Scientists capture the very first image of an electron’s orbit within an exciton

In a world first, researchers at the Okinawa Institute of Science and Technology Graduate University (OIST) have mapped the internal orbits, or spatial distribution, of particles in an exciton – a goal that had eluded scientists for nearly a century. Their findings are published in Science Advances.

Excitons are excited states of matter found in semiconductors – a class of materials that are 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 within materials than other particles such as electrons. Their presence can really change the way a material reacts to light,” said Dr. Michael Man, co-first author and staff scientist in the OIST Femtosecond Spectroscopy unit. “This work brings us closer to fully 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 voids called gaps in the lower energy level. The oppositely charged electrons and holes attract and rotate around each other, creating the excitons.

Excitons are critical within semiconductors, but so far scientists have only been able to detect and measure them in limited ways. One problem lies in their vulnerability – it takes relatively little energy to break up the exciton into free electrons and holes. Moreover, they are fleeting in nature – in some materials excitons are extinguished in about a few thousandths of a billionth of a second after they are formed, when the excited electrons ‘fall’ back into the holes.

“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 recently, people generally only had 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 hole every other, can only be described theoretically. “

However, in December 2020, scientists from the OIST Femtosecond Spectroscopy Unit published a paper in Science describes a revolutionary technique for measuring the momentum of the electrons within the excitons.

Report now in Science Advancesthe team used the technique to create the very first image 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 recently discovered class of materials that are only a few atoms thick and harbor more robust excitons.

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

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

“The technique has similarities to the collider experiments of high-energy physics, in which particles with intense amounts of energy are compressed and broken open. By measuring the orbits of the smaller internal particles produced in the collision, scientists can begin to work together the internal structure. of the original intact particles, ”said Professor Dani. “Here we do something similar: we use photons from extreme ultraviolet light to separate excitons and measure the trajectories of the electrons to get a picture of what’s inside.”

“This was no mean feat,” Professor Dani continued. “The measurements had to be performed with extreme care – at low temperature and low intensity to avoid heating of the excitons. It took a few days to get a single image.”

Ultimately, the team managed to measure the exciton’s wave function, which gives the probability of where the electron is likely to be around the hole.

“This work is a significant advance in the field,” said Dr. Julien Madeo, co-lead author and staff scientist at the OIST Femtosecond Spectroscopy Unit. “By visualizing the internal orbits of particles as they form larger composite particles, we can 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. . ”