scattered waves. In the vicinity of the scattering air rods phase singularities are found with a topological charge of plus or minus one.     

Robust adaptive cruise control of high speed trains

The cruise control problem of high speed trains in the presence of unknown parameters and external disturbances is considered. In particular a Lyapunov-based robust adaptive controller is presented to achieve asymptotic tracking and disturbance rejection. The system under consideration is nonlinear, MIMO and non-minimum phase. To deal with the limitations arising from the unstable zero-dynamics we do an output redefinition such that the zero-dynamics with respect to new outputs becomes stable. Rigorous stability analyses are presented which establish the boundedness of all the internal states and simultaneously asymptotic stability of the tracking error dynamics. The results are presented for two common configurations of high speed trains, i.e. the DD and PPD designs, based on the multi-body model and are verified by several numerical simulations.     

En bloc optical sectioning of resin-embedded specimens using a confocal laser scanning microscope

Reconstruction of 3D structures of specimens embedded for light or electron microscopy is usually achieved by cutting serial sections through the tissues, then assembling the images from each section to reconstruct the original structure or feature. This is both time-consuming and destructive, and may lead to areas of particular interest being missed. This paper describes a method of examining specimens which have been fixed in glutaraldehyde and embedded in epoxy resin, by utilising the autofluorescence preserved or enhanced by aldehyde fixation, and by using a confocal laser scanning microscope to section optically such specimens in the block down to a depth of about 200 μm. In this way, the accurate estimation of the depth of particular features could be used to facilitate subsequent sectioning at the light microscope or electron microscope level for more detailed studies, and 3D images of tissues/structures within the block could be easily prepared if required.