Microscopy Theory Research Articles

Role of the probe quadrupole moment in apertureless near-field optical microscopy

A theory of an apertureless scanning near-field optical microscope is developed that takes into account the quadrupole moment of the probe in forming the near field and the scattered wave in the far-field zone. It is shown that the quadrupole moment can lead to a change in the conditions for the resonant coupling of the probe tip with the sample surface, as well as to the appearance of additional resonances in the frequency dependence of the response of a near-field microscope. The theory developed is found to be in good agreement with known experimental data in the region of resonant tip—sample coupling.

A far-field optical microscope with nanometre-scale resolution based on in-plane surface plasmon imaging

A new far-field optical microscopy technique capable of reaching nanometer-scale resolution has been developed recently using the in-plane image magnification by surface plasmon polaritons. This microscopy is based on the optical properties of a metal-dielectric interface that may, in principle, provide extremely large values of the effective refractive index n up to 100-1000 as seen by the surface plasmons. Thus, the theoretical diffraction limit on resolution becomes lambda/2n, and falls into the nanometer-scale range. The experimental realization of the microscope has demonstrated the optical resolution better than 50 nm for 502 nm illumination wavelength. However, the theory of such surface plasmon-based far-field microscope presented so far gives an oversimplified picture of its operation. For example, the imaginary part of the metal dielectric constant severely limits the surface-plasmon propagation and the shortest attainable wavelength in most cases, which in turn limits the microscope magnification. Here I describe how this limitation has been overcome in the experiment, and analyze the practical limits on the surface plasmon microscope resolution. In addition, I present more experimental results, which strongly support the conclusion of extremely high spatial resolution of the surface plasmon microscope.

Low-cost, frequency-domain, fluorescence lifetime confocal microscopy.

We describe the theory and implementation of a frequency-domain fluorescence lifetime confocal microscope using switched diode laser illumination. Standard, communications-type, radio-frequency electronics are used to provide inexpensive modulation references and to perform phase-sensitive detection. This allows the rapid acquisition of fluorescence intensity and lifetime images and their display in real time. We show fluorescence lifetime images of bead objects and fluorescence lifetime images of biological specimens from a single confocal scan.

FEATHERS AT A FINE SCALE

IN 1668, ANTOINE von Leeuwenhoek improved the crude microscopes that were being produced in Europe to better study small biological objects (Madigan et al. 1997). Although von Leeuwenhoek's microscope revolutionized biology-giving credence to, among other things, the theory of the cell-the microscope's potential escaped the attention of most ornithologists. Since the late 19th and 20th centuries, a small group of researchers have used microscopes to study feathers (for historical reviews, see Fox 1976, Prum 1999); but it was only recently that a consideration of feathers (and the organisms that live on them) has been united with traditional studies of colors and structures on a macroscopic level. The result is a new appreciation of the importance of the bacterial flora of feathers and their potential to serve as a selective force that can affect the colors of feathers. Microbes were isolated from feathers more than 40 years ago (e.g. Gierl0ff et al. 1961; Pugh and Evans 1970a, b), but feather bacteria went largely unstudied until Burtt and Ichida (1999) isolated feather-degrading Bacillus spp. from the feathers of several species. Shawkey et al. (2003a) subsequently cultured 13 distinct isolates from the feathers of House Finches (Carpodacus mexicanus). More comprehensive surveys, using both culture-based and cultureindependent methods (see Amann et al. 1995 for a review of those methods and their importance in detecting microbial diversity), have revealed even greater microbial diversity on feathers (M. D. Shawkey et al. unpubl. data). Although it is now clear that feathers are capable of harboring a diverse microflora, the ecological role(s) of that microflora remain largely a mystery. Goldstein et al. (2004) improves our understanding of these roles. Using standard microbiological methods, they demonstrate that feather-degrading bacteria degrade unmelanized white feathers more quickly and completely than melanized black

The Theory-Dependence of the Use of Instruments in Science

The idea that the use of instruments in science is theory-dependent seems to threaten the extent to which the output of those instruments can act as an independent arbiter of theory. This issue is explored by studying an early use of the electron microscope to observe dislocations in crystals. It is shown that this usage did indeed involve the theory of the electron microscope but that, nevertheless, it was possible to argue strongly for the experimental results, the theory of dislocations being tested, and the theory of the instrument, all at the same time.

Theory of a magnetic microscope with nanometer resolution

We propose a theory for a type of apertureless scanning near-field microscopy that is intended to allow the measurement of magnetism on a nanometer length scale. A scanning probe, for example a scanning tunneling microscope (STM) tip, is used to scan a magnetic substrate while a laser is focused on it. The electric field between the tip and substrate is enhanced in such a way that the circular polarization due to the Kerr effect, which is normally of order 0.1%, is increased by up to two orders of magnitude for the case of a Ag or W tip and an Fe sample. Apart from this there is a large background of circular polarization which is nonmagnetic in origin. This circular polarization is produced by light scattered from the STM tip and substrate. A detailed retarded calculation for this light-in-light-out experiment is presented.

