Method could expand the reach of electron microscopes

Passing electrons through a nanometre-scale grating, the scientists imparted the resulting electron waves with so much orbital momentum that they maintained a corkscrew shape in free space.

The development opens the possibility of adapting transmission electron microscopy, which can see tinier details than optical microscopy and can study a wider range of materials than scanning probe microscopy, for quick and inexpensive imaging of a larger set of magnetic and biological materials with atomic-scale resolution.

’The spiral shape and angular momentum of these electrons will let us look at a greater variety of materials in ways that were previously inaccessible to TEM users,’ said NIST’s Ben McMorran. ’Outfitting a TEM with a nanograting [such as the one] we used in our experiment could be a low-cost way to dramatically expand the microscope’s capabilities.’

Although NIST researchers were not the first to manipulate a beam of electrons in this way, their device was much smaller, separated the fanned-out beams 10 times more widely than previous experiments and spun up the electrons with 100 times the orbital momentum. This increase in orbital momentum enabled them to determine that the electron corkscrew, while remarkably stable, gradually spreads out over time.

A beam of corkscrew-shaped electrons, when interacting with a specimen, can exert torque on the material by exchanging angular momentum with its atoms. In this way, the corkscrew electrons could obtain more information in the process than beams with ordinary electrons, which do not carry this orbital angular momentum.

The technique could also help improve TEM images of transparent objects such as biological specimens. Biological material can be difficult to image in ordinary TEMs because electrons pass through it without deflecting. But by using corkscrew electron beams, researchers hope to provide high-contrast, high-resolution images of biological samples by looking at how the spiral wavefronts get distorted as they pass through such transparent objects.

While these imaging applications have not yet been demonstrated, producing corkscrew electrons with nanogratings in a TEM provides a significant step toward expanding the capabilities of existing microscopes.