An electron’s orbit probability cloud in an exciton. Image: Okinawa Institute of Science and Technology
New research into excitons from the Okinawa Institute of Science and Technology could help pave the way for new technology or quantum states of matter.
Excitons are excited states of matter found within semiconductors, making them a vital part of modern technological devices such as LEDs, lasers, smartphones and solar cells.
Since the discovery of excitons around 90 years ago, scientists have been trying to learn more about the momentum and orbit of these quasiparticles. Now, researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) have successfully captured an image of an exciton’s internal orbits for the first time.
According to the researchers, it has been difficult to get to this point because excitons are fragile and fleeting. They break apart very easily into free electrons and holes, and in some materials are extinguished in just a few thousandths of a billionth of a second after they form.
When semiconductors absorb photons of light, negatively charged electrons jump from their lower energy level to a higher energy level, leaving positively charged empty spaces called holes behind them. The electrons and holes, which have opposite charges, attract one another and orbit each other, creating the exciton.
Electrons and holes orbiting each other in excitons. Image: OIST
“Up until very recently, one could generally access only the optical signatures of excitons; for example, the light emitted by an exciton when extinguished,” said Prof Keshav Dani, a senior author of the research paper and head of the Femtosecond Spectroscopy Unit at OIST.
“Other aspects of their nature, such as their momentum and how the electron and the hole orbit each other, could only be described theoretically.”
The group generated excitons by sending a laser pulse of light at a two-dimensional semiconductor. Then, they broke the excitons apart using a laser beam with ultra-high energy photons and kicked them out of the semiconductor material. They were sent into a vacuum space within an electron microscope.
Using the electron microscope, researchers could measure the angle and energy of the electrons as they exited the semiconductor and, ultimately, work out the initial momentum of the electrons when they were bound to the holes within the exciton. This allowed them to measure the exciton’s wave function, which showed them the probability of where the electrons were likely to be located around the holes.
“The technique has some similarities to the collider experiments of high-energy physics, where particles are smashed together with intense amounts of energy, breaking them open,” Dani explained. “By measuring the trajectories of the smaller internal particles produced in the collision, scientists can start to piece together the internal structure of the original intact particles.
“Here, we are doing something similar. We are using extreme ultraviolet light photons to break apart excitons and measuring the trajectories of the electrons to picture what’s inside.”
Dani added that the researchers had to carry out the measurements with “extreme care” at low temperature and low intensity to avoid heating up the excitons.
“This was no mean feat. It took a few days to acquire a single image,” he said.
Dr Julien Madéo, co-first author and staff scientist in the OIST Femtosecond Spectroscopy Unit, described the work as an “important advancement in the field”.
“Being able to visualise the internal orbits of particles as they form larger composite particles could allow us to understand, measure and ultimately control the composite particles in unprecedented ways. This could allow us to create new quantum states of matter and technology based on these concepts.”
