Scientists first capture an image of an orbit of an electron within an excitation

Exitons are technically not particles, but quasi-particles (quasi-meaning “almost” in Latin). They are formed by the electrostatic attraction between excited, negatively charged electrons, and positively charged holes. Holes are spaces left behind by excited electrons and are themselves a kind of quasi-particle. Credit: OIST

In world first, researchers at the Okinawa Institute for Graduate Science and Technology University (OIST) captured an image showing the internal orbits, or spatial distribution, of particles in excitation — a goal that has eluded scientists for nearly a century. Their results are published in Scientific 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.

“Exitons are really unique and interesting particles; they are electrically neutral, which means they behave very differently within materials of other particles like 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 Spectroscopic Unit. “This work brings us closer to fully understanding the nature of excitons.”

Excitons form 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, at the lower energy level. The oppositely charged electrons and holes attract and begin to orbit each other, creating the excitons.

Excitons are very important within semiconductors, but so far scientists have only been able to detect and measure them in limited ways. One thing is their fragility – it takes relatively little energy to separate the excitation into free electrons and holes. In addition, they are transient in nature – in some materials, excitons are extinguished in about a few trillionth of a second of a second after their formation, when the excited electrons “fall” back into the holes.

Scientists first capture an image of an orbit of an electron within an excitation

The instrument uses an initial pump pulse of light to excite electrons and generate excitons. This is quickly followed by a second light pulse, which used extreme ultraviolet photons to kick the electrons inside excitons from the material and into the vacuum of an electron microscope. The electron microscope then measures the energy and angle that the electrons left the material to determine the momentum of the electron around the hole inside the excitation. Credit: OIST

“Scientists first discovered excitons about 90 years ago,” said Professor Keshav Dani, a senior author and head of the Femtosecond Spectroscopic Unit at OIST. “But until very recently one could only access the optical signatures of excitons – for example, the light emitted by excitation extinguished. Other aspects of their nature, such as their momentum, and how the electron and the hole orbit each other, could only be described. theory. “

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

Now report on Scientific Advances, the team used the technique to capture the first image, which shows the distribution of an electron around the hole within excitation.

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

Scientists first capture an image of an orbit of an electron within an excitation

In the physics of the tiny, strange quantum concepts apply. Electrons act as particles and waves and therefore it is not possible to know both the position and the momentum of an electron at the same time. Instead, the probable cloud of excitation shows where the electron is most likely to be located around the hole. The research team generated an image of the probable cloud of excitation by measuring the wavefunction. Credit: OIST

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 connected to a hole inside the exciton.

“The technique has some similarities to the collision experiments of high-energy physics, where particles are shattered along with intense amounts of energy, breaking them. By measuring the trajectories of the smaller internal particles produced in the collision, scientists can start a piece. together the internal structure of the original intact particles, ”said Professor Dani. “Here we’re doing something similar – we’re using extreme ultraviolet light photons to separate excitons and measure the trajectories of the electrons to visualize what’s inside.”

“This was not a bad feat,” continued Professor Dani. “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 excitation, which gives the probability where the electron is likely to be located around the hole.

“This work is a major breakthrough in the field,” said Dr. Julien Madeo, co-author and employed scientist in the OIST Femtosecond Spectroscopic Unit. “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. This could allow us to create new quantum states of matter and technology based on these concepts. ”

Researchers are initiating a revolutionary new method for directly observing dark excitations

Additional information:
“Experimental measurement of the internal excitation wave function” Scientific Advances (2021). …. .1126 / sciadv.abg0192

Granted by Okinawa Institute of Science and Technology

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