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Plasmonics

Can antenna spacing tune terahertz plasmons in a topological insulator?

Viti L, Schiattarella C, Sichert L, et al. · Light, science & applications · 2025

Open access · cc by · source: Europe PMC

Placing a second topological-insulator antenna at the right distance beside the first shortens the terahertz plasmon wavelength and, at higher frequencies, roughly halves its relative losses.

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Study at a glance

Design
Other — THz scattering-type near-field microscopy with quantum cascade laser self-mixing detection on single, doublet and triplet Bi2Se3 antennas, plus finite-element simulations.
N
No sample N; single antennas and coupled doublets/triplets at three gap sizes, probed at three laser frequencies.
Population
Lithographically patterned antennas etched from an MBE-grown Bi2Se3 topological insulator film on sapphire
Outcome
Complex plasmon-polariton wavevector (wavelength and damping length) versus gap and frequency

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Key findings

Single antennas behaved as expected, with the plasmon wavelength compressed about tenfold relative to free space. With a gap of 0.8 micrometres the plasmon wavevector rose by over 20% in pairs and triples, while a very small gap slightly lowered it and a large gap left it unchanged, so coupling strength peaked at an intermediate gap. At the optimal gap and higher frequencies the propagation length relative to wavelength was about doubled, meaning lower losses.

Methodology

The researchers etched rectangular antennas, 16 by 4 micrometres, from a thin Bi2Se3 film and arranged them singly or as side-by-side pairs and triples with different gaps. A terahertz laser focused on a vibrating microscope tip launched surface plasmons, and the reflected signal fed back into the laser itself, letting them map the plasmon phase along each antenna. They fitted these phase profiles to extract the plasmon wavevector and damping, compared them to a conductivity model, and ran electromagnetic simulations of coupled antennas.

Limitations

Only three gap values, three frequencies and one film thickness were tested, so the full dependence on geometry is sketched rather than mapped. The explanation for the non-monotonic gap effect and the loss reduction is qualitative, and the simulations showed some sensitivity to the assumed sheet conductivity. No functional device such as a tunable filter or modulator was demonstrated.

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