Holographic electron microscopy.
Using an energy-dispersive x-ray detector, an electron gun, and a computer-controllable sample stage, a multiple-energy hologram of the atomic arrangement around the Ti atom in SrTiO3 was obtained and 3D atomic images of the elements strontium (Sr), titanium (Ti), and oxygen in SrTiO3 were clearly visualized.
To understand the significance of this, we should return to the roots of holography and Gabor's 1947 invention when he was trying to improve resolution in the electron microscope. After describing his method of recording images from which the amplitude and phase components could be separately extracted, Gabor encountered an obstacle when he tried to use the technique. The electron microscopes of his era did not produce an electron wave with sufficient coherence to permit the proper degree of interference required to make a useful hologram. Similarly, holograms could not be produced from ordinary light because typical light sources of the time produced beams that spread over large angles or had a wide range of wavelengths. Thus, holography did not become practical until the invention of the laser, which produces light of a single wavelength moving in one direction.
From 1948 to 2011
The first attempts to perform holography with electron waves were made by Haine and Muley in 1952 when they made a holographic recording of zinc oxide crystals with a resolution of about 1nm. In 1955 G. Mollenstedt and H. Duker invented the biprism for electrons and the recording of electron holograms in off-axis scheme became possible. Recently, the development of transmission electron microscopes using highly coherent field-emission electron sources have made Gabor's original dream come true.
Progress in developing electron holography techniques was slow for a number of years. Electron holography was not pursued strongly until the introduction of the field-emission electron gun on electron microscopes in the late 1970s. Holography at Tubingen was then carried on primarily by Hannes Lichte, who trained under Wahl and Mollenstedt.
At the same time, Akira Tonomura from the Hitachi Corporation in Japan studied at Tubingen and returned to Hitachi to lead a major developmental effort in electron holography. He deserves the credit for developing the cold field-emission electron gun; the electronic equivalent of a laser. This culminated in the construction of a 350-kilovolt (kV) instrument designed to be optimized for electron holography. This instrument is housed in the new Hitachi Advanced Research Laboratory in a northern suburb of Tokyo. It was used for research conducted in a recently completed 5-year, $20-million program supported by the Japan Research Development Corporation.
Holo Hard Copies?
As a result of Hitachi's introduction of a commercial 200-kV version of this instrument in 1989, followed by the introduction of 200-kV and 300-kV instruments by other manufacturers, the number of laboratories that can undertake electron holography has increased from only a couple to several dozen. So this raises the possibility of optical holographic hardcopies of atoms, molecules, viruses and the world of sub micron structures.