First Terahertz Detection In Graphene

Researchers at the National University of Singapore have found that terahertz light causes unique electron flow in doped graphene.

When light interacts with certain materials, especially those with a property called photoresistance, it can alter their electrical conductivity. Graphene is one such material; exposure to light excites its electrons, changing its photoconductive properties.

Researchers at the National University of Singapore have discovered an unusual photoresistive response in doped metallic graphene. The team demonstrated that under continuous-wave terahertz (THz) radiation, Dirac electrons in this material can become thermally isolated from the lattice, resulting in a hydrodynamic flow of electrons.

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The main goal of the team was to delve deeper into the fluid-like behavior of graphene’s electrons. Specifically, they aimed to investigate whether the viscous electron flow observed in graphene could offer a solution to a long-standing challenge in optoelectronics: the detection of terahertz (THz) radiation.

To investigate the impact of THz waves on graphene’s electrical conductivity, the team began by preparing single-layer graphene samples “doped” with extra electrons, giving them metallic-like properties. However, for effective sensing in these samples, further processing was required, as graphene’s electrical conductivity is generally insensitive to heating from THz radiation.

To tackle this issue, the researchers designed their samples with a narrow constriction, allowing viscous effects to alter the conductivity of the samples exposed to THz radiation. Using high-precision measurement tools, they monitored changes in electron movement and electrical resistance within the graphene as it interacted with the THz waves.

Interestingly, they observed that under THz light, the viscosity of the fluid-like electrons in the doped metallic graphene decreased, enabling the electrons to flow more easily through the material, with reduced resistance.

The team documented this phenomenon in newly developed viscous electron bolometers, devices capable of detecting shifts in electrical conductivity at exceptionally high speeds.

The team’s recent study could significantly impact the advancement of ultra-fast, high-performance THz technologies. Their findings could guide the development of next-generation wireless communication (6G and beyond), enhance navigation systems for autonomous vehicles, and improve tools for capturing high-resolution astronomical images.

Reference: M. Kravtsov et al, Viscous terahertz photoconductivity of hydrodynamic electrons in graphene, Nature Nanotechnology (2024). DOI: 10.1038/s41565-024-01795-y