Effect of a physical phase plate on contrast transfer in an aberration-corrected transmission electron microscope

Abstract

In this theoretical study we analyze contrast transfer of weak-phase objects in a transmission electron microscope, which is equipped with an aberration corrector (C(s)-corrector) in the imaging lens system and a physical phase plate in the back focal plane of the objective lens. For a phase shift of pi/2 between scattered and unscattered electrons induced by a physical phase plate, the sine-type phase contrast transfer function is converted into a cosine-type function. Optimal imaging conditions could theoretically be achieved if the phase shifts caused by the objective lens defocus and lens aberrations would be equal to zero. In reality this situation is difficult to realize because of residual aberrations and varying, non-zero local defocus values, which in general result from an uneven sample surface topography. We explore the conditions--i.e. range of C(s)-values and defocus--for most favourable contrast transfer as a function of the information limit, which is only limited by the effect of partial coherence of the electron wave in C(s)-corrected transmission electron microscopes. Under high-resolution operation conditions we find that a physical phase plate improves strongly low- and medium-resolution object contrast, while improving tolerance to defocus and C(s)-variations, compared to a microscope without a phase plate.

Advances in electron microscopy: A qualitative view of instrumentation development for macromolecular imaging and tomography

“…It has been discussed [39] that the combination of aberration correction and physical phase plates will in the future provide ideal imaging for any weak phase object, either for high resolution imaging of macromolecular complexes embedded in a thin ice layer or in a whole vitrified cell. Such a physical phase plate copies the original ideas of Frits Zernike for light microscopy [40] and has long been discussed as the ideal contrast enhancement device for TEM (see reviews [41,42]).…”

Wave-function Reconstruction by Phase-plate Transmission Electron Microscopy

“…If an aberration-corrected microscope is used, it is possible to correct all aberrations as precisely as possible. Since phase contrast is produced by the phase plates, it is not necessary to have other than residual aberrations for phase contrast transfer [19]. In this case no WTF-correction is needed if the wave aberration function is flat over the reconstructed part of the Fourier space.…”

Improved specimen reconstruction by Hilbert phase contrast tomography

“…Therefore, it is advantageous to improve image contrast by the use of a physical phase plate. It provides strong in-focus phase contrast and a microscope PSF which corresponds to the ideal contrast transfer (Gamm et al, 2008).…”