Researchers at the University of Eastern Finland have uncovered two complementary mechanisms that govern coherence in miniaturised lasers composed of metallic nanoparticle arrays incorporated with an optical gain material, also known as plasmonic lattice lasers.
In one study, they showed that these nanostructures can generate phase-locked ultrafast laser pulse modulation through the synchronisation of multiple lasing modes. In the other study, they demonstrated that structures supporting lasing modes with different topologies and polarisations can sustain independent channels without mutual coherence. Together, these findings establish a unified physical picture of coherence formation in plasmonic lasers and provide new design principles for nanoscale photonic devices.
The studies also connect plasmonic nanolasing with concepts from topological photonics, a field that applies ideas from topology, a branch of mathematics that describes properties that remain robust against small perturbations. Previously, topology in physics has been utilised for generating exotic, topological phases of matter (David Thouless, Duncan Haldane and Michael Kosterlitz, 2016 Physics Nobel Prize). In photonic systems, topology provides a way to generate unidirectional propagation of light similar to electrical diodes and to classify lasing modes into distinct categories, including topologically trivial and nontrivial states.
Published in Laser & Photonics Reviews and ACS Nano, the studies reveal how the topology and polarisation of lasing modes can influence whether different modes synchronise or remain independent. The work advances the fundamental understanding of plasmonic lasing and opens new opportunities for applications ranging from chip-scale ultrafast light sources to low-crosstalk photonic technologies.
Ultrafast pulse modulation through phase locking
In the first study, published in Laser & Photonics Reviews, the researchers demonstrated that plasmonic superlattices (i.e., lattices composed of two or more periodicities) supporting topologically trivial lasing modes can generate mode-locked ultrafast laser pulse modulation.
When combining the nanoparticle lattice with a liquid dye gain medium and optical pumping, two lasing modes became phase locked through near-field interactions, leading to ultrafast pulse modulation.
“A major challenge was developing a measurement setup capable of resolving these ultrafast dynamics,” says Doctoral Researcher Janne Heikkinen, lead author of the study. “Once we achieved this, we could directly observe how different lasing modes synchronise to produce ultrafast modulation.”
Numerical simulations revealed the physical origin of the phase locking. The calculations showed that femtosecond-scale correlations emerge within the shared gain medium at plasmonic hotspots, providing a mechanism that synchronises the lasing modes.
“The simulations allowed us to study the gain dynamics and identify the origin of the phase correlations,” says Postdoctoral Researcher Roman Calpe. “We found that correlations formed at the plasmonic electric field hotspots around the nanoparticles enabled phase locking between multiple lasing modes.”
Topologically distinct modes remain independent
The second study, published in ACS Nano, revealed fundamentally different behaviour when the lasing occurred in modes belonging to different topological classes.
The researchers fabricated arrays of gold nanoparticles with varying diameters, enabling the simultaneous formation of topologically trivial dipolar modes and topologically nontrivial quasi-bound-state-in-the-continuum (qBIC) modes. Although these modes lased simultaneously, measurements showed that they remained mutually incoherent. In contrast to the first study, no ultrafast pulse modulation was observed, indicating the absence of phase locking between the modes.
Further experiments confirmed that the modes were not mutually phase-correlated and simulations provided insight into the origin of this behaviour. In addition to belonging to distinct topological classes, the modes were found to possess orthogonal polarisations at the sample plane, in the close vicinity of the nanoparticles.
“We initially set out to investigate the coherence properties of these nanolasers in greater detail and were surprised to find no correlations between modes with different topologies and polarisations,” says Assistant Professor Antti Moilanen. “The key realisation was that the absence of mutual coherence was itself an important discovery.”
The finding could prove valuable for applications that require multiple optical channels to operate independently without interfering with one another.
“The ability to engineer mutually incoherent lasing modes is particularly exciting for sensing and optical communications,” says Doctoral Researcher Laura Yrjänheikki, lead author of the second study. “It provides a clean and robust way to separate channels while minimising unwanted crosstalk.”
A new design principle for plasmonic nanolasers
Taken together, these studies reveal a fundamental principle for coherence engineering in plasmonic nanolasers, which can be designed through nanoparticle geometry, lattice structure, mode topology and polarisation. Topologically trivial modes can synchronise through shared gain dynamics, while topologically distinct modes with orthogonal polarisations remain incoherent. The findings enable the rational design of multimode plasmonic nanolasers for applications ranging from ultrafast pulse generation to optical information processing, sensing, and communications.
“Coherence in plasmonic nanolasers is not simply an accidental property; it is something we can engineer through design,” says Professor Tommi Hakala, who led the research. “By designing nanostructures with specific mode topologies and polarisations, we can realise either ultrafast mode-locked pulse modulation or independent lasing channels. These studies demonstrate how the coherence properties of nanoscale lasers can be controlled through their underlying physics.”
The research combines advanced nanofabrication, plasmonic lattice engineering, ultrafast spectroscopy, and topological photonics, demonstrating how fundamental control over light-matter interactions can translate into new functionalities for nanoscale laser technologies. The studies were carried out at the Center for Photonics Sciences, Department of Physics and Mathematics, University of Eastern Finland, Joensuu.
For further information, please contact:
Tommi Hakala, Professor, University of Eastern Finland, tommi.hakala(at)uef.fi
Publications
J. I. Heikkinen, R. Calpe, L. Yrjänheikki, A. Halder, A. J. Moilanen, M. Koivurova and T. K. Hakala, Mode-Locked Pulse Generation in Plasmonic Lattices, Laser & Photonics Reviews 20(7), e02215 (2026). https://doi.org/10.1002/lpor.202502215
L. Yrjänheikki, R. Calpe, M. Nečada, J. I. Heikkinen, M. Koivurova, A. J. Moilanen, and T. K. Hakala, Tailorable Topological Multimode Nanolaser with Mutually Incoherent Modes, ACS Nano 20(27), 19191-19201 (2026). https://doi.org/10.1021/acsnano.5c22211
Image caption:
Artistic illustration of two plasmonic laser designs. A metallic nanoparticle superlattice (left) generates ultrafast mode-locked laser pulse modulation from modes with the same topology. In contrast, a simple square lattice (right) supports lasing in two modes that have different topologies and polarisations, enabling separate, crosstalk-free laser channels. Image: Laura Yrjänheikki and Tommi Hakala.
