Scientists first take an image of an electron’s orbit inside an exit

In the first place, researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) captured an image showing the internal orbits or spatial distribution of particles in an exhibit – a goal that scientists have evaded for almost a century . Their findings are presented in Scientific progress.

Excitons are excited states of matter found within semiconductors – a class of materials that are the key to many modern technological devices, such as solar cells, LEDs, lasers, and smartphones.

“Excitons are really unique and interesting particles; they are electrically neutral, which means that they behave very differently in materials than other particles. Their presence can change the way a material reacts to light,” says Dr. Michael Man, first author and personnel scientist in the OIST Femtosecond Spectroscopy Unit. “This work brings us closer to understanding the nature of excitons.”

Excitons are formed when semiconductors absorb photons of light, causing negatively charged electrons to jump from a lower energy level to a higher energy level. This leaves positively charged empty spaces, called holes, in the lower energy level. The opposite charged electrons and holes pull and they begin to orbit each other, creating the excitement.

Excitations are of great importance within semiconductors, but so far scientists have been able to detect and measure them only in limited ways. One issue lies in their fragility – it takes relatively little energy to break the exciton into free electrons and holes. Furthermore, it is volatile in nature – in some materials, excitons are extinguished in a few thousandths of a billionth of a second after being generated, when the generated electrons “fall” into the holes again.

“Scientists first discovered excitons about 90 years ago,” said Professor Keshav Dani, senior author and head of the Femtosecond Spectroscopy Unit at OIST. “But until very recently, one only gained access to the optical signatures of excitons – for example, the light emitted by an exciton when it is extinguished. Other aspects of their nature, such as their momentum, and how the electron and the hole each orbits others, can only be described theoretically. “

In December 2020, however, scientists in the OIST Femtosecond Spectroscopy Unit published an article in Science describe a revolutionary technique for measuring the momentum of the electrons within the excitons.

Well, reported in Scientific progress, the team used the technique to record the first image ever showing the distribution of an electron around the hole in an exciton.

The researchers first generated excitons by sending a laser pulse of light to a two-dimensional semiconductor – a newly discovered class of materials that have only a few atoms in thickness and contain more robust excitons.

After the excitons were formed, the team used a laser beam with ultra-high energy photons to break the excitons apart and kick the electrons straight out of the material into the vacuum space inside an electron microscope.

The electron microscope measured the angle and energy of the electrons as they flew out of the material. From this information, the scientists were able to determine the initial momentum of the electron when it was bound to a hole in the exciton.

“The technique has some similarities with the collider experiments of high-energy physics, where particles are crushed with intense amounts of energy and break open. By measuring the orbits of the smaller internal particles produced during the collision, “Scientists can start making pieces together with the internal structure of the original intact particles,” said Professor Dani. state what’s in it. “

“It was no small feat,” continued Professor Dani. “The measurements had to be done with extreme care – at low temperature and low intensity to prevent the excitons from heating up. It took a few days to obtain a single image.”

Eventually, the team succeeds in measuring the exciton’s wave function, which gives the probability where the electron is likely to be around the hole.

“This work is an important advancement in the field,” said Dr. Julien Madeo, co-first author and staff scientist of the OIST Femtosecond Spectroscopy Unit, said. “If we can visualize the internal orbits of particles as they form larger composite particles, we can understand, measure and ultimately control the constituent particles in unprecedented ways. This can enable us to create new quantum states of matter and technology based on these concepts. ”