Friday, February 17, 2012

York researchers create ‘tornados’ inside electron microscopes

Researchers from the University of York are pioneering the development of electron microscopes which will allow scientists to examine a greater variety of materials in new revolutionary ways. 

The team, headed by Professor Jun Yuan and Professor Mohamed Babiker, from the University’s Department of Physics has created electron beams with orbital angular momentum – electron vortex beams – which will open the way to many novel applications including the more efficient examining of magnetic materials.

Electron microscopes use a beam of electrons to illuminate a specimen and produce a magnified image, allowing scientists to investigate atomic arrangements. Compared to conventional electron beams, electron vortex beams improve the resolution and sensitivity of imaging, which is key when determining the structure of biological specimens such as proteins. They also have applications in the manipulation of nano-scale objects such as atoms and molecules.

As the electron vortex consists of moving charged particles, there is a magnetic field associated with the vortex. This magnetic field will be invaluable in examining magnetic materials, enabling the nanoscale magnetic structure to be imaged.

The York team has created a design for a holographic mask to generate an electron vortex beam and now plans to use this to improve the imaging capabilities of the electron microscope in its York-JEOL nanocentre.

Arxiv - Quantised orbital angular momentum transfer and magnetic dichroism in the interaction of electron vortices with matter

Following the very recent experimental realisation of electron vortices, we consider their interaction with matter, in particular the transfer of orbital angular momentum in the context of electron energy loss spectroscopy, and the recently observed dichroism in thin lm magnetised iron samples. We show here that orbital angular momentum exchange does indeed occur between electron vortices and the internal electronic-type motion, as well as center of mass motion of atoms in the electric dipole approximation. This contrasts with the case of optical vortices where such transfer only occurs in transitions involving multipoles higher than the dipole. The physical basis of the observed dichroism is explained.

In conclusion, we have shown by direct analysis that it
is possible to transfer OAM between an electron vortex
beam and the internal electron states of an atom in the
dipole transition and we have checked by direct analysis that (orbital angular
momentum) OAM transfer occurs for quadupole transitions and in principle in the case of all higher multipoles. This is in direct contrast to the case of optical OAM transfer in the interaction with similar systems. It has been demonstrated both theoretically and experimentally that optical vortices are not speci c in their interaction with chiral matter. Here we have shown that although orbital angular momentum transfer can occur between electron vortices and matter in electric dipole transitions for a given topological charge, there is no intrinsic di fference in absorption for the two opposite helicities. We have concluded that the OAM dichroic electron energy loss spectroscopy of the type performed by Verbeeck et al. shows a dichroism due to the magnetic nature of the material, in which magnetic sublevels would be unequally populated.

Details of York’s latest work - part of the research by second year PhD student Sophia Lloyd - showing that orbital angular momentum of electron beams with vortex structure are more efficient than light for probing atomic magnetism, are published in the February edition of the Physical Review Letters.

Professor Yuan said: “The introduction of vortex beams into electron microscopy, with its screw-like revolving wave front – much like tornados, will revolutionise the study of magnetic nanostructures, as well as creating new applications in terms of nanoparticle manipulation and trapping, and edge contrast detection.”

Professor Babiker, an expert in light vortex research, added: “Optical vortex beams, created using beams of light photons, have been studied for the past 20 years. They have found a great many applications, most notably in fine scale manipulation of single molecules and nano-objects in so-called optical tweezers and optical spanners.

“Research being carried out at York is intended to further current understanding of electron vortices so that a similarly broad range of applications can be realised.”

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