AIP's 75th anniversary,
PNU will run a series of occasional comparisons between noteworthy
physics topics from 1931 and 2006. This is the first article of the series.
Germany have taken a crucial step towards achieving sharper
electron microscope images
of biological samples and other "weak-contrast" objects.
microscope images of samples made of low-weight elements like
hydrogen, carbon, nitrogen, and oxygen, are characterized by poor
contrast. In the new approach, contrast will be improved for a
transmission electron microscope (TEM) by imposing a large relative
phase shift to the electron waves scattered from samples.
of a beam of electrons as an illumination source for microscopy was
pioneered in the early 1930s by Ernst Ruska, who won a Nobel Prize
for the effort half a century later. Since then, electron
microscopes have been a workhorse for imaging small things, often
with a spatial resolution superior to that available with light
microscopes. Nevertheless, even electron microscopes have
In a TEM device most of the electrons pass
through the thin electron-transparent sample without scattering.
Scattering of electron waves, when it does happen, occurs not
because of absorption -- the amplitude of the electron beam is
largely undiminished -- but through the shifting of the electron
Scattered and unscattered waves are focused and recombine
downstream of the sample in a recording medium, typically a charged
coupled device (CCD).
Unfortunately, in weak-phase objects the phase shifting is slight,
resulting in poor contrast. What scientists at the University of
Karlsruhe and the Max-Planck Institute for Biophysics in Frankfurt
have done to remedy this situation is to interpose a special
free-suspended, micro-scaled electrostatic lens beyond the sample;
this electrostatic lens has the effect of shifting the phase of the
unscattered waves by a further 90 degrees but leaving the scattered
waves unshifted (see figures at
Physics News Graphics).
This dramatically improves the contrast in the resultant
images. This electrostatic lens is called a Boersch phase plate in
honor of Hans Boersch, who proposed the technique in 1947. It has
not been achieved until now because of its demanding size
Schultheiss et al., Review of Scientific Instruments, March 2006
Contact Fabian Perez-Willard, firstname.lastname@example.org
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