For humans, an ideal temperature can mean a lot to a day's work; our productivity can struggle under stifling heat or chilling winds. For two-dimensional (2D) semiconductive materials, however, an ideal temperature might be hundreds of degrees below freezing, making it challenging to use their potent light-interaction properties in everyday devices and at ambient temperatures.
“Often mere atoms thick, 2D materials have unique light-matter interactions not seen in bulkier materials, such as strong exciton binding energy and dangling bond-free surfaces, which allow easier integration with other materials,” said Jinghua Teng, Senior Principal Scientist and Senior Group Leader from A*STAR’s Institute of Materials Research and Engineering (IMRE).
Using 2D materials, researchers are developing metaoptics: nanoscale device components with strong light-manipulating powers, thanks to tiny surface structures smaller than the wavelength of light itself. When light hits these structures, it bends, focuses or scatters, with changes to its colour, intensity or even polarisation. These fascinating properties mean metaoptics built from 2D materials can support new ultra-compact optoelectronics designs for advanced displays, sensors, photonic circuits, communications and more.
However, existing device designs either need large amounts of power or very low temperatures to operate; or respond weakly to optical tuning. “It’s crucial to be able to dynamically control how a 2D metaoptic reflects light using electricity and at room temperature," said Teng.
To pave the way for innovative, yet practical 2D metaoptics applications, Teng and colleagues from IMRE, Nanyang Technological University Singapore, and University College London, UK, explored a phenomenon known as exciton-trion conversion in transition metal dichalcogenides (TMDCs), a class of 2D materials.
Many TMDCs are packed full of excitons (bound states of electron-hole pairs carrying energy) and trions (charged excitons with an additional electron or hole). Studies have shown that the balance between excitons and trions in a TMDC affects how it interacts with light, and that electricity can convert excitons to trions. However, little data exists on how to electrically control that conversion in an everyday setting, as the effect is normally too weak to detect at room temperatures.
To better understand electrically-controlled exciton-trion conversion under ambient conditions, Teng and colleagues designed a vertical optical cavity using a single-molecule layer of tungsten disulfide (WS2), a TMDC, tucked between an ionic gel and layers of aluminium and aluminium oxides. The cavity acted like a tiny echo chamber, making the dynamics between light and the metaoptic easier to observe.
“The optical cavity provides strong light-matter interactions that boost excitonic resonance and absorption in monolayer WS2 at specific exciton energies, enhancing exciton-trion conversion in the WS2 monolayer,” said Zeng Wang, a Senior Scientist in IMRE and the first author of the paper.
When the researchers applied an electric force to their device, they found it could convert nearly all energy carried by the excitons to trions at ambient temperature, causing dramatic changes to the metasurface’s reflection and refractive indices.
“We showed that monolayer WS2’s optical properties are highly tuneable through electrical modulation," said Teng. “These findings could contribute to innovations in telecommunications, imaging systems and information processing, where adaptability and miniaturisation are key.”
Moving forward, the team is delving deeper into applications of TMDC-based metasurfaces in new optical devices and systems, focusing on commercial applications such as lasers and sensors.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).
University College London, UK, and Nanyang Technological University, Singapore, contributed to this work through joint training of A*STAR Research Attachment Programme (ARAP) and Singapore International Graduate Award (SINGA) students, respectively.