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

In a world premiere, researchers at the Okinawa Institute of Graduate University of Science and Technology (OIST) captured an image showing the internal orbits, or spatial distribution, of particles in an exciton – a target that had eluded scientists for nearly a year. century. Their findings are published in Scientific advances.

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

“Excitons are truly unique and interesting particles; they are electrically neutral, which means they behave very differently inside materials than other particles, such as electrons. Their presence can really change the way a material responds to light, “said Dr. Michael Man, co-author and first 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 light photons, which causes negatively charged electrons to jump from a lower energy level to a higher energy level. This leaves behind positively charged empty spaces, called holes, in the lower energy level. Opposite charged electrons and holes attract and begin to orbit each other, which creates excitations.

Excitons are of crucial importance in semiconductors, but so far scientists have been able to detect and measure them in limited ways. One problem is their fragility – it takes relatively little energy to break the exciton into electrons and free holes. Moreover, they are transient in nature – in some materials, the excitons go out in about a few thousandths of a second of a second after they form, when the excited electrons “fall” back into the holes.

“Scientists first discovered excitons about 90 years ago,” said Professor Keshav Dani, lead author and head of the Femistosecond Spectroscopy Unit at OIST. “But until recently, only the optical signatures of excitons could generally be accessed – 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 orbit orbit each other, could only be described theoretically. “

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

Now report in Scientific advances, the team used the technique to capture the first image showing the distribution of an electron around the hole inside an exciton.

Researchers first generated excitons by sending a laser pulse of light to a two-dimensional semiconductor – a newly discovered class of materials that are only a few thick atoms and harbor more robust excitons.

After excitations formed, the team used an ultra-high-energy photon laser beam to break excitations and throw electrons right out of the material into the vacuum space of an electron microscope.

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

“The technique has some similarities with high-energy physics collision experiments, in which particles are broken together with intense amounts of energy, breaking them open. By measuring the trajectory of smaller internal particles produced in the collision, scientists can begin to piece together the internal structure of the original intact particles, “said Professor Dani.” Here, we do something similar – we use photons with extreme ultraviolet light to break excitations and measure the trajectory of the electrons to imagine what is inside ”.

“It was not a vicious deed,” Professor Dani continued. “The measurements had to be done with extreme care – at low temperature and low intensity to avoid heating the excitons. It took a few days to get a single image.”

Finally, the team was able to measure the wave function of the exciton, which provides the probability where the electron is likely to be located around the orifice.

“This work is an important step forward in the field,” said Dr. Julien Madeo, co-lead author and scientist at the OIST Femtosecond Spectroscopy Unit. “The ability to visualize the internal orbits of particles as they form larger composite particles would allow us to finally understand, measure, and control composite particles in unprecedented ways. This would allow us to create new quantum states of matter and technology based on these concepts. . ”