Schematic of the experimental setup used to shape electrons into chiral coils of mass and charge. Credit: Dr. Yiqi Fang, University of Konstanz; Edited by
Physicists at the University of Konstanz have discovered a way to use laser light to imprint the chirality of electrons in a never-before-seen geometric shape, creating a chiral coil of mass and charge.
This breakthrough in manipulating electron chirality has profound implications for quantum optics, particle physics and electron microscopy, paving the way for new scientific exploration and technological innovation.
Understanding Chirality and Its Meaning
Have you ever placed the palm of your right hand on the back of your left hand, with all your fingers pointing in the same direction? If you have, you know that the thumb on your right hand cannot touch the thumb on your left hand. You cannot turn your left hand into a right hand or vice versa, no matter how you rotate, translate, or combine them. This feature is called chirality.
Scientists at the University of Konstanz have now 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 a left- or right-handed coil of mass and charge. Such engineered elementary particles with inherent non-spin chirality shapes could have implications for fundamental physics as well as be useful in a variety of applications, including quantum optics, particle physics and electron microscopy.
“We are opening up new possibilities for scientific research that have not been considered before,” says Peter Baum, corresponding author of the study and head of the Light and Materials Research Group at the University of Konstanz.
Chirality single Particles and Composites
Chiral objects play an important role in nature and technology. In the subatomic realm, one of the most important chiral phenomena is spin. Although spin is often likened to the self-rotation of a particle, it is in fact a purely quantum mechanical property with no classical analogue. For example, electrons have half a spin and therefore often exist in two potential states: right-handed and left-handed. This fundamental aspect of quantum mechanics gives rise to many important phenomena in the real world, such as nearly all magnetic phenomena and the periodic table of elements. Electron spin is also essential for the development of advanced technologies such as quantum computers and superconductors.
But there are also compound chiral objects in which none of the components are chiral by themselves. For example, our hands are composed of atoms that have no particular chirality, but are still chiral objects, as we learned before. The same is true for many molecules, where chirality emerges without the need for chiral components. Whether a molecule is left- or right-handed can make the difference between a therapeutic and a harmful substance. Both versions can have very different biological effects because of their different three-dimensional shapes.
In materials science and nanophotonics, chirality influences the behavior of magnetic materials, MetamaterialsThis gives rise to phenomena such as topological insulators and chiral dichroism. The ability to control and manipulate the chirality of composite materials composed of achiral components provides a wealth of avenues to tailor the properties of materials for different applications.
Advances in electronic operation technology
Is it possible to shape a single electron into a chiral three-dimensional object in terms of charge and mass? In other words, is it possible to induce chirality in an electron without the need for spin? So far, researchers have only managed to move the electron along a spiral trajectory or create electron vortex beams in which the phase of the de Broglie wave rotates around the center of the beam while the charge and mass remain constant. In contrast, the chiral matter wave object that the Konstanz physicists report in their Science paper has flat de Broglie waves, but the expectation values of the charge and mass are shaped into a chiral shape.
To create the object, the researchers used an ultrafast transmission electron microscope and combined it with laser technology. They first generated femtosecond electron pulses, which they then interacted with precisely modulated laser waves with a spiraling electric field to shape them into chiral patterns. Normally, in such experiments, electrons and laser photons do not interact because energy and momentum cannot be conserved. However, a silicon nitride film, which is transparent to electrons but changes the phase of the laser light, facilitated the interaction in their experiment.
The helical electric field of the laser wave accelerates or decelerates the incoming electrons around the center of the beam, depending on their azimuthal position. In the second half of the beam, the accelerated or decelerated electrons eventually catch up with each other and their wave function is transformed into a chiral coil of mass and charge. “We then used attosecond electron microscopy to obtain detailed tomographic measurements of the electron’s expectation value, that is, its probability of being somewhere in space and time,” says Baum, explaining how the generated shapes were measured. In the experiment, right- or left-handed single or double coils were generated. Neither spin nor angular momentum nor helical trajectories were needed to generate this purely geometrical chirality.
To see whether the interaction of the three-dimensional electron coil with other chiral materials would preserve chirality, the researchers placed gold nanoparticles with chiral electromagnetic fields in an electron microscope and measured the scattering dynamics using the chiral electron coil. Depending on whether the researchers fired left-handed electrons into right-handed nanophotonic objects or vice versa, the results showed constructive or destructive rotational interference phenomena. In a sense, the overall chirality never disappeared.
A whole new world of possibilities
The ability to shape electrons into chiral coils of mass and charge opens up new avenues of scientific exploration and technological innovation. For example, engineered chiral electron beams should be useful for chiral electron optical tweezers, chiral sensor technologies, quantum electron microscopes, or probing and creating rotational motions of atoms or nanostructured materials. In addition, they will contribute to particle physics and quantum optics in general.
“Though so far we have only modulated one of the simplest elementary particles, the electron, this method is versatile and can be applied to almost any particle or matter wave. What other elementary particles have or could have such chiral shapes, and could they have cosmological implications?” Baum says. The researchers’ next step is to use the chiral electrons in attosecond electron imaging and two-electron microscopy to further unravel the complex interactions between chiral light and chiral matter waves, with applications in future technologies.
Reference: Yiqi Fang, Joel Kuttruff, David Nabben, Peter Baum, “Structured Electrons with Chiral Mass and Charge,” July 11, 2024, Science.
DOI: 10.1126/science.adp9143
Professor Peter Baum heads the Light and Materials research group at the Department of Physics of the University of Konstanz, whose 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 the Dr. K. H. Eberle Foundation.
