Future technologies based on the principles of quantum mechanics could revolutionize information technology. But to realize the devices of tomorrow, today’s physicists must develop precise and reliable platforms to trap and manipulate quantum-mechanical particles.
In a published Feb. 25 in the journal , a team of physicists from the 天美影视传媒, the University of Hong Kong, the Oak Ridge National Laboratory and the University of Tennessee reports that they have developed a new system to trap individual . These are bound pairs of electrons and their associated positive charges, known as holes, which can be produced when semiconductors absorb light. Excitons are promising candidates for developing new quantum technologies that could revolutionize the computation and communications fields.
The team, led by , the UW’s Boeing Distinguished Professor of both physics and materials science and engineering, worked with two single-layered 2D semiconductors, and , which have similar honeycomb-like arrangements of atoms in a single plane. When the researchers placed these 2D materials together, a small twist between the two layers created a “superlattice” structure known as a 鈥� a periodic geometric pattern when viewed from above. The researchers found that, at temperatures just a few degrees above absolute zero, this moir茅 pattern created a nanoscale-level textured landscape, similar to the dimples on the surface of a golf ball, which can trap excitons in place like eggs in an egg carton. Their system could form the basis of a novel experimental platform for monitoring excitons with precision and potentially developing new quantum technologies, said Xu, who is also a faculty researcher with the UW’s 聽and the .
Excitons are exciting candidates for communication and computer technologies because they interact with 鈥� single packets, or quanta, of light 鈥� in ways that change both exciton and photon properties. An exciton can be produced when a semiconductor absorbs a photon. The exciton also can later transform back into a photon. But when an exciton is first produced, it can inherit some specific properties from the individual photon, such as spin. These properties can then be manipulated by researchers, such as changing the spin direction with a magnetic field. When the exciton again becomes a photon, the photon retains information about how the exciton properties changed over its short life 鈥� typically, about a hundred nanoseconds for these excitons 鈥� in the semiconductor.
In order to utilize individual excitons’ “information-recording” properties in any technological application, researchers need a system to trap single excitons. The moir茅 pattern achieves this requirement. Without it, the tiny excitons, which are thought to be less than 2 nanometers in diameter, could diffuse anywhere in the sample 鈥� making it impossible to track individual excitons and the information they possess. While scientists had previously developed complex and sensitive approaches to trap several excitons close to one another, the moir茅 pattern developed by the UW-led team is essentially a naturally formed 2D array that can trap hundreds of excitons, if not more, with each acting as a , a first in quantum physics.
A unique and groundbreaking feature of this system is that the properties of these traps, and thus the excitons, can be controlled by a twist. When the researchers changed the rotation angle between the two different 2D semiconductors, they observed different optical properties in excitons. For example, excitons in samples with twist angles of zero and 60 degrees displayed strikingly different magnetic moments, as well as different helicities of polarized light emission. After examining multiple samples, the researchers were able to identify these twist angle variations as 鈥渇ingerprints鈥� of excitons trapped in a moir茅 pattern.
In the future, the researchers hope to systematically study the effects of small twist angle variations, which can finely tune the spacing between the exciton traps 鈥� the egg carton dimples. Scientists could set the moir茅 pattern wavelength large enough to probe excitons in isolation or small enough that excitons are placed closely together and could 鈥渢alk鈥� to one another. This first-of-its-kind level of precision may let scientists probe the quantum-mechanical properties of excitons as they interact, which could foster the development of groundbreaking technologies, said Xu.
“In principle, these moir茅 potentials could function as arrays of homogenous quantum dots,” said Xu. “This artificial quantum platform is a very exciting system for exerting precision control over excitons 鈥� with engineered interaction effects and possible properties, which could lead to new types of devices based on the new physics.”
“The future is very rosy,” Xu added.
Co-lead authors on the paper are Pasqual Rivera and Kyle Seyler, who conducted this research as UW doctoral students in physics and are now postdoctoral researchers at the UW and the California Institute of Technology, respectively. Co-corresponding author is Wang Yao at the University of Hong Kong. Co-authors are Hongyi Yu at the University of Hong Kong; Nathan Wilson and Essance Ray at the UW; and David Mandrus and Jiaqiang Yan at the Oak Ridge National Laboratory and the University of Tennessee. The research was funded by the U.S. Department of Energy, the CEI, the Croucher Foundation and the Research Grants Council of Hong Kong.
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For more information, contact Xu at xuxd@uw.edu or 206-543-8444.
Grant numbers: DE-SC0018171, HKU17302617