Sandwich-style construction: towards ultra-low-energy exciton electronics

The breakthrough, led by PhD candidate Matthias Wurdack at ANU, demonstrated robust, dissipationless propagation of an exciton mixed with light bouncing between the high-quality mirrors.

Conventional electronics relies on flowing electrons, or holes (a hole is the absence of an electron, i.e. a positively-charged quasi-particle).

Instead, FLEET’s ANU team investigated an alternative future electronic technology using excitons because, in principle, they could flow in a semiconductor without losing energy by forming a collective superfluid state. Also, excitons in novel, actively-studied atomically-thin semiconductors are stable at room temperature.

Despite this promise for low-energy electronics and sensors, the properties of atomically-thin semiconductors, including the flow of excitons, are strongly affected by disorder or imperfections, which can be introduced during fabrication.

The ANU-led FLEET team – with Centre colleagues at Swinburne University of Technology and FLEET Partner institution Wroclaw University – coupled the excitons in an atomically-thin material to light, to demonstrate for the first time their long-range propagation without any dissipation of energy, at room temperature.

Trapping light between two parallel high-quality mirrors in an optical microcavity an exciton (matter) to bind with a photon (light), forming the hybrid particle an exciton-polariton. Microcavities are micrometre-scale structures with an optical medium sandwiched between ultra-reflective mirrors, used to confine light such that it forms exciton-polaritons.

In the 2021 study, a new ‘sandwich-style’ fabrication process for the optical microcavity allowed the researchers to minimise damage to the atomically-thin semiconductor and to maximise the interaction between the excitons and the photons. The exciton-polaritons formed in this structure were able to propagate without energy dissipation across tens of micrometres, the typical scale of an electronic microchip.

A high-quality optical microcavity ensuring the longevity of the light (photonic) component of exciton-polaritons was the key to these observations. “We found that exciton-polaritons can be made remarkably stable if microcavity construction avoids damage of the fragile semiconductor sandwiched between the mirrors during fabrication,” says Matthias.

“The choice of the atomically-thin material in which the excitons travel is far less important than the construction.”

“We fabricate the entire top structure separately, and then place it on top of the semiconductor mechanically, like making a sandwich. Thus we avoid any damage to the atomically-thin semiconductor, and preserve the properties of its excitons.”

The researchers optimised this sandwiching method to make the cavity very short, maximising the exciton-photon interaction.

“This demonstration, for the first time, of dissipationless transport of room-temperature polaritons in atomically-thin TMDCs is a significant step towards future, ultra-low-energy exciton-based electronics,” says group leader Prof Elena Ostrovskaya (ANU).

Furthermore, the researchers confirmed that exciton-polaritons can propagate in the atomically-thin semiconductor for tens of micrometres (easily far enough for functional electronics) without scattering on material defects. (In fact, the travel length of excitons in these materials is dramatically reduced by these defects.) The exciton-polaritons’ high coherence bodes well for their potential as information carriers.

“This long-range, coherent transport was achieved at room temperature, which is important for development of practical applications of atomically-thin semiconductors,” said Matthias.

If future excitonic devices are to be a viable, low-energy alternative to conventional electronic devices, they must be able to operate at room temperature, without the need for energy-intensive cooling.

Back to Research theme 2: Exciton superfluid