Recent Developments in Electron Holography for Phase Microscopy

/ Electron Microsc 44: 425-435 (1995) Review Recent Developments in Electron Holography for Phase Microscopy Akira Tonomura Advanced Research Laboratory, Hitachi, Ltd., Hatoyama, Saitama, 350-03 Japan The phase information of an electron wave can be obtained from an interference pattern. The first macroscopic electron interference pattern, Fresnel fringes, was observed by Boersch1' in 1940, while the first electron interferometer was developed in 1952 by Marton. 2 ' This was a Mach-Zehnder-type interferometer that used Bragg reflections on three sheets of single-crystalline films. The idea for this interferometer came from a result in the preceding year by Mitsuishi et al., 3 ' who observed the electron interference fringes between transmitted beams and Bragg-reflected beams on two single-crystalline films. This kind of interferometer is no longer used for general practical applications for electron beams but is instead used for X-rays and neutron beams. 4 ' The electron interferometer now widely used is an electron biprism devised by Mollenstedt and Diiker5' in 1955. From then until the early 1970s, extremely interesting experiments were carried out, first at Tubingen University 6 ' and CNRS Toulouse 7 ' and later at Technical University of Berlin,8' Tohoku University,9' Bologna University, 10 ' PTB Braunschweig, 11 ' Hitachi 12 ' and elsewhere. These experiments investigating the inner and contact potentials, 67 ' quantized magnetic fluxes,8' and magnetic fields1012' are summarized in a review article by Missirolli et a!.13' These electron-interference experiments were carried out using a transmission electron microscope equipped with an electron biprism.5' The development of a coherent field-emission electron beam,14' however, has made it possible to produce bright and high-contrast interference fringes. As a result, electron interference patterns that could previously be recorded on film only after a long exposure have become directly observable on the fluorescent screen with the naked eye, and the total number of interference fringes recordable on film has increased from 300 to more than 3,000. 15) This development has not only made conventional electron interferometry feasible but has also opened up new possibilities using electron holography. Vol. 44, No. 6, 1995 425 New features which have become possible by holographic interference microscopy are as follows: (1) When a biprism is used, the phase distribution is displayed only in deviations from regular interference fringes, as an interferogram. By using electron holography, however, it is also possible to obtain phase contours. (2) The precision in the phase measurement can be increased from 1/4 of the wavelength in the case of the biprism to 1/100 of the wavelength by using a phase-amplification technique peculiar to holography. 16 ' (3) The brighter interference patterns make real-time observation possible. 1718 ' The coherent electron beam brought about new possibilities also in Lorentz microscopy, where spatial gradients in the phase distribution can be manifested as intensity variations simply by defocusing an electron micrograph under a collimated electron illumination. By using this technique, vortices in superconductors have recently been observed dynamically. 19 ' EXPERIMENTAL METHOD Electron holography is a two-step imaging method. A hologram is formed by the interference between an object electron wave and a reference electron wave, and then an image is reconstructed optically by illuminating a laser beam onto the hologram. Holographic interference microscopy In the first step in holography (Fig. l(a)), an interference pattern is formed between an object wave and a reference wave, usually in a field-emission transmission electron microscope, and recorded on film as a hologram. This hologram film is subsequently illuminated by a collimated laser beam (Fig. l(b)). The exact image is produced three-dimensionally in a diffracted beam. An additional, so called conjugate image is also produced in holography; the amplitude is the same but it has the opposite sign. An interference micrograph, or contour map of the This paper reports on the recent remarkable progress made in electron phase microscopy, especially due to the development of both a "coherent" field-emission electron beam and the related image processing techniques. With these techniques, the phase distribution of an electron beam transmitted through a specimen can now be measured with a precision of within 1/100 of the electron wavelength to observe the thickness distribution of a uniform specimen at the atomic level, the magnetic domain structures in a ferromagnetic thin film, and individual vortices in a superconducting thin film. Vortices in superconducting thin films have become dynamically observable by Lorentz microscopy. Key words: electron holography, interference microscopy, field emission, magnetic line of force, vortex A. Tonomura 426 Electron Hologram formation Optical reconstruction (a) Hologram formation Hologram Light Image I (b) reconstruction I Hologram t> Fig. 1. Principle behind electron holography, (a) Electron hologram formation, (b) Optical image reconstruction. Reconstructed wavefront Plane wave Conjugate wavefront Amplified contour map (b) Fig. 2. Principle behind phase amplification, Twice-amplified contour map. Fig. 3. Real time electron holography using liquid crystal panel. Image (a) Contour map. (b) wavefront, can be obtained by simply overlapping an optical plane wave with this reconstructed wave (see Fig. 2(a)). This kind of micrograph cannot be obtained using an electron microscope equipped with an electron biprism, where the phase distribution is displayed only as deviations from regular fringes, or in the form of an interferogram. If a conjugate image instead of a plane wave overlaps this reconstructed image, the phase difference becomes twice as large, and it is as if the phase distribution were amplified two times, as shown in Fig. 2(b). By repeating this technique, a phase shift even as small as 1/100 of the wavelength can be detected. The optical reconstruction method described above is simple, but must be done off-line due to the timeconsuming process of developing the film. Therefore, on-line or real-time reconstruction techniques that use computers and optical techniques are now being developed. For example, electron holograms are directly recorded on a CCD camera or TV camera attached to an electron microscope. An image is numerically reconstructed from the hologram by carrying out Fourier transformation twice. Since both the phase and amplitude of the image are independently obtained, they can be displayed as the phase image, the amplitude image, or the interference micrograph. The time required to get such images is fairly short with a fast computer, but is still not as short as that for real-time reconstruction. In the real-time method recently developed, the image signal (...truncated)