2D Material for Single Photon Emitters

Schematic of a laser-illuminated nano-optical probe investigating a strained nanobubble of tungsten diselenide. The single layer is sitting on a layer of boron nitride. (Source: N. Borys, Montana State U.)

Researchers at Columbia Engineering and Montana State Uni­versity have found that placing sufficient strain in a 2D material – tungsten diselenide – creates localized states that can yield single-photon emitters. Using sophis­ticated optical micro­scopy techniques developed at Columbia over the past three years, the team was able to directly image these states for the first time, revealing that even at room tempera­ture they are highly tunable and act as quantum dots, tightly confined pieces of semi­conductors that emit light.

“Our discovery is very exciting, because it means we can now position a single-photon emitter wherever we want, and tune its proper­ties, such as the color of the emitted photon, simply by bending or straining the material at a specific location,” says James Schuck. “Knowing just where and how to tune the single-photon emitter is essential to creating quantum optical circuitry for use in quantum computers, or even in quantum simu­lators that mimic physical pheno­mena far too complex to model with today’s computers.”

Developing quantum techno­logies such as quantum computers and quantum sensors is a rapidly developing field of research as researchers figure out how to use the unique properties of quantum physics to create devices that can be much more efficient, faster, and more sensitive than existing techno­logies. For instance, quantum information would be much more secure. Light-based quantum technologies rely on the creation and mani­pulation of indi­vidual photons. „For example, a typical green laser pointer emits over 10 quadril­lion photons every second with the mere push of a button,“ notes Nicholas Borys, assistant professor of physics at Montana State University. “But developing devices that can produce just a single control­lable photon with a flip of a switch is extremely difficult.”

Researchers have known for five years that single-photon emitters exist in ultrathin 2D materials. Their discovery was greeted with much excitement because single-photon emitters in 2D materials can be more easily tuned, and more easily integrated into devices, than most other single-photon emitters. But no one under­stood the underlying material proper­ties that lead to the single-photon emission in these 2D materials. “We knew that the single-photon emitters existed, but we didn’t know why,” says Schuck.

In 2019, the group of Frank Jahnke, a professor at the Institute for Theo­retical Physics at the University of Bremen, Germany, theorized how the strain in a bubble can lead to wrinkles and localized states for single-photon emission. Schuck, who focuses on sensing and engi­neering phenomena emerging from nano­structures and interfaces, was imme­diately interested in collaborating with Jahnke. He and Borys wanted to focus in on the tiny, nanoscale wrinkles that form in the shape of doughnuts around bubbles that exist in these ultrathin 2D layers. The bubbles, typically small pockets of fluid or gas that get trapped between two layers of 2D materials, create strain in the material and lead to the wrinkling.

Schuck’s group, and the field of 2D materials, faced a major challenge in studying the origins of these single-photon emitters: the nano­scale strained regions, which emit the light of interest, are much smaller than can be resolved with any conven­tional optical micro­scope. “This makes it difficult to understand what specifi­cally in the material results in the single-photon emission: is it just the high strain? Is it from defects hidden within the strained region?” says Tom Darlington, who is a postdoc and former graduate researcher with Schuck. “You need light to observe these states, but their sizes are so small that they can’t be studied with standard micro­scopes.”

Working with other labs at the Columbia Nano Institute, the team drew upon their decades-long expertise in nanoscale research. They used sophis­ticated optical microscopy techniques, including their new micro­scopy capa­bility, to look not just at the nano-bubbles, but even inside them. Their advanced nano-optical micro­scopy techniques enabled them to image these materials with ~10 nm resolution, as compared to approximately 500 nm resolution achievable with a conventional optical micro­scope.

Many researchers have thought that defects are the source of single-photon emitters in 2D materials, since they usually are in 3D materials such as diamond. To rule out the role of defects and show that strain alone could be respon­sible for single-photon emitters in 2D materials, Schuck’s group studied the ultralow-defect materials developed by Jim Hone’s group at Columbia Engineering. They also leveraged new bilayer structures developed within the Program­mable Quantum Materials Center, which provided well-defined bubbles in a platform that was easily studied with Schuck’s optical nano­scopes.

“Atomic-scale defects are often attributed to localized sources of light emission in these materials,” says Jeffrey Neaton, a professor of physics at UC Berkeley and Associate Labora­tory Director for Energy Sciences, Lawrence Berkeley National Labora­tory, who was not involved in the study. “The emphasis in this work on the fact that strain alone, without the need for atomic-scale defects, poten­tially impact[s] appli­cations ranging from low-power light-emitting diodes to quantum computers.”

Schuck, Borys, and their teams are now exploring just how strain can be used to precisely tailor the specific properties of these single-photon emitters, and to develop paths towards engi­neering addressable and tunable arrays of these emitters for future quantum techno­logies. “Our results mean that fully tunable, room-tempera­ture single-photon emitters are now within our grasp, paving the way for controllable and practical quantum photonic devices,” Schuck observes. “These devices can be the foundation for quantum techno­logies that will profoundly change computing, sensing, and infor­mation techno­logy as we know it.” (Source: Columbia U.)

Reference: T. P. Darlington et al.: Imaging strain-localized excitons in nanoscale bubbles of monolayer WSe2 at room temperature, Nat. Nano., online 13 July 2020; DOI: 10.1038/s41565-020-0730-5

Link: Nanolight group (J. Schuck), Dept. of Mechanical Engineering, Columbia University, New York, USA

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