Teaching Microscopy and Microscope Theory Based on Remote Instrument Access and Instrument Automation

Abstract The intense use of computers to operate electron microscopes as well as the ability to control microscopes remotely over the Internet, is increasingly changing the way electron microscopes are being used and how microscopy is being taught. Practically all of the top-of-the-line electron microscopes offered by the microscope vendors today are fully computer controllable, and provide for digital imaging. The combination of both features with the ever increasing speed of computers has created a situation that has and will continue to change the way electron microscopists work. Lehigh University together with Oak Ridge National Laboratory have collaborated since 1996 to make the instrumentation in the High Temperature Materials Laboratory at ORNL available for teaching purposes. The primary instrument being used is the Hitachi HF-2000 field emission TEM, which is controlled by Gatan's DigitalMicrograph™ (DM) software and uses a Ik by Ik CCD camera for digital imaging.

Electric field effects in scanning tunneling microscope imaging

We present a high-voltage extension of the Tersoff– Hamann theory of scanning tunneling microscope (STM) images, which includes the effect of the electric field between the tip and the sample. The theoretical model is based on first-principles electronic structure calculations and has no adjustable parameters. We use the method to calculate theoretical STM images of the monohydrate Si(100)−H(2×1) surface with missing hydrogen defects at −2 V and find an enhanced corrugation due to the electric field, in good agreement with experimental images. First-principles electronic structure calculations have become an important tool in interpreting scanning tunneling microscope (STM) experiments. Calculations of theoretical STM images are often based on the theory by Tersoff and Hamann [1], which states that the the STM current is proportional to the local density of states (LDOS) of the sample. In this theory it is assumed that the potential is flat between the tip and the sample and the vacuum level given by the sample workfunction. However, for relatively high biases (> 2 V), which are often used in STM experiments on semiconductor surfaces, the electric field strength in the tunnel region can be relatively high and must be included in the theoretical model [2]. In this paper we extend the Tersoff–Hamann formalism to include the electric field in the tunnel region, and apply the theory to calculate the corrugation of a single missing hydrogen defect on the monohydride Si(100)−H(2×1) surface. We find that the corrugation is strongly increased by the electric field, mainly due to polarization effects and partly due to changes in the tunnel barrier. The organization of the paper as follows. In Sect. 1 we present the basic theory for calculating field-dependent STM images, and in Sect. 2 we show how the electric field effect can be included in the first-principles calculation. In Sect. 3 we apply the formalism to calculate the corrugation of a missing hydrogen defect on the monohydrate Si(100)−H(2×1) surface and in Sect. 4 we conclude. 1 Theory In this section we present the basic theory for calculating field-dependent STM images. The derivation will follow Chen’s [3] closely. Figure 1 shows the tunnel junction between the tip and sample. Using the modified Bardeen approach [3], the tunnel current is given by

Analysis of spin-dependent tunnelling of electrons in solid state structures using the transfer-Hamiltonian method

The transfer-Hamiltonian (t-H) method is applied to study spin-dependent low-rate transfer of electrons between magnetically ordered metallic electrodes separated by a potential barrier. A weak coupling of the electrodes through the potential barrier is described with help of a t-H treated as a perturbation. Spin-dependent tunnelling probability amplitudes are expressed by a matrix with elements evaluated through an overlap between the spinor components of the electron wavefunctions from both sides of the potential barrier, resulting in spin-dependent tunnelling current and tunnelling conductance. In particular, the magnetic valve effect in tunnelling between two ferromagnets and elements of the theory of a spin-polarized scanning tunnelling microscope with a ferromagnetic tip are analysed. The t-H method is useful also in applications to tunnelling in magnetic nanostructures and multilayers.

Theory of the scanning tunneling microscope: Xe on Ni and Al.

We present a theory for the scanning tunneling microscope (STM) current based on a Keldysh Green function formalism. In our formalism, we solve self-consistently an ab initio linear combination of atomic orbitals Hamiltonian within a local density formalism. Total energy calculations for xenon deposited on metal surfaces are performed to obtain the equilibrium position, and the Green functions needed to compute the current are obtained at the same time. Structural and nonstructural effects that can influence the correct interpretation of experimental STM results are studied. We find good agreement between our calculations and experimental images taken under highly controlled conditions, and we conclude that STM images should be analyzed by comparing iteratively the theory and the experiment, much in the same way as it is usually done for other surface sensitive techniques like low-energy electron diffraction, photoelectron diffraction, surface-extended x-ray-absorption fine structure spectroscopy, etc. \textcopyright{} 1996 The American Physical Society.

Theory of a scanning tunneling microscope with a two-protrusion tip.

