EPFL researchers create electron vortices
In the famous Jules Verne classic Twenty Thousand Leagues Under the Sea, the iconic submarine Nautilus disappears into the Maelstrom, a powerful whirlpool off the Norwegian coast. In space, stars spiral around black holes; on Earth, the raging phenomena that rage take different forms: cyclones, tornadoes and dust swirls.
All these manifestations have in common a particular morphology: the vortex. Milky galaxies that are stirred in their coffee, vortices are present everywhere in nature, even in the subatomic world, when a stream of elementary particles or energy spirals around a fixed axis, such as the tip of a corkscrew.
When particles move in this way, they form what are called "vortex beams". These are very interesting because they imply that the particle has a well-defined orbital angular momentum that describes its rotation around a fixed point.
This means that vortex beams can offer new ways of interacting with matter, for example by increasing the sensitivity of sensors to magnetic fields or by generating new absorption channels that facilitate interactions between radiation and tissue during treatments. medical (such as radiotherapy). In addition, vortex beams make possible new pathways in the basic interactions between elementary particles, giving hope for new insights into the internal structure of particles such as neutrons, protons and ions.
The matter is astonishing as it is both particulate and undulatory. This means that it is possible to transform large particles into vortex beams by simply modulating their wave function. This operation can be carried out with a device called "passive phase mask", which can be thought of as an obstacle rising in the sea. When the waves collide against this obstacle, their wave characteristics are modified and they form vortices. Until now, physicists have used the passive phase mask method to create vortex beams of electrons and neutrons.
However, scientists at EPFL's Fabrizio Carbone laboratory questioned this idea. By demonstrating for the first time that it was possible to use light to dynamically deform the wave function of an electron, the researchers managed to generate an extremely short electron vortex beam and modify its vorticity at the attosecond scale (10-18 seconds).
To do this, they have exploited one of the fundamental rules that govern the interaction between particles at the nanoscale: the conservation of energy and momentum. This means that the sum of the energies, masses and velocities of two particles is the same before and after their collision. This law implies that an electron obtains an orbital angular momentum during its interaction with a bright field prepared for this purpose, that is to say a chiral plasmon.
Experimentally speaking, the scientists sent circular polarized ultrafast laser pulses through a nanotrode pierced in a metal film. These pulses generated a strong localized magnetic field (the chiral plasmon), with which individual electrons were interacted.
Scientists have used an electron microscope in ultrafast transmission to observe the resulting electron phase profiles. They discovered that, during the interaction of electrons with the magnetic field, the wave function of electrons was subject to "chiral modulation," a movement on the right or left that can be actively controlled by adjusting the polarization of the laser pulses.
