Twisted Layers of 2D Materials Can Be Used to Propagate and Control Light

An international scientific team has demonstrated that twistronics — the science of layering and twisting 2D materials to control their electrical properties — can be used to manipulate the flow of light in extreme ways. The findings could help drive advances in light-driven technologies including nanoimaging, optical computing, and biosensing. The research team was led by scientists at the Advanced Science Research Center at City University of New York (CUNY ASRC), in collaboration with National University of Singapore, University of Texas at Austin, and Monash University.

The scientists drew inspiration from a recent discovery of superconductivity in a pair of stacked graphene layers rotated to the “magic twist angle” of 1.1° that showed how the careful control of rotational symmetries can unveil unexpected material responses.

The team discovered that an analogous principle could be applied to manipulate light in highly unusual ways. At a specific rotation angle between two ultrathin layers of molybdenum trioxide, the researchers were able to prevent optical diffraction and enable robust light propagation in a tightly focused beam at desired wavelengths.

A bilayer of molybdenum trioxides supports highly collimated, directive, and diffractionless propagation of nanolight when the two layers are aligned at the photonic “magic twist angle.” Courtesy of FLEET/Monash University.

The researchers stacked two thin sheets of molybdenum trioxide and rotated one of the layers with respect to the other. When they excited the materials with a tiny optical emitter, they observed widely controllable light emission over the surface as they changed the rotation angle. At the photonic “magic twist angle,” the configured bilayer supported robust, diffraction-free light propagation in tightly focused channel beams over a wide range of wavelengths.

“While photons — the quanta of light — have very different physical properties than electrons, we have been intrigued by the emerging discovery of twistronics, and have been wondering if twisted two-dimensional materials may also provide unusual transport properties for light, to benefit photon-based technologies,” CUNY professor Andrea Alù said.

When Alù and the team stacked two thin layers of molybdenum trioxide on top of each other and controlled their relative rotation, they observed dramatic control of the light guiding properties. “At the photonic magic angle, light does not diffract, and it propagates very confined along straight lines. This is an ideal feature for nanoscience and photonic technologies,” Alù said.

Twisted bilayer α-MoO₃. Courtesy of FLEET/Monash University.

Although this discovery was based on a specific material and wavelength range, with advanced nanofabrication many other material platforms could be patterned to replicate the unusual optical features over a wide range of light wavelengths, researcher Guangwei Hu at the National University of Singapore said.

According to researchers at Monash University, this work is the first application of moiré physics and twistronics to the light-based technologies of nanophotonics and polaritonics, and could lead to opportunities for extreme photonic dispersion engineering and robust control of polaritons on 2D materials.

“Our experiments were far beyond our expectations,” said researcher Qingdong Ou, who led the experimental component of the study at Monash University. “By stacking ‘with a twist’ two thin slabs of a natural 2D material, we can manipulate infrared light propagation, most intriguingly, in a highly collimated style.” The researchers will continue to explore the science of twistronics and its applications for photonics.

The research was published in Nature (www.doi.org/10.1038/s41586-020-2359-9).