An optical method for polarizing free electrons in a laboratory setting

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Credit: Pan and Xu

Polarized electrons are electrons in which the spins have a “preferred” orientation or are preferentially oriented in a specific direction. The realization of these electrons has significant implications for physics research, as it may pave the way for the creation of promising materials and enable new experiments.

Researchers from East China Normal University and Henan Academy of Sciences recently introduced a new method for polarizing free electrons in a laboratory setting using near-field optical techniques, which involves applying beams of light from a positioned optical device close to a sample. Their article, published in Physical Review Lettersit could open up exciting new possibilities for high-energy physics, the development of quantum technology and materials science.

“The initial idea for this study took hold two years ago while I was a postdoctoral researcher in the group of Prof. Francisco Javier Garcia de Abajo, renowned for their theoretical work on optical excitations in electron beams,” Deng Pan, one of the researchers who conducted the study, told “Since then, the field of photon-induced near-field electron microscopy (PINEM) has gained momentum, emerging as a leading topic in electron microscopy.”

PINEM is a promising microscopy technique that could allow researchers to manipulate the quantum properties of electrons, which could potentially unravel new quantum computing mechanisms that rely on free electrons. Previous work has mainly sought to use this technique to manipulate the orbitals and momentum of electrons. In their study, Pan and his colleague Hongxing Xu set out to explore its potential use to polarize free electrons.

“I began to wonder if a similar approach could be employed to alter the spin state of electrons or even polarize electron beams,” Pan said. “Engaging in discussions with several electron beam theory experts, most of them believed that such an effect was undetectable, given that the magnetic field within the electromagnetic field is significantly weaker than the electric field component. Consequently, it was both surprising and unexpected that my calculations finally demonstrated the substantial effect of electron polarization within the near optical range.”

The optical method introduced by Pan and Xu is inspired by PINEM, as it is based on a similar approach. The researchers used an array of nanowires (i.e. a periodic nanostructure) with a carefully designed lattice constant. Their design ensured a near-field match between the speed of the incoming electron and the structure, ensuring a strong interaction between them.

“There is a fundamental difference between my proposal and the PINEM scheme,” Pan explained. “The PINEM effect is induced by the electric field component parallel to the electron beam. In contrast, we used a transverse electric near field (TE), which only has the electric field perpendicular to the electron beam. Why we opted for this TE near-field? The answer is closely related to another fascinating topic of nanophotonics known as transverse spin angular momentum or spin-orbit interaction in optical near-fields.”

The near-field TE applied by the researchers has a circularly polarized magnetic field. Consequently, it can be used to manipulate the spins of electrons, just as other fields can be used to control quantum spins.

“A spin electron beam proves to be an invaluable tool in the study of magnetic materials and high-energy physics, among other areas,” Pan said. ‘Our research also sheds light on the realization that, although weaker than the electric field component, the magnetic field component in the near optical field can be exploited to produce unexpected results.’

Pan and Xu are among the first to introduce a reliable optical method for preparing spin electrons in a laboratory setting. In the future, their approach could be adapted and used by other research groups to create spin-polarized electron beams, potentially also inspiring the development of new quantum computing approaches that exploit both electron and photon spins.

“With the direction provided by this work, there is still a large theoretical space to explore, such as the potential for quantum information processing combining electron and photon spins,” Pan added. “However, I believe that the most crucial aspect is the experimental demonstration of the proposal outlined in this study. I have already discussed this work with several experimenters, and they have expressed optimism regarding the feasibility of achieving our results in experiments.”

More information:
Deng Pan et al, Polarizing free electrons in near optical fields, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.186901

About the magazine:
Physical Review Letters

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