Atomic-resolution Imaging Using Cs-corrected Vortex Beams

Phase gratings have been shown to produce electron beams with orbital angular momentum as demonstrated by numerous groups, and show promise for electron magnetic circular dichroism (EMCD) at the atomic scale [1, 2]. A linear diffraction grating will produce diffracted beams with a separation determined by the pitch or spacing between grating lines. A grating with a central line that forks into j + 1 central lines will produce a set of diffracted beams each containing discrete units of orbital angular momentum m = j × n in the n th diffraction order [2]. The discontinuity in the center of the grating imparts a “vortex”-type phase on the diffracted beams. We have built such a forked grating with one discontinuity, a radius of 30 m, and a grating pitch of 80 nm using focused ion beam patterning on a 50 nm thick SiN window. Figure 1 shows a low-magnification SEM image of the grating, and the inset shows the discontinuity (fork) in the center at higher magnification. The SiN thin film was patterned as a phase grating rather than an amplitude grating, and thus the 30 nm trench depth does not extend through the full thickness of the SiN thin film. Amplitude gratings have a theoretical maximum diffraction efficiency of 10.1% into the first order, but this grating achieves ~20% efficiency due to the phase grating design. High diffraction efficiency is a critical consideration for the application of diffractive optics in STEM imaging and spectroscopy.