Coherently controlling the interaction between electrons and photons is the fundamental requirement for quantum technologies. The dipole approximation limit describes situations where the exchange of linear or angular momentum between electrons and photons is impossible.
This limit applies in many cases involving light-matter interaction. However, one scenario beyond this limit and remains experimentally challenging is when chiral electrons interact with vortex light. In this case, the orbital angular momentum of light can be transferred to electrons, but this phenomenon has been difficult to observe experimentally.
In a recent paper, scientists introduced a groundbreaking method to control quantum interactions between light and matter. They demonstrated a novel technique in which light imparts a “spinning kick” to electrons.
Their results demonstrated that a light beam can reliably transfer orbital angular momentum to itinerant electrons in graphene.
Deric Session, a postdoctoral researcher at JQI and the University of Maryland (UMD) and the lead author of the new paper, said, “The interaction of light that has orbital angular momentum with matter has been thought about since the 90s or so. But there have been very few experiments demonstrating the transfer.”
A key challenge in controlling light-matter interactions lies in the size mismatch between the light beams and the objects researchers want to manipulate, such as electrons. For electrons to feel the momentum transfer from a light beam, they must interact with the beam’s variations as it passes.
However, the light beam’s wavelength is much larger than the target matter. For example, atoms and their orbiting electrons are about 1,000 times smaller than the light wavelengths typically used in experiments.
Light propagates as waves of electric and magnetic fields, with the wavelength defining the size of these waves. The wavelength also determines the energy of individual photons. Since the wavelength of light is much larger than the atoms, photons can transfer energy and momentum to the atom as a whole.
Still, the internal components—such as the nucleus and electrons—cannot detect the changes in the beam. As a result, it becomes difficult to transfer orbital angular momentum specifically to the electrons within the atom.
One common approach to overcoming the challenge of the size mismatch between light and electrons is to shrink the light’s wavelength. However, this increases photon energy, making atoms unreliable targets. In a new experiment, researchers, including JQI Fellows Mohammad Hafezi and Nathan Schine, JQI Co-Director Jay Sau, and JQI Adjunct Fellow Glenn Solomon, explored an alternative strategy.
Instead of shrinking the wavelength, they expanded the electrons’ spatial extent. While electrons bound to atoms have limited movement, electrons in conductive materials like graphene can move more freely. The researchers turned to graphene, a highly conductive material, to make electrons occupy more space, enabling more effective manipulation in their experiment.
In their experiment, the researchers cooled a graphene sample to just 4 degrees above absolute zero and applied a strong magnetic field. This caused the normally free-moving electrons to become trapped in tight loops called cyclotron orbits. As the magnetic field strength increased, the orbits shrank until they were packed so tightly that no more electrons could fit.
While tight, these orbits were still much larger than the electron orbitals in atoms, making them ideal for interacting with light-carrying orbital angular momentum.
The graphene sample was set up with electrodes—one in the center and another forming a ring around the edges. Earlier theoretical work by former JQI graduate student Bin Cao and others in 2021 predicted that electrons in such orbits could gain angular momentum from incoming light, causing their orbits to expand and eventually get absorbed by the electrodes.
This provided the basis for the researchers’ experiment, enabling them to manipulate the electrons with light in a previously challenging way.
Session said, “You can change the size of the cyclotron orbits by adding or subtracting orbital angular momentum from the electrons, thus effectively moving them across the sample and creating a current.”
In their new paper, the research team observed a stable graphene current under various experimental conditions. When the sample was exposed to light carrying orbital angular momentum that circulated clockwise, a current flowed in one direction. The current direction flipped accordingly when the light’s angular momentum was reversed (counterclockwise).
They also observed that reversing the magnetic field caused the current to reverse, which was expected, as this also changed the direction of electrons’ cyclotron orbits. Additionally, varying the voltage across the electrodes still produced a clear difference between currents generated by clockwise and counterclockwise vortex light.
The researchers also tested the effect of circularly polarized light, which carries intrinsic angular momentum, but found it generated little to no current. This confirmed that the current only appeared in the presence of light with orbital angular momentum, and the current’s direction was linked to the light’s momentum. This result culminated years of work, overcoming sample fabrication and data collection challenges.
Ultimately, researchers reached out to a group they had worked with before. They were able to come through and make the samples that they worked well. But still, they had trouble aligning their twisted light with the sample to observe the current.
Mahmoud Jalali Mehrabad, a postdoctoral researcher at JQI and UMD and a paper co-author, said, “The signal we were looking at was not quite consistent. Then one day, with Deric, we started to do this spatial sweep. And we kind of mapped the sample with really high accuracy. Once we did that—once we nailed down the very peak, optimized position for the beam—everything started to make sense.”
“Within a week or so, we collected all the data they needed and could pick out all the signals of the current’s dependence on the orbital angular momentum of the beam.”
“In addition to demonstrating a new method for controlling matter with light, the technique might also enable fundamentally new measurements of electrons in quantum materials. Specially prepared light beams, combined with interference measurements, could be used as a microscope that can image the spatial extent of electrons—a direct measurement of the quantum nature of electrons in a material.”
“Being able to measure these spatial degrees of freedom of free electrons is an important part of measuring the coherence properties of electrons in a controllable manner—and manipulating them. Not only do you detect, but you also control. That’s like the holy grail of all this.”
Journal Reference:
- D. Session, M. Jalali Mehrabad, N. Paithanker, T. Grass, C. Eckhardt, B. Cao, D. Gustavo Suarez Forero, K. Li, M. S. Alam, G. S. Solomon, N. Schine, J. Sau, R. Sordan, and M. Hafezi. Optical pumping of electronic quantum Hall states with vortex light. Nature Photonics. DOI: 10.1038/s41566-024-01565-1