Schematic representation of the experimental setup used to shape electrons into chiral coils of mass and charge. Credit: Dr. Yiqi Fang, University of Konstanz, edited
Physicists at the University of Konstanz have discovered a way to imprint a previously unseen geometric form of chirality onto electrons using laser light, creating chiral coils of mass and charge.
This advance in manipulating electronic chirality has broad implications for quantum optics, particle physics, and electron microscopy, opening the way to new scientific explorations and technological innovations.
Understanding Chirality and Its Implications
Have you ever placed the palm of your right hand on the back of your left hand, so that all fingers point in the same direction? If so, you probably know that your right thumb will not touch its left counterpart. Neither rotations nor translations nor their combinations can turn a left hand into a right hand and vice versa. This characteristic is called chirality.
Scientists at the University of Konstanz have succeeded in imprinting such three-dimensional chirality onto the wave function of a single electron. They used laser light to shape the electron’s matter wave into left- or right-handed coils of mass and charge. Such artificial elementary particles with chiral geometries other than their intrinsic spin have implications for fundamental physics but can also be useful for a whole range of applications, such as quantum optics, particle physics or electron microscopy.
“We are opening up new perspectives for scientific research that have not been considered before,” says Peter Baum, corresponding author of the study and head of the Light and Matter research group at the University of Konstanz.
Chirality of Bachelor Particles and composites
Chiral objects play a crucial role in nature and technology. In the realm of elementary particles, one of the most important chiral phenomena is spin, often compared to the autorotation of a particle, but which is in fact a purely quantum property with no classical equivalent. An electron, for example, has a spin equal to half and therefore often exists in two potential states: a right-handed and a left-handed one. This fundamental aspect of quantum mechanics gives rise to many important real-world phenomena, such as almost all magnetic phenomena or the periodic table of elements. Electron spin is also essential for the development of advanced technologies such as quantum computers or superconductors.
There are, however, composite chiral objects in which none of the constituents are chiral by themselves. Our hand, for example, is composed of atoms without any particular chirality, but it is nevertheless a chiral object, as we have seen previously. The same is true for many molecules in which chirality arises without any chiral constituent being necessary. Whether a molecule is in the left-handed or right-handed geometry can mean the difference between a curative drug and a harmful substance – the two versions can have very different biological effects due to their different three-dimensional geometries.
In materials science and nanophotonics, chirality influences the behavior of magnetic materials and
” data-gt-translate-attributes=”({“attribute”:”data-cmtooltip”, “format”:”html”})” tabindex=”0″ role=”link”>metamaterialsleading to phenomena such as topological insulators or chiral dichroism. The ability to control and manipulate the chirality of composite materials composed of achiral constituents thus offers a rich tool for adjusting material properties according to application needs.
Advances in electron manipulation techniques
Is it possible to transform a single electron into a three-dimensional object that is chiral in terms of charge and mass? In other words: can chirality be induced in an electron without the need for spin? So far, researchers have only moved electrons along spiral paths or created electron vortex beams in which the phase of the de Broglie wave rotates around the center of the beam at constant charge and mass. In contrast, the chiral matter wave object that the Konstanz physicists describe in their scientific paper has a flat de Broglie wave but the expected values of charge and mass are shaped into a chiral form.
To create this object, they used an ultrafast transmission electron microscope and combined it with laser technology. The researchers first generated femtosecond electron pulses and then shaped them into chiral patterns by interacting with laser waves precisely modulated by spiral electric fields. Normally, electrons and laser photons do not interact in such an experiment because energy and momentum cannot be conserved. However, silicon nitride membranes, which are transparent to electrons but change the phase of the laser light, facilitated the interaction in the experiment.
The spiraling electric fields of the laser wave accelerate or decelerate the incoming electron around the center of the beam, depending on the azimuthal position. Later in the beam, the accelerated or decelerated electrons eventually catch up, and the wave function transforms into a chiral coil of mass and charge. “We then used attosecond electron microscopy to obtain a detailed tomographic measurement of the electron’s expectation value, that is, the probability of being somewhere in space and time,” Baum says, explaining how they measured the generated shapes. Single or double coils, right or left, appeared in the experiment. Neither spin, nor angular momentum, nor spiral trajectories were necessary to produce this purely geometric chirality.
To determine whether the interaction of three-dimensional electron coils with other chiral materials could preserve chirality, the researchers placed gold nanoparticles with chiral electromagnetic fields in their electron microscope and used the chiral electron coils to measure the scattering dynamics. Depending on whether the researchers fired a left-handed electron at a right-handed nanophotonic object or vice versa, the results showed either constructive or destructive rotational interference phenomena. In a sense, the overall chirality never disappeared.
A whole new world of possibilities
The ability to transform electrons into chiral coils of mass and charge opens new avenues for scientific exploration and technological innovation. For example, the engineered chiral electron beams are expected to be useful for chiral electronic optical tweezers, chiral sensor technologies, quantum electron microscopy, or for probing and creating rotational motion in atomic or nanostructured materials. In addition, they will contribute to general particle physics and quantum optics.
“Although we have so far only modulated the electron, one of the simplest elementary particles, the method is general and applicable to almost all matter particles or waves. What other elementary particles have or can have such chiral forms, and are there possible cosmological consequences?” says Baum. The researchers’ next step is to use their chiral electrons in attosecond electron imaging and two-electron microscopy to better elucidate the complex interplay between chiral light and chiral matter waves for applications in future technologies.
Reference: “Structured Electrons with Chiral Mass and Charge” by Yiqi Fang, Joel Kuttruff, David Nabben, and Peter Baum, July 11, 2024, Science.
DOI: 10.1126/science.adp9143
Professor Peter Baum heads the Light and Matter research group at the Department of Physics at the University of Konstanz. His team was recently awarded the Helmholtz Prize for Basic Research for the development of an innovative attosecond microscopy technique.
Funding: German Research Foundation (DFG; SFB 1432) and Dr. KH Eberle Foundation