Flat photonic bands, in which light effectively stands still, are attractive for lasers, but structures that support them are hard to design. Researchers in Päivi Törmä's Quantum Dynamics group at Aalto University have introduced lattices made of chains of nanoparticles, whose long-range coupling produces completely flat bands at predictable wavelengths across the full angular range.

The team demonstrated lasing in single nanoparticle chains and explained how the emission changes to conventional single-mode lasing as more chains are added. Square and triangular lattices built from chains gave partially coherent emission that depends on pump power and polarisation. The authors, Rebecca Heilmann, Joel Lehikoinen, Sioneh Eyvazi, Evgeny Mamonov and Päivi Törmä, suggest the approach could lead to bright, narrowband and linearly polarised light sources.

Figure: Heilmann et al., Nano Lett. (2026); image from arXiv:2602.21825, CC BY 4.0