The inside of an exciton has been captured for the first time

April 21, 2021
Ever heard of an exciton? Scientists have finally captured a picture of one. (OIST)

Ever heard of an exciton? Scientists have finally captured a picture of one. (OIST)

An exciton has been intimately pictured for the first time in a major development for the field of physics, as the phenomenon is one of many quasiparticles that require scientists to go to significant lengths to make any sort of observation.

Understanding excitons is vital because they're found in many technical devices due to their association with semiconductors, and they're also thought to have implications for quantum information. A highly precise experiment, explained in a paper published April 21 in Science Advances, produced an image that may finally elucidate this quasiparticle's counterintuitive qualities.

Excitons are peculiar in nature because they're like a metaphysical derivative of an electron. When light is shined on a semiconductor-based electron, the particle reaches a higher energy state. That means it exits its previous state, leaving behind a hole, which then takes on a life of its own and continues to interact with the electron.

"A hole is positively charged; an electron is negatively charged," lead author Keshav Dani told The Academic Times. "Opposites attract. They attract each other to form a bound system."

He added, "It's like the Earth and the sun — with the Earth bound to the sun, going around it."

The system, collectively referred to as an exciton, is considered a great option to hold quantum information, because its electron — the same particle that can become a qubit, or quantum bit — behaves in a protected way. That's due to the system's neutral state as it is made up of two opposing charges; the electron, theoretically, shouldn't interact with external substances.

But Dani, who is head of the Femtosecond Spectroscopy Unit at Okinawa Institute of Science and Technology, explained that excitons are also incredibly difficult to observe.

"The ways in which we could look at an exciton were quite limited," he said. "Though we've learned a lot about them — and they've been extensively studied — you could only see the excitons that directly interact with light."

Excitons are no exception to the notion that angular momentum is always conserved. Previously, scientists studying excitons took advantage of this concept, which states that anytime an object hits another with a force, that force transfers to the second object, prompting it to move.

The law manifests in an exciton when the exciton acts in reverse, meaning the electron recombines with the hole. That produces an opposite effect — the emission of light, or a photon.

"All these years, the most common way you would study an exciton is … you shined in light and created the exciton. Then, when the exciton recombined and gave out light, you would measure the outgoing light and learn about the exciton," Dani explained. 

However, he noted that around 70% or 80% of excitons are so-called dark excitons, which can't give off light.

"[For] those excitons where the electron and hole … recombine and they give out a photon, the momentum of the original exciton would have to now go into the photon," Dani said. "But if the photon cannot carry that much momentum, that process is not going to happen."

That means light isn't given off, so the dark exciton can't be studied. Addressing this, the team behind the new paper presents a method that can tackle even these mysterious excitons. It's similar to the technique used at the European Council for Nuclear Research, where scientists are scrutinizing antimatter.

"You send in a photon of very high energy — high enough energy that you break apart the exciton — and you kick the electron out of the material," Dani said. "Then you measure the angle at which the electron comes out."

That angle can inform the researchers about the momentum of the electron prior to its expulsion. If that measurement is done over and over again, a map of the momentum distribution is attained, which eventually leads to the calculation of the electron's exact position over time. The result is what the researchers obtained: a depiction of the electron's orbit.

According to Dani, the experiment was done over a period of 24 hours, or 86,400 seconds. There were 1 million photon pulses per second. He traces his novel finding back to his days as a Ph.D. studying under professor Daniel Chemla. 

"I've known about this particle since I started my Ph.D. at Berkeley; my advisor was one of the preeminent scientists studying these excitons," Dani said. "Early in my Ph.D., he had a stroke and we could never communicate with words thereafter — we ended up communicating with gestures and barely readable, scribbled notes. We grew very close this way."

He continued, "He passed away shortly after I finished my Ph.D., but I was aware of his legacy: studying excitons. And now the idea that 20 years later, I'm able to go back and image the inside of the exciton — it's amazing, it's humbling; I wish I could share the picture with him."

The paper, "Experimental measurement of the intrinsic excitonic wave function," was published April 21 in Science Advances. It was authored by Michael K. L. Man, Julien Madéo, Chakradhar Sahoo, Keshav M. Dani, Vivek Pareek, Arka Karmakar, E Laine Wong, Abdullah Al-Mahboob, Nicholas S. Chan, David R. Bacon, Xing Zhu and Mohamed M. M. Abdelrasoul, Okinawa Institute of Science and Technology; Kaichen Xie and Ting Cao, University of Washington; Marshall Campbell and Xiaoqin Li, The University of Texas at Austin; and Tony F. Heinz and Felipe H. da Jornada, Stanford University.

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