High-resolution STEM imaging with a quadrant detector--conditions for differential phase contrast microscopy in the weak phase object approximation.

Differential phase contrast is a contrast mechanism that can be utilized in the scanning transmission electron microscope (STEM) to determine the distribution of magnetic or electric fields. In practice, several different detector geometries can be used to obtain differential phase contrast. As recent high resolution differential phase contrast experiments with the STEM are focused on ring quadrant detectors, we evaluate the contrast transfer characteristics of different quadrant detector geometries, namely two ring quadrant detectors with different inner detector angles and a conventional quadrant detector, by calculating the corresponding phase gradient transfer functions. For an ideal microscope and a weak phase object, this can be done analytically. The calculated phase gradient transfer functions indicate that the barely illuminated ring quadrant detector setup used for imaging magnetic fields in the specimen reduces the resolution limit to about 2.5Å for an aberration corrected STEM. Our results show that the resolution can be drastically improved by using a conventional quadrant detector instead.