Schematic representation of the experimental setup used to shape electrons into chiral coils with mass and charge. Credit: Dr. Yiqi Fang, University of Konstanz, edited
Physicists at the University of Konstanz have discovered a way to imprint a never-before-seen geometric form of chirality in electrons using laser light, creating chiral coils of mass and charge.
This breakthrough in the manipulation of electronic chirality has major implications for quantum optics, particle physics and electron microscopy, paving the way for 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 in such a way that all the fingers point in the same direction? If you have, then you probably know that your right thumb will not touch its left counterpart. Neither rotations nor translations nor their combinations can turn the left hand into the right and vice versa. This property is called chirality.
Scientists at the University of Konstanz have now succeeded in embedding such three-dimensional chirality in the wave function of a single electron. They used laser light to shape the electron matter wave into coils of mass and charge to the left or right. Such elementary particles designed with chiral geometries different from their intrinsic spin have implications for fundamental physics, but may also be useful for a variety of applications, such as quantum optics, particle physics or electron microscopy.
“We are opening up new potentials 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 i Single Particles and compounds
Chiral objects play a crucial role in nature and technology. In the field of elementary particles, one of the most important chiral phenomena is spin, which is often compared to a self-rotation of a particle, but in fact it is a purely quantum-mechanical property without classical analogues. An electron, for example, has a spin of 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. The spin of the electron is also critical to the development of advanced technologies such as quantum computers or superconductors.
However, there are also chiral compound objects in which none of the components are themselves chiral. Our hand, for example, is composed of atoms with no particular chirality, but it is nevertheless a chiral object, as we learned earlier. The same is true for many molecules in which chirality occurs without the need for any chiral components. Whether a molecule is in left- or right-handed geometry can make the difference between a healing drug and a harmful substance—the two versions can have very different biological effects because of their different three-dimensional geometry.
In materials science and nanophotonics, chirality affects the behavior of magnetic materials and metamaterials, leading to phenomena such as topological insulators or chiral dichroism. The ability to control and manipulate the chirality of composite materials composed of achiral components thus provides a rich key to tune the properties of the materials as required for applications.
Advances in Electron Manipulation Techniques
Is it possible to form a single electron into a three-dimensional chiral object in terms of charge and mass? In other words: Can chirality be induced in an electron without the need for spin? Until now, researchers have only moved electrons along spiral trajectories or created vortex electron beams in which the de Broglie wave phase rotates around the center of the beam with constant charge and mass. In contrast, the chiral matter wave object that the Konstanz physicists report in their Science 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 precisely modulated laser waves with helical 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 shift the phase of laser light, facilitated the interaction in the experiment.
The helical electric fields in the laser wave either 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 caught up with each other and the wave function became a chiral spiral of mass and charge. “We then used attosecond electron microscopy to get a detailed tomographic measurement of the electron’s expectation value, i.e. the probability of being somewhere in space and time,” says Baum, explaining how they measured the shapes created. . In the experiment, single or double coils were displayed with the right or left hand. Neither rotation, nor angular momentum, nor helical trajectories were necessary to produce this purely geometrical chirality.
To investigate whether an interaction of three-dimensional electron coils with other chiral materials would 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 released a left-handed electron into a right-handed nanophotonic object or vice versa, the results showed constructive or destructive spin interference phenomena. In a sense, overall chirality never disappeared.
A whole new world of possibilities
The ability to shape electrons into chiral coils of mass and charge opens new avenues for scientific exploration and technological innovation. For example, engineered chiral electron beams should be useful for chiral electron-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 so far we have only modulated the electron, one of the simplest elementary particles, the method is general and applicable to almost any particle or wave of matter. 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 further elucidate the complex interaction between chiral light waves and chiral matter for applications in future technologies.
Reference: “Structured Electrons with Chiral Mass and Charge” by Yiqi Fang, Joel Kuttruff, David Nabben and Peter Baum, 11 July 2024, science.
DOI: 10.1126/science.adp9143
Prof. 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. Foundation. KH Eberle