Scientists have just experimentally measured the wave function of excitation, and they are happy about it – because they have been waiting for a century.
An excitation is an excited state of matter, created when an electron gains energy and jumps to a higher energy level. The negatively charged electron leaves a positively charged empty space (“hole”), and the two are attracted to each other and begin to orbit each other – together forming an excitation. These electrically neutral “quasi-particles” are crucial for semiconductors, which are key to applications such as solar cells, lasers and LEDs.
So far excitons have been difficult to detect and measure because they are delicate and transient, sometimes lasting only a few trillionth of a second.
But now, for the first time, researchers at the Okinawa Institute for Graduate Science and Technology University (OIST) in Japan have visualized the inner orbits of particles in excitement.
“Scientists first discovered excitons about 90 years ago,” says Keshav Dani, lead author of the study, published in Scientific Advances.
“But until very recently one could generally access only the optical signatures of excitons – for example, the light emitted by excitation when extinguished. Other aspects of their nature, such as their momentum, and how the electron and the hole orbit each other, could only be theoretically described. “
Then last year a technique was discovered to measure the momentum of electrons inside the excitons.
The OIST team used this technique in their new study. After generating excitons by firing a laser at a 2D superconducting material, the researchers used high-energy UV photons to separate the particles again and force the electrons to fly away – into the vacuum space inside an electron microscope.
The microscope measured the angle and energy of the electrons, allowing the equipment to reconstruct the initial momentum and therefore where the electrons are relative to the positively charged hole within the exciton.
“The technique has some similarities to the high-energy physics collision experiments, where particles are shattered along with intense amounts of energy, breaking them down,” Dani says.
“By measuring the trajectories of the smaller internal particles produced in the collision, scientists can begin to put together the internal structure of the original intact particles. Here we do something similar – we use extreme ultraviolet light photons to separate excitons and measure the trajectories of the electrons to visualize what is inside. “
The experiment had to be done at low temperature and low intensity, and it took a few days to get just one image, capturing the wave function of the excitation. This gives the probability of an electron location around the hole.
According to OIST co-author Julien Madeo, this is a major breakthrough.
“Being able to visualize 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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