Physics and Applications of Electron Vortex Beams

This research proposal is about new investigations to be carried out at York concerned with the physics and applications of a very recent development, namely the controlled creation of electron vortex (EV) beams. EV beams are a brand new type of electron beams which differ from common electron beams in that they are endowed with a twisting (vortex) property vaguely akin to a tornado vortex. They bear resemblance to optical vortices (OVs), which have been much researched over the last two decades or so. OVs have found applications in optical tweezers and spanners and have other potential applications as, for example, in quantum information processing. Associated with the twisting property in both OVs and EVs is a physical property called orbital angular momentum (OAM). However, EVs differ significantly from OVs in that an electron carries electric charge and mass and possesses another intrinsic twisting property, called spin, which can be vaguely visualised as a rotation about its own axis. Furthermore, as electrons also possess wave properties, their wavelength is much smaller than that of visible light. It is this feature that makes them potentially superior in their ability as EVs to enable much better images in an electron microscope to be taken than currently possible. It is also this same property that makes an EV an excellent probe of tiny matter at the sub-nanoscale and EVs in general are expected to be excellent probes of matter at the individual molecular and atomic levels. The electron spin has been utilised in probing the properties of magnetic materials, but the orbital angular momentum content of EV beams presents new properties. The electron orbital motion relative to a nucleus has been vital in understanding the electronic motion within atoms and molecules, but, until recently, has not been considered to be a property normally associated with electron beams such as those existing inside cathode ray tubes and in electron microscopes. This proposal aims to take advantage of the recent technological advance of EVs to explore the extensive properties of such electron beams and to carry out investigations in both fundamental studies and practical applications. Specifically, we will develop ways to fabricate filters and convertors to generate various kinds of EV beams inside electron microscopes and to study their potential in fundamental research and ways of tailoring them for practical applications.
We plan to investigate a number of many, as yet, unexplored phenomena associated with the processes of the quantized transfer of orbital angular momentum between the orbital angular motions of the EV beam and that of the sample to explore the chiral specific properties of materials, such as magnetic and plasmonic transitions. We will explore the phenomena of electron vortices residing in the phase structure within the beam to develop new electron microscopic methods for revealing phase structures such as biological molecules. We will exploit the interesting 'diffraction-free' effect of the Bessel beams, i.e. pencil-like narrow beams, to develop 3D scanning microscopy tomography of nanostructures with better resolutions. We will also explore the complex structured intensities of the EV beams to develop efficient atom trapping and nanolithographic tools.

Electron microscopy, electron energy-loss spectroscopy and electron beam lithography are interdisciplinary research areas that have benefited society by imaging objects below the resolving power of optical microscopy, determining the chemical composition of nanomaterials down to atomic levels and engineering complex surface structures in microelectronics down to nanoscale. These developments have given us the structures of viruses, the discovery of nanotubes as well as world's smallest transistors. In our research, we seek to boost greatly the power of these electron beam based techniques by introducing vortex phase structures. The impact is envisaged also to be both diverse and wide ranging.
As a development of an enabling technology, the immediate impact will be in academic research areas such as condensed matter physics, materials science, stereochemistry, and biomedical science. Specifically, we expect the EV beams to markedly improve sub-nanometer resolution in the imaging of materials, including biological specimens of materials with low absorption contrast, because the sensitivity of the EV beams to local phase gradients. The angular momentum content also allows investigations of chiral processes more directly than has been possible to date. Novel results in electron energy-loss spectroscopy are expected to emerge, associated with the exchange of the orbital angular momentum (OAM) property of these beams. Fort example, the OAM can also be used to reveal the magnetism of different atoms in a compound or in a device. It can also be used to test for optical activity of chiral transitions. EV beams could have applications in characterizing chiral metamaterials in optics, distinguishing different enantiomers (left and right hand copies) of chiral molecules, drugs or biomolecules. The intrinsic magnetic and Coulomb fields the EV beams carry, applicable within atomic length scales, are significant and unique features, allowing possibilities of applications in nanomanipulation and nanofabrication. The tiny length scales also allow manipulation of individual atoms and sub-atomic constituents in a manner not possible before, involving mechanical translational and rotational forces and associated torques. The ability to prepare pure quantum states of EV beams and manipulate their interaction among themselves and their coupling to atoms can also open ways for experiments on quantum information processing, with impact on quantum cryptography and quantum computing.
The direct economical impact of our research will be introducing new functionality to electron microscopy and electron spectroscopy. This will impact on the industrial sectors such as the manufacture of electron microscopes. Indirect economical impact will result when EV beams allow biologists and chemists to develop new health diagnostic techniques, design new drugs and opticians to develop new devices based on chiral metamaterials. EV beams also allow materials scientists to develop complex nanostructures by more efficient electron beam lithography tools. Hard disk manufacturers can use EV beams to study nanoscale magnetism in their recording media and read/write heads in order to improve their magnetic recording devices.
The research will contribute directly to impact in society by training students and postdoctors in this new developing area and in enriching our understanding of the concepts of charged particle vortices.
Our strategy of developing impact is through publication in high impact journals, collaborative research to develop the applications of EV beams in materials science, in biological structure determination and in protecting intellectual property through patents and in promoting its transfer to the relevant industry or spin-off companies. We will develop the social impact by training physicists in this field and developing outreach programs on the EV beam resaerch and applications.