Monash researchers are part of an international collaboration applying 'twistronics' concepts (the science of layering and twisting 2D materials to control their electrical properties) to manipulate the flow of light in extreme ways.

The findings, published in the journal Nature, hold the promise for leapfrog advances in a variety of light-driven technologies, including nano-imaging devices; high-speed, low-energy optical computers; and biosensors.

This is the first application of Moire physics and twistronics to the light-based technologies, photonics and polaritonics, opening unique opportunities for extreme photonic dispersion engineering and robust control of polaritons on 2D materials.

Applying Twistronics To Photons
The team took inspiration from the recent discovery of superconductivity in a pair of stacked graphene layers that were rotated to the 'magic twist angle' of 1.1 degrees.

In this stacked, misaligned configuration, electrons flow with no resistance, while separately, each of the two graphene layers shows no special electrical properties.

The discovery has shown how the careful control of rotational symmetries can unveil unexpected material responses.

The research team was led by Andrea Alu at the Advanced Science Research Center at the Graduate Center, CUNY, Cheng-Wei Qiu at National University of Singapore and Qiaoliang Bao formerly at Monash University.

The team discovered that an analogous principle can 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.

Typically, light radiated from a small emitter placed over a flat surface expands away in circles very much like the waves excited by a stone that falls into a pond. In their experiments, the researchers stacked two thin sheets of molybdenum-trioxide and rotated one of the layers with respect to the other. When the materials were excited by a tiny optical emitter, they observed widely controllable light waves over the surface as the rotation angle was varied.

In particular, they showed that at the photonic 'magical twist angle' the configured bilayer supports 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," said Andrea Alu.

"To unveil this phenomenon, we used thin layers of molybdenum trioxide. By stacking two of such layers on top of each other and controlling their relative rotation, we have 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."

"Our experiments were far beyond our expectations," said Dr 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."

"Our study shows that twistronics for photons can open truly exciting opportunities for light-based technologies, and we are excited to continue exploring these opportunities," said National University of Singapore graduate student Guangwei Hu, who led the theoretical component.

"Following our previous discovery published in Nature in 2018, we found that biaxial van der Waals semiconductors like a-MoO3 and V2O5 represent an emerging family of material supporting exotic polaritonic behaviors," said A/Prof Qiaoliang Bao, "These natural-born hyperbolic materials offer an unprecedented platform for controlling the flow of energy at the nanoscale."

Development Of Twistronics And Magic Angles In Graphene
Novel electronic properties in 'misaligned' graphene sheets was first predicted by National University of Singapore Professor (and FLEET Partner Investigator) Antonio Castro Neto in 2007, and the 'magic angle' of 1.1 degrees was theorised by FLEET PI (University of Texas in Austin) in 2011. Superconductivity in twisted graphene was experimentally demonstrated by Pablo Jarillo-Herrero (MIT) in 2018.

Research Report: "Topological polaritons and photonic magic angles in twisted a-MoO3 bi-layers"

Related Links
ARC Centre Of Excellence In Future Low-Energy Electronics Technologies
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