Researchers from NUS have developed terahertz (THz)-sensitive devices by harnessing the viscous, liquid-like flow of electrons in graphene, paving the way for ultra-fast light sensors.
Electron flow gives rise to the electric currents that power nearly all devices in use today. In conventional materials, electrons are typically modelled as independent particles that move freely between, and scatter off, atoms. However, in advanced 2D materials, or quantum materials, electrons may interact with one another and flow together in a fluid-like manner that resembles highly viscous liquids like oil.
This fluid-like behaviour has garnered considerable attention in recent years, as it challenges a century-long understanding of electron behaviour in circuits.
Now, a research team led by Assistant Professor Denis Bandurin from NUS Materials Science and Engineering and the NUS Institute for Functional Intelligent Materials, have described how this fluid-like electron behaviour could address a key challenge in modern optoelectronics - the ability to efficiently detect terahertz (THz) radiation.
This work was published in
Nature Nanotechnology in October 2024.
Applications of THz radiation spanning medical imaging, communications and self-driving navigation
The THz range (from 0.03 mm to 3 mm) of the electromagnetic spectrum is crucial for technologies such as medical imaging, beyond 5G communication, observational astronomy, industrial quality control, as well as mapping the environment and detecting obstacles in self-driving navigation. However, THz radiation is challenging to detect because it lies between the microwave (1 m to 1 mm) and infrared ranges (0.75 to 15 µm) on the electromagnetic spectrum. This is essentially a technological frequency gap, where frequencies are too fast for traditional semiconductor chips to operate on, yet too slow to be detected by conventional optoelectronic devices.
Currently, specialised devices are required to convert THz wavelengths into measurable signals. This is necessary as most existing technologies do not offer the short response time required for data encoding.
If we need to pass information at THz frequencies, we need to switch it off and on with ultra-high speed so that we can encode our data in a series of zeros and ones. Thus, we need ultrafast detectors which can see these on and off sequences of information.
Leveraging on the properties of graphene’s electron flow, the team led by Asst Prof Bandurin created viscous electron bolometers, which are devices used to measure electromagnetic radiation by detecting minute changes in temperature, causing a measurable change in its electrical resistance. Their bolometers are capable of ultra-fast THz detection, with their performance hinging on its innovative design.
A schematic diagram of the bolometer device created by the team
Within each device, graphene is shaped as a narrow constriction to enhance observation of the viscous flow regime, and is sandwiched between crystal, hexagonal boron nitride, which serves to protect graphene and improve performance. The devices also included a specially designed bow-tie antenna designed to efficiently detect THz radiation by concentrating and directing the electromagnetic energy onto the graphene constriction.
The researchers discovered that THz radiation interacts with electrons in graphene by significantly reducing the viscosity of the electron fluid giving rise to the increase in device conductance that can be measured. This reduction in viscosity allows electrons to flow more freely, becoming more ‘liquid’ and becoming highly sensitive to temperature changes induced by THz radiation.
By streaming this fluid-like electron flow through narrow graphene constrictions, the team observed a significant increase in electrical conductance, a phenomenon known as superballistic transport. When exposed to THz radiation, the electrons in graphene are heated, which reduces electrical resistance and enhances sensitivity to any THz radiation-induced temperature change.
This fluid-like electron flow property in graphene allows absorbed THz radiation to be converted into measurable resistance changes.
“Our innovation represents the first practical application of electron hydrodynamics, a concept that was previously considered purely fundamental,” said Mikhail Kravtsov, first author and PhD student at NUS Materials Science and Engineering.
It is hoped that through these discoveries, new graphene-based technologies that exploit this unique property may be unlocked. Ultra-fast light sensors capable of isolating and capturing light in the THz range is just one example. This work also promises to advance sensing and signalling technologies.
“We are actively working on optimising these devices for practical applications, aiming to harness their full potential for various technological advancements. One potential application is receivers for 6G band to enable ultra-fast communications in the future,” Asst Prof Bandurin added.
Ultimately, the development of a compact, all-in-one, multi-wavelength imaging sensor that spans the X-ray spectrum to the near-infrared region can open up new possibilities for advanced imaging applications across various fields, including wearable microscopes, smartphone-integrated hyperspectral cameras, and industrial automation.
References
Kravtsov, M., Shilov, A. L., Yang, Y., Pryadilin, T., Kashchenko, M. A., Popova, O., ... & Bandurin, D. A. (2024). Viscous terahertz photoconductivity of hydrodynamic electrons in graphene. Nature Nanotechnology, 1-6.
