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Physical: Letter Production of electron vortex beams carrying large orbital angular momentum using spiral zone plates Koh Saitoh 1, *, Yuya Hasegawa 2 , Nobuo Tanaka 1 and Masaya Uchida 3 1 EcoTopia Science Institute, 2 Department of Crystalline Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan and 3 Instrument and Research Technology Center, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan *To whom correspondence should be addressed. E-mail: saitoh@esi.nagoya-u.ac.jp Abstract We report the production of electron vortex beams carrying large orbital angular momentum (OAM) using micro-fabricated spiral zone plates. A series of the spherical waves, focussing onto different positions along the propagating direction of the electron beam, were observed. The nth order vortex beam has an OAM n times larger than that of the first-order vortex beam. We observed an electron vortex with an OAM up to 90h  in a high-order diffracted wave. A linear dependence of the diameter of the vortex beam on the OAM was observed, being consistent to numerical simulations. Keywords electron vortex, orbital angular momentum, Fresnel zone plate, STEM Received 5 December 2011, accepted 2 February 2012; online 6 March 2012 Optical vortex beams carrying orbital angular mo- mentum (OAM) are extensively studied in modern optics and have found a wide range of applications in optical manipulation, quantum information and astronomy [1,2]. The formation of such vortex beams is not limited to a light wave but is attributed to the nature of waves. The first demonstration of the production of an electron vortex beam was reported by Uchida and Tonomura [3]. They used a phase plate in which the thickness of graphite films is gradually increased around a point in a spiral way. After that, Verbeeck [4] showed that a grating mask with a fork dislocation generates electron vortex beams and an application of the vortex beams to energy-loss magnetic circular dichroism. The grating mask produced a series of diffracted vortex beams aligned in a line perpendicular to the propagation direction of the electron beam, where the nth order diffracted vortex beam has a topo- logical charge n times larger than that of the first- order beam. A realization of a large OAM up to 100h  was demonstrated by a micro-fabricated binary mask with a fork dislocation having a Burgers vector of 25 [5]. Such a large OAM may play a great role in the significant enhancement of magnetic scattering because electron vortex beams carry a magnetic moment proportional to the OAM [6–8]. Another type of the binary mask producing electron vortex beams is a spiral zone plate, which was first realized for visible light and soft X-ray [9,10] and was very recently reported for an elec- tron beam [11]. One of the characteristic features of the zone plate is that the diffracted waves are not plane waves but spherical waves, which are conver- ging to (or diverging from) different positions on the optical axis of the instruments. The spiral zone plate may be suitable for the application to scan- ning transmission electron microscopy (STEM) rather than the grating mask as the diffracted beams with different orders can never be focussed on the specimen at the same time. In the present paper, we report the production of electron vortex beams formed by spiral zone plate masks designed for such spherical vortex beams. We observe how ........................................................................................................................................................................................................................................................ Journal of Electron Microscopy 61(3): 171–177 (2012) doi: 10.1093/jmicro/dfs036 ........................................................................................................................................................................................................................................................ © The Author 2012. Published by Oxford University Press [on behalf of Japanese Society of Microscopy]. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com Downloaded from https://academic.oup.com/jmicro/article-abstract/61/3/171/1989004 by guest on 22 May 2020