G. Matteucci

ELECTRON HOLOGRAPHY OF LONG-RANGE ELECTRIC AND MAGNETIC FIELDS

The influence of the perturbed reference wave in electron holography is considered for the case of static electromagnetic microfields, whose extension around the observed specimen cannot be neglected. These microfields are called ‘‘long‐range’’ to distinguish them from the ‘‘short‐range’’ ones, whose extension is strictly limited within the object wave and hence do not perturb the reference wave. Optical reconstructions of experimental holograms of simple electrostatic or magnetic long‐range fields have been modeled and simulated. The results indicate that perturbation effects must be taken into account when long‐range microfields are investigated by electron holography; it is also shown that their influence can be minimized by increasing the interference distance between the object and reference wave.

Observation of electrostatic fields by electron holography: The case of reverse-biased p-n junctions

Abstract The technique of electron holography is applied to the investigation of microelectric fields such as those associated with reverse-biased p-n junctions. Suitable electron-optical conditions were adopted in order to minimize the effect of the electrostatic fringing field on the reference wave. The electron holograms were optically processed by the method of differential interferometry.

Electron holography in the study of the electrostatic fields : the case of charged microtips

Abstract The use of electron holography has been tested to map the electrostatic field around a charged microtip. Theoretical and experimental results show that whenever a modulated reference wave is used the equiphase lines observed in the final maps are not directly or simply related to the potential distribution. A set of double-exposure holograms has been used to circumvent the limited brightness and coherence of the field emission gun so as to obtain a larger useful field.

Mapping of microelectric and magnetic fields with double‐exposure electron holography

The double‐exposure electron holographic technique has been put into practical use for the first time. By this method an accurate recording of the distribution of electric and magnetic fields can be directly obtained by the electron microscope without resorting to sophisticated optical manipulation of the holograms. Problems concerning the operative definition of the contour maps are discussed. Experimental results are presented.

Distinguishing between Fraunhofer and Fresnel diffraction by the Young's experiment

Theoretical considerations, simulations and experiments have been carried out to distinguish between Fresnel and the Fraunhofer diffraction regarding the formation of interference patterns by a conventional Youngs double-pinhole arrangement with variable separation, which is illuminated by a coherent plane wave. We show that the optical path difference introduced by this setup fits the Fresnels phase difference between the pinholes. Consequently, it is possible to determine the number of Fresnel zones subtended by a circle centered on one of the pinholes and with radius equals to the pinhole separation. Then, we propose a criterion for distinguishing between Fresnel and Fraunhofer diffraction based on this number of Fresnel zones, which is applicable for diffracting apertures with any shape.

A new off-axis fresnel holographic method in transmission electron microscopy: An application on the mapping of ferromagnetic domains. III

Abstract The two-beam lattice fringe system, produced by a single crystal placed in the normal position of a conventional electron microscope, is modulated by an object inserted at the level of the selector aperture plane. The image, recorded under suitable electron optical conditions, can be considered an off-axis Fresnel hologram of the object and processed on an optical bench. The method is here applied to the investigation of the magnetization structure in thin permalloy films. By holographic interferometry in the reconstruction step, it can be easily observed that the contour map shows the trend of the magnetic lines of force in regions a few microns wide. The obtained results are compared and discussed with reference to similar observations carried out by means of an electron biprism.

Amplitude division electron interferometry

The lattice fringes produced by a single crystal placed in the normal specimen level are modulated by a specimen (phase object) located at the level of the diffraction aperture of a high resolution electron microscope. The advantages of this amplitude division interferometer compared with a microscope equipped with an electrostatic biprism are higher intensities, as extended sources can be used, and larger interference field.

Electron holography of gas-phase condensed Fe nanoparticles

Electron holography observations were performed on Fe nanoparticles with a mean size of about 50 nm synthesized by gas-phase condensation. Phase maps were obtained which represent the magnetic field both inside and around nanoparticle chains. The results suggest the presence of flux-closure magnetic configurations inside the particles, in agreement with recent micromagnetic calculations.

Build-up of interference patterns with single electrons

A conventional transmission electron microscope, equipped with a fast recording system able to measure the electron arrival time and the position of single electrons, is used to show the build-up of interference patterns. Two experiments are presented. The first is the electron version of the Grimaldi and Young experiments performed with light, where single electrons strike on an opaque thin wire. Interference fringes are observed in the geometrical shadow of the wire and diffraction effects are clearly displayed at the wire edges. The second, original experiment reports the build-up of two-slit interference patterns with single electrons.

Electron-optical analysis of the electrostatic Aharonov-Bohm effect

Abstract The electrostatic field distribution due to the contact potential difference in a bimetallic wire with no net charge introduces a quantum constant phase difference which has been detected by means of diffraction and interference experiments carried out in a transmission electron microscope. The dependence of this effect on the boundary conditions, the wire shape and its net charge is investigated by means of rather simple two-dimensional models. This analysis shows that the non-cylindrical shape of the wire is responsible for an additional phase difference, which may mask the basic quantum effect. The experimental results are herewith presented and their theoretical interpretation discussed. It is shown that only interference electron microscopy allows the unequivocal detection of the quantum phase difference due to the contact potential.