Twist-engineered acoustic plasmon nanocavities enable deep-nanoscale terahertz molecular fingerprinting

Terahertz radiation is capable of identifying substances through their unique spectral “fingerprints,” yet detecting nanoscale samples has long posed a major scientific challenge due to the vast scale difference between terahertz wavelengths and target molecules. Addressing this fundamental limitation, a research team from the Terahertz Technology Innovation Research Institute at Shanghai University of Technology has developed an innovative twisted bilayer graphene platform that achieves unprecedented compression of terahertz waves into nanoscale volumes. Their approach theoretically enables the detection and spectral identification of molecular layers as thin as 1 nanometer. The study was published in the high-impact journal PhotoniX under the title "Twist-engineered acoustic plasmon nanocavities enable deep-nanoscale terahertz molecular fingerprinting".

“Comparing terahertz waves to molecules is like using a meter stick to measure a grain of sand — their scale difference makes high-sensitivity sensing extremely difficult,” explains Prof. Chen Shu, co-corresponding author of the study. “Our method effectively creates a nanoscale ‘ruler’ that can probe molecular vibrations at the single-layer level in the terahertz band.”

The team’s breakthrough centers on twisted double-layer graphene plasmonic metasurfaces (t-DL-GPMs). By rotating two graphene layers to specific angles and harnessing interlayer plasmon coupling, the researchers constructed 3D acoustic plasmon nanocavities (APCs), which acts like a compact 3D trap, concentrating terahertz wave into an extremely small volume between the two graphene layers, thereby dramatically enhancing the electric field and field confinement.

“Think of it as a super-compact, ultra-powerful lens that can focus terahertz wave down onto a single-layer molecules,” Prof. Chen Shu explained. “This nanocavity allows us to see the vibrational signals of molecules at THz frequency.”

• The t-DL-GPM platform exhibited remarkable performance in both refractive index sensing and molecular fingerprint detection: 1) a sensitivity of 5.2 THz/RIU and figure of merit of 6.4 in refractive index sensing, outperforming conventional single-layer and untwisted double-layer structures by tens of times; 2) successful identification of terahertz vibrational fingerprints of γ-aminobutyric acid (GABA) molecules at just 1 nm thickness; 3) Dynamic control over resonance and field enhancement via electrical gating and geometric tuning.

Professor Zhu Yiming, the other co-corresponding author, highlights the cross-disciplinary nature of the work: "This research  integrates terahertz photonics, plasmonic nanotechnology, low-dimensional materials science and spectroscopy. It provides a new insight for investigating THz wave–matter interactions at extreme scales.”

This advancement not only paves the way for a new generation of terahertz sensing devices but also provides a robust platform for fundamental studies of ultra-strong light–matter interactions at the nanoscale at THz frequency.