Exciting exciton breakthrough

Scientists have just experimentally measured the wave function of an exciton, and they’re excited about it – because they’ve been waiting a century.

An exciton is an excited state of matter, created when an electron gains energy and jumps to a higher energy level. The negatively charged electron leaves behind a positively charged empty space (a “hole”), and the two are attracted to each other and begin to orbit each other – together forming an exciton. These electrically neutral “quasiparticles” are crucially important to semiconductors, which are key to applications such as solar cells, lasers and LEDs.

Until now excitons have been difficult to detect and measure, as they are fragile and fleeting, sometimes lasting just a few thousandths of a billionth of a second.

But now, for the first time, researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) in Japan have imaged the internal orbits of particles in an exciton.

“Scientists first discovered excitons around 90 years ago,” says Keshav Dani, senior author of the study, published in Science Advances.

“But up until very recently, one could generally access only the optical signatures of excitons – for example, the light emitted by an exciton when extinguished. Other aspects of their nature, such as their momentum, and how the electron and the hole orbit each other, could only be described theoretically.”

Then last year, a technique was discovered enabling the measurement of the momentum of electrons within the excitons.

The OIST team used this technique in their new study. After generating excitons by firing a laser pulse at a 2D superconductor material, the researchers used high-energy UV photons to break the particles apart again and force the electrons to fly away – into the vacuum space within an electron microscope.

The microscope measured the angle and energy of the electrons, allowing the team to reconstruct the initial momentum and therefore where the electrons are in relation to the positively charge hole within the exciton.

“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 says.

“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.”

The experiment had to be conducted at low temperature and low intensity, and it took several days to acquire just one image, capturing the wave function of the exciton. This gives the probability of an electron’s location around the hole.

According to OIST co-author Julien Madeo, this is an important advance.

“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,” he says. “This could allow us to create new quantum states of matter and technology based on these concepts.”

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