Electron holography surmounts resolution limit of electron microscopy.

In high resolution electron{oo-axis holography, the complete information about amplitude and phase of the complex electron image wave is captured in a single hologram, fed to a computer, numerically reconstructed and analysed using methods of wave optical image processing. Speciically, the blurring eeect due to the aberration of the objective lens of the electron microscope is corrected under reconstruction. The presented rst results, achieved with a Philips CM30FEG electron microscope specially developed for the needs of high resolution electron holography, reveal that the point resolution of modern electron microscopes is signiicantly improved. In contrast to the light optical case, in electron microscopy the lateral resolution is not limited by the diiraction error, i.e. by the wavelength of the electrons, but instead is governed by the spherical aberration of the objective lens. As shown by Scherzer 1] already in 1936, this error can not be avoided as long as common rotational symmetric lens designs are used. For example, in the case of the Philips CM30FEG high resolution electron microscope applied in this work, the best point resolution is 0.198nm, about two orders of magnitude worse than the diiraction limit of the = 1:98pm wavelength, 300keV electrons. In electron microscopy we deal with a complex electron wave o(r) = a(r) expi'(r)] modulated in both amplitude and phase due to the interaction with the object. During the imaging process from this object wave to the recordable image wave, the aberrations of the objective lens lead to a blurring of the available information. The backpropagation from the aberration{corrupted image wave b(r) = A(r) exppi(r)] to the level of the object is possible following the wave laws given by the Kirchhoo diiraction integral. Prerequisites are the registration of the image wave amplitude and phase as well as a suucient knowledge of the lens aberrations. This approach { called holography { was proposed by Gabor already in 1948 2] but it took nearly 50 years until electron holography nally achieved this goal. From the various forms of electron holography 3] under investigation, the oo{axis technique has proven to be most promising 4]. Using a Moellenstedt biprism, the image wave is coherently superimposed with a plane reference wave, and the resulting interference pattern-the hologram { reveals a cosinusoidal intensity distribution I(r) = 1 + A(r) 2 + 2A(r) cos(2q c r + (r)): (1)