How Twistronics Manipulates the Flow of Light

A bilayer of molybdenum trioxide supports highly unusual light propagation along straight paths when the two layers are rotated with respect to each other at the photonic magic angle. (Source: ASRC)

A research team led by scientists at the City University of New York in colla­boration with National Univer­sity of Singapore, Univer­sity of Texas at Austin and Monash University, has employed twistro­nics concepts – the science of layering and twisting two-dimen­sional materials to control their electrical properties – to manipulate the flow of light in extreme ways. The findings 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.

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 confi­guration, electrons flow with no resistance. Separately, each graphene layer shows no special electrical properties. The discovery has shown how the careful control of rotational symmetries can unveil unexpected material responses. The research 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 emission 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 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 Alù, founding director of the CUNY ASRC’s Photonics Ini­tiative. “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 discovery was based on quite a specific material and wavelength range, but with advanced nano­fabrication we can pattern many other material platforms to replicate these unusual optical features over a wide range of light wavelengths,” said National University of Singapore graduate student Guangwei Hu. “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 NUS-researcher C.W. Qiu. (Source: ASRC / CUNY)

Reference: G. Hu et al.: Topological polaritons and photonic magic angles in twisted α-MoO3 bi-layers, Nature 582, 209 (2020); DOI: 10.1038/s41586-020-2359-9

Links: Dept. of Electrical and Computer Engineering, National University of Singapore, Singapore, Singapore Photonics Initiative, Advanced Science Research Center, City University of New York, New York, USAARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET), Monash University, Clayton,  Australia

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