Type of presentation: Oral

IT-11-O-1464 Split-illumination electron holography

Tanigaki T.1, Aizawa S.1, Park H. S.1, Matsuda T.2, Harada K.3, Murakami Y.1,4, Shindo D.1,4
1Center for Emergent Matter Science (CEMS), RIKEN, Saitama, Japan, 2Japan Science and Technology Agency, Saitama, Japan , 3Central Research Laboratory, Hitachi Ltd., Saitama, Japan, 4Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan
tanigaki-toshiaki@riken.jp

  Off-axis electron holography [1] has been used for observing microscopic distributions of magnetic fields, electrostatic potentials and strains at nanoscale level and for aberration-corrected electron microscopy by detecting phase shifts of electron waves. An off-axis electron hologram is formed by overlapping an object wave transmitted through a sample with a reference wave passed through the reference area. The inherent problem with this method is that the distance D between the object and reference waves, or the hologram width W, is limited by the lateral coherence length R or by the brightness of the illuminating electron waves.

 

  We solved this long-standing problem by developing split-illumination electron holography (SIEH). Experiments were performed using a 300-kV cold field emission transmission electron microscope (TEM) (HF-3300X, Hitachi High-Technologies Co.).

 

  In our SIEH (Fig. 1), we can illuminate a sample by using two highly separated and yet coherent electron waves without reducing the density of electron and form high-contrast holograms at regions far from the sample edge. The separation distance D can be controlled by a condenser biprism in the illuminating system. The fringe spacing s and the width W of the hologram can be independently controlled as in double-biprism electron interferometry [2]. Using SIEH, a fringe contrast of 50% can be attained even if the object wave is as far as 17 μm from the reference wave in the sample plane [3].

 

  Recently, in order to improve precision of phase measurement in SIEH, double condenser biprism type SIEH without Fresnel fringes was developed (Fig. 2) [4]. Since demanded phase shifts to be measured in nanoscale are becoming smaller and smaller, it is important to improve precision of phase measurements to broaden the applications of the off-axis electron holography. The developed methods are used for varieties of applications and will be used for revealing electromagnetic phenomena in atomic scale.

 

References:

[1] A. Tonomura, “Electron holography”, (Springer-Verlag, 1999).

[2] K. Harada et al., Appl. Phys. Lett. 84 (2004) 3229.

[3] T. Tanigaki et al., Appl. Phys. Lett. 101 (2012) 043101.

[4] T. Tanigaki et al., Ultramicroscopy 137 (2014) 7.

The authors are grateful to the late A. Tonomura for his valuable discussions. This research was supported by a grant from JSPS through the “FIRST Program”, initiated by CSTP.

Fig. 1: Schematic diagrams of electron-optical method and fringe contrasts of holograms. (a) Conventional electron holography. (b) Split-illumination electron holography in which a coherent electron wave is split into two coherent partial waves. (c) Measured fringe contrasts C of holograms as function of distance D between object and reference waves.

Fig. 2: Schematic diagram of double condenser biprism (CB) type split-illumination electron holography without Fresnel fringes (a) and holograms (b, c) and phase images (d, e) of charged latex particles. (b, d): double CB, (c, e): single CB.