Optical Defects as Sources of Quantum Light

Researchers at Stanford University are investigating light-emitting defects in materials that could someday enable quantum-based technologies. Once understood, these defects could be controlled and potentially provide bright, on-demand, predictable sources of quantum light.

The researchers are specifically investigating hexagonal boron nitride (hBN), a material that can emit bright light one photon at a time at room temperature. Hexagonal boron nitride emits light in a rainbow of hues that currently cannot be controlled. “We wanted to know the source of the multicolor emission, with the ultimate goal of gaining control over emission,” researcher Fariah Hayee said.

The scientists were able to trace the material’s emission to specific atomic defects. They directly correlated hBN quantum emission with local strain using a combination of photoluminescence, cathodoluminescence, and nanobeam electron diffraction.

Using a modified electron microscope developed by the Stanford team, the scientists matched the local, atomic-scale structure of hBN with its unique color emission. Over the course of hundreds of experiments, they bombarded the material with electrons and visible light and recorded the pattern of light emission. They found that at least four distinct defect classes were responsible for the observed emission range, and all four classes were stable upon both optical and electron illumination. They also studied how the periodic arrangement of atoms in hBN influenced the emission color.

“The challenge was to tease out the results from what can seem to be a very messy quantum system. Just one measurement doesn’t tell the whole picture,” Hayee said. “But taken together, and combined with theory, the data is very rich and provides a clear classification of quantum defects in this material.”

A Harvard University team led by professor Prineha Narang developed a new theory to predict the color of defects by accounting for how light, electrons, and heat interacted in the material. “We needed to know how these defects couple to the environment and if that could be used as a fingerprint to identify and control them,” Harvard researcher Christopher Ciccarino said.

In addition to the specific findings about types of defect emissions in hBN, the process the team developed to collect and classify these quantum spectra could, on its own, be transformative for a range of quantum materials. “Materials can be made with near atomic-scale precision, but we still don’t fully understand how different atomic arrangements influence their optoelectronic properties,” professor Jennifer Dionne said.

Although the researchers are currently focused on understanding which defects give rise to certain colors of quantum emission, the eventual aim is to control the defects’ properties.

The work brought together materials scientists, physicists, and electrical engineers. “We were able to lay the groundwork for creating quantum sources with controllable properties, such as color, intensity, and position,” Dionne said. “Our ability to study this problem from several different angles demonstrates the advantages of an interdisciplinary approach.”

The research was published in Nature Materials (www.doi.org/10.1038/s41563-020-0616-9).