Ultrahigh-resolution imaging may be achieved using modifications of
the off-axis holography scheme in a scanning transmission electron
microscopy (STEM) instrument equipped with one or more electrostatic
biprisms in the illuminating system. The resolution is governed by the
diameter of a reference beam, reduced by channeling through a line of
atoms in an atomic-focuser crystal. Alternatively, the off-axis
holography may be combined with the Rodenburg method in which a
four-dimensional data set is obtained by recording a nanodiffraction
pattern from each point of the specimen as the incident beams are
scanned. An ultrahigh-resolution image is derived by computer
processing to give a particular two-dimensional section of this data
set. The large amount of data recording and data processing involved
with this method may be avoided if the two-dimensional section is
derived by recording the hologram while the four beams produced by two
perpendicular biprisms are scanned in opposing directions across the
specimen by varying the voltages on the biprisms. An equivalent scheme
for conventional TEM is also possible. In each case, the complex
transmission function of the specimen may be derived and resolutions of
about 0.05 nm may be expected.