For the first time, scientists have successfully imaged the electron orbitals within an exciton, a quasiparticle. The result allowed them to finally measure the excitonic wave function of the spatial distribution of electron momentum within the quasiparticle.
This achievement, which has been pursued since the discovery of excitons in the 1930s, may sound abstract, but it has helped advance a variety of technologies, including quantum applications.
“Excitons are very unique and interesting particles; they are electrically neutral, which means that they behave very differently in materials than other particles such as electrons. Their presence can really change the way materials respond to light,” said Michael Man, a physicist in the Femtosecond Spectroscopy Group at the Okinawa Institute of Science and Technology (OIST) in Japan.
“This work brings us closer to our goal of fully understanding the nature of excitons.”
Excitons are not really particles, but rather a quasi-particle. They occur when the collective behavior of particles causes them to act in a particle-like manner. Excitons appear in semiconductors, materials that are more conductive than insulators, but not enough to be considered conductors.
Semiconductors are useful in electronics because they allow finer control of the flow of electrons. Although difficult to observe, excitons play an important role in these materials.
Excitons are formed when a semiconductor absorbs a photon that raises a negatively charged electron to a higher energy level; that is, the photon “excites” the electron, leaving a positively charged gap, called an electron hole. Negative electrons and positive holes join together in a mutual orbit; an exciton is an “electron-electron-hole” pair in such an orbit.
But excitons are very short-lived and fragile, as electrons and holes can recombine in a fraction of a second, so seeing them is not an easy task.
“Scientists first discovered excitons 90 years ago,” says Keshav Dani, a physicist with OIST’s Femtosecond Spectroscopy Group, “but until recently, people usually only had access to the optical characteristics of excitons – for example, the light emitted when an exciton annihilates. Other aspects of their properties, such as their momentum and how electrons and holes operate with each other, could only be described theoretically.”
It’s a problem the researchers have been grappling with. Last December, they found a way to directly observe electron momentum. Now, they have successfully used this method.
The new technique uses a two-dimensional semiconductor material of tungsten diselenide, which is housed in a vacuum chamber, then cooled and held at 90 Kelvin.
A laser pulse is used to generate excitons in this material; then a second ultra-high-energy laser is used to knock the electrons completely away into the vacuum chamber, which is monitored by an electron microscope.
The instrumentation measures the velocity and trajectory of the electrons, and this information is then used to calculate the initial orbit of the particle out of the exciton system.
Although it was a delicate, time-consuming task, the team was eventually able to measure the exciton’s wave function.
With adjustments, the team’s research represents a giant leap forward in the field of exciton research. It can be used to measure wave functions for different exciton states and configurations and to probe exciton physics processes in different semiconductor materials and systems.
“This work is an important advance in the field,” said Julien Madeo, a physicist in the OIST Femtosecond Spectroscopy Group.
“The ability to visualize the internal orbit of particles as they form larger composite particles allows us to understand, measure and ultimately control composite particles in a way that has never been possible before. This could allow us to create new quantum states of matter and technologies based on these concepts.”
The team’s research has been published in Science Advances.
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