We consider a scanning tunneling microscope (STM) such that tunneling occurs through two atomically sharp protrusions on its tip. When the two protrusions are separated by at least several atomic spacings, the differential conductance of this STM depends on the electronic transport in the sample between the protrusions. Furthermore two-protrusion tips commonly occur during STM tip preparation. We explore possible applications to probing dynamical impurity potentials on a metallic surface and local transport in an anisotropic superconductor.

Imaging of test quartz gratings with a photon scanning tunneling microscope Experiment and theory

We use the differential formalism of the electromagnetic theory of gratings to interpret the images of test sinusoidal or lamellar quartz gratings obtained with a photon scanning tunneling microscope. The period of the grating is 0.5 μm, and the height of the rule is 0.2 μm. It is shown that the images depend strongly on several parameters, such as polarization or angle of incidence, with respect to the ruling direction. A systematic study of the isointensity lines above the gratings as a function of polarization is presented, and it is shown that the image contrast can be increased or decreased depending on the sample–probe distance. To model the interaction of the fiber probe with the electromagnetic field, we consider a second grating facing the grating under study, and by varying the periodicity of the second grating we calculate the intensity of the collected light and compare it with the experimental results.

Plasma-heating induced intensity-dependent gain in semiconductor lasers

Starting from a set of equations derived from a microscope theory we show that the familiar nonlinear gain of the form a/(1+I/Is) introduced in semiconductor laser rate equations phenomenologically should be replaced by a more general form of a/(1+I/Is)b. The new scaling exponent b depends on the relaxation constant γT that describes the rate of heat dissipation from plasma to the lattice due to carrier-phonon scattering.

Theory of scanning tunneling microscopy

To overcome the drawbacks of the transfer Hamiltonian method of Bardeen, which is predominantly used in the theory of the scanning tunneling microscope (STM), we develop a new system of formulation in three dimensions accounting for the explicit correlation between the measured current and the electronic states of both the tip and the sample surfaces with the coupling between them being taken into account. It is found that the contrast or the corrugation of the STM image can be essentially modified by the nature of tip electronic state and that the resolution of the STM is determined by the coupling between the tip and the sample along with the tip electronic state. Comparison with other theories are given.

Microscopic theory of scanning tunneling microscope for finite electric field and current

A new method of obtaining a self-consistent surface electronic structure for a finite electric field and current within the local density functional formalism is presented. In the method, two different chemical potentials are assigned to the left and right electrodes, and the scattering waves are calculated by the step-by-step recursion matrix method. This method is applied to the Na bimetallic planar junction. Effective potential, electric current, and charge density distribution are presented for given values of the separation and the bias voltage.

Theory of vacuum tunneling and its application to the scanning tunneling microscope

In the first part of the paper a short review is given on the current stage of the tunneling theory and of one of its most important applications, the theory of the scanning tunneling microscope (STM). The second part contains new results. The transfer Hamiltonian approach is used to deduce an analytical solution for the tunneling current in the STM. Real three-dimensional systems are considered. The complicated atomic structure of the tip and sample surface is taken into consideration. The wave functions of the electrodes are expanded in atomic exponential type orbitals. Introducing Wigner-Seitz cells an interface midway between the electrodes is constructed. The interface integration is exactly carried out. The current formula depends on the parameters characterizing the tip and sample surfaces (local densities of states, atomic structure, electronic structure of the surfaces atoms). An exponential dependence of the current on the atomic separation between the current carrying tip and sample atoms is found. The analytical solution for current provides some guidance to an understanding of the operation of STM and in interpretation of the resulting images.

Scanning Electron Microscope EDucatorS

Abstract Can you imagine the motivational value of a young adult being exposed to images of common day objects that they have an interest in, magnified ten, one hundred, or one thousand times? Then imagine the student learning the theory and operation of a scanning electron microscope. Finally, imagine walking into a laboratory to see these same students operating an SEM and taking their own micrographs. For the past three years I have had the pleasure of teaching the laboratory portion of the SEMEDS program. The program's goal is not just to motivate high school students in science, but to expose them to the wonders of microscopy and how it affects their everyday life. The program currently has three phases. During phase one, teachers from local high schools are contracted about the program. The teachers volunteer at the beginning of the school year then become students themselves during a four hour class.

Theory of the point source electron microscope

The theory of the point source electron microscope including multiple scattering events is formulated. Images are calculated and analyzed for carbon clusters, varbon fibers and large metal films. A Fourier-like transform is then shown to be appropriate for the reconstruction of the object with atomic resolution. Effects due to higher partial waves, multiple scattering and finite image size are examined in detail.

Theory of the scanning tunneling microscope

Theory of the scanning tunneling microscope

Low Energy Point Source Electron Microscopy

AbstractThe theory of the point source low energy electron microscope is reviewed. Images are calculated for a carbon fibre, a small cluster of MgO and a double helix of carbon atoms. A Kirchoff-Helmholtz transform is used for reconstruction. The importance of image size is stressed and the chemical specificity of the method is demonstrated