Flat bands, where light's group velocity vanishes, are useful for slow light, nonlinear optics and lasing, but had not been reported in lattices whose sites couple over long distances through radiation. QMAT researchers at Aalto University, with colleagues at the University of Turku, show that lattices of superposed, equally spaced chains produce flat bands of purely diffractive origin.

The energies and angles of these bands are set by the geometry of the lattice; the bands extend over all angles, can be only a few nanometres wide and are linearly polarised. The team observed them at the predicted energies in gold nanoparticle lattices in the near infrared. The work comes from Päivi Törmä's Quantum Dynamics group, with first author Joel Lehikoinen, and Konstantinos Daskalakis's team in Turku.

The preprint has not yet been peer reviewed.

Figure: Lehikoinen et al., arXiv:2602.21830 (2026), CC BY 4.0