Preface for Proceedings of Workshop on Atomic Resolution Holography

Since Dénes Gábor invented holography in 1947 [patent GB685286, 1947, D. Gabor: Nature 161, 777 (1949)], many methods of electron holography and optical holography have been developed. In the experimental technique of holography a three-dimensional image of an object can be reconstructed from a hologram by irradiating a reference wave on the hologram. A hologram is a coherent interference pattern between a reference wave and an object wave. One of split reference wave is irradiated on a three-dimensional object, and the scattered wave (called an object wave) is merged with the other part of the reference wave, then the interference pattern (hologram) is created. The most important point is that this interference includes not only the intensity of the scattered wave but also its phase relative to the reference wave. Hence the so-called phase problem, which is a difficulty existing in x-ray or electron diffraction analysis, doesn't exist, and a simple irradiation of a reference wave or a simple Fourier transformation on the hologram can reproduce the three-dimensional object shape directly.

The coherency of the reference wave is important in this technique to make a clear hologram. Optical holography succeeded in the early stage of holography to reconstruct three-dimensional image of object because coherent photon beam was easily obtained by the invention of laser. After this invention of laser, the optical holography became commercially available and widely used.

The coherency of the electron beam gradually increased and Tonomura applied electron beam holography to verify the Aharonov-Bohm Effect [A. Tonomura, N. Osakabe, T. Matsuda, T. Kawasaki, J. Endo, S. Yano, and H. Yamada, Phys. Rev. Lett. 56 (1986): 792-795], and to observe magnetic vortex movement in superconductors [T. Matsuda, S. Hasegawa, M. Igarashi, T. Kobayashi, M. Naito, H. Kajiyama, J. Endo, N. Osakabe, A. Tonomura, and R. Aoki, Phys. Rev. Lett. 62 (1989): 2519-2522]. However, atomic resolution has not been obtained in this type of electron holography.

Because the hologram is an interference pattern the resolution of holography is limited by the wave length of the wave and also the coherency of the wave. In order to obtain atomic resolution the wave length should be shorter than inter-atomic distance, which is realized for more than several hundred eV electrons, usual x-rays and neutrons. The wave having enough coherency for atomic-resolution can be realized by the wave emitted from one atom, such as photoelectrons from inner core state, Auger electrons, or fluorescent X-rays from an atom. Atomic resolution holographies using these atomic waves have been developed in 1980’s such as photoelectron holography [A. Szöke, AIP Conf. Proc. 147, 361 (1986)], Auger electron holography [B. P. Tonner, Zei-Lan Han, G. R. Harp, and D. K. Saldin,Phys. Rev. B 43, 14423 (1991)], and fluorescent x-ray holography [M. Tegze and G. Faigel, Europhys. Lett. 16, 41 (1991)]. In these holographies, the waves emitted from a specific atom, such as photoelectron, Auger-electron, or fluorescent x-ray, are used as a reference wave, and the waves scattered by nearby atoms are used as object waves. Because the emitter atom itself is a point source with a size of atom, a three-dimensional arrangement of atoms surrounding the emitter atom is obtained in atomic resolution.

However the reconstruction by numerical Fourier transformation of these holograms so far has not been so successful in spite of development of many reconstruction methods. The reason of the difficulty in the reconstruction in photoelectron holography has been that the scattered wave has a forward focusing peak which has a strong anisotropy in intensity and phase. The difficulty in the x-ray fluorescence holography has been that the hologram signal is very weak.

Recently, many methods have been developed on holography and phase retrieval methods of X-ray, electron and neutron scatterings. Recent development of synchrotron radiation and improvement of techniques of data acquisition reduced the data acquisition time and also improved the quality of hologram. These improvements realized the resolution to be smaller and reconstruction area size to be several times larger than before. This volume includes recent topics on electron and X-ray emission holography, surface (interface) scattering holography, and coherent electron and X-ray diffractive imaging methods. Because most of these new techniques of holography can analyze local structure of specific atoms, such as dopant, which cannot be analyzed by usual x-ray diffraction technique, this Renaissance of holography will contribute to effective development of new functional materials.

To advance this method, we founded a research group of “atomic resolution X-ray excited holography” under SPring-8 users society in 2008. For scientific exchange in this field, we held the first workshop on the atomic resolution holography at Sendai, Japan in 12—13 November 2010. Topics on electron and X-ray emission holography, surface (interface) scattering holography, and coherent electron and X-ray diffractive imaging methods were intensively discussed during the workshop. Toward examining middle range local structures of materials contributable to green and life sciences, we set a new goal for the improvements of these methods.

Some of the papers presented in this workshop were collected in e-Journal of Surface Science and Nanotechnology as proceedings. Based on such as our activity, we hope to move the science of the 3D atomic imaging forward. In near future, we will hold the 2nd workshop of atomic resolution holography or 3D atomic imaging.

The Guest Editors of this collection of proceedings papers are Kouichi Hayashi, Tomohiro Matsushita and Fumihiko Matsui. Kouichi Hayashi and Akiko Kikuchi are greatly acknowledged for their effort in organizing the workshop.

[1] D. Gabor, Nature 161, 777 (1949).

[2] A. Tonomura, N. Osakabe, T. Matsuda, T. Kawasaki, J. Endo, S. Yano, and H. Yamada, Phys. Rev. Lett. 56, 792 (1986).

[3] T. Matsuda, S. Hasegawa, M. Igarashi, T. Kobayashi, M. Naito, H. Kajiyama, J. Endo, N. Osakabe, A. Tonomura, and R. Aoki, Phys. Rev. Lett. 62, 2519 (1989).

[4] A. Szöke, AIP Conf. Proc. 147, 361 (1986).

[5] B. P. Tonner, Zei-Lan Han, G. R. Harp, and D. K. Saldin, Phys. Rev. B 43, 14423 (1991).

[6] M. Tegze and G. Faigel, Europhys. Lett. 16, 41 (1991).

Hiroshi Daimon

Nara Institute of Science and Technology (NAIST)

8916-5 Takayama, Ikoma, Nara 630-0192, Japan