Science1 publisher2 min readPublished
Gold nanoparticle lattices phase-lock two lasing modes through femtosecond hotspot correlations
In two companion papers from the University of Eastern Finland, modes sharing a topological class locked together into ultrafast pulse modulation, while modes of differing topology and polarization lased side by side with no mutual coherence.
The Scientist · Science desk
What happened
- Researchers at the University of Eastern Finland reported two complementary mechanisms that decide whether the lasing modes of a metal nanoparticle array embedded in a gain material share coherence.
- In the Laser & Photonics Reviews study, a nanoparticle lattice with a liquid dye gain medium under optical pumping produced two lasing modes that phase-locked through near-field interactions and gave ultrafast pulse modulation.
- In the ACS Nano study, gold nanoparticle arrays lased simultaneously in two mode families that stayed mutually incoherent, with no ultrafast pulse modulation and no sign of phase locking.
- Those non-locking modes also carried orthogonal polarizations at the sample plane close to the nanoparticles, on top of belonging to different topological classes.
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Why it matters
- capability Two lasing channels on one array with no coherence between them is the ingredient a low-crosstalk on-chip link needs, and here it came out of the particle geometry itself.
- decision Someone designing a nanoscale source can now treat synchronized pulsed output and independent channels as a layout choice made at fabrication, since the two behaviours came from different lattice arrangements.
- constraint The comparison that showed no locking changed topology and polarization together, so an engineer cannot yet tell from it which of the two has to be broken to keep channels apart.
- constraint Anyone costing a chip-scale ultrafast source has to reproduce the phase locking outside a liquid dye pumped by another laser, and that has not been shown.
Phase locking needs something to couple the modes. In the superlattice device that coupling came from the gain medium the two modes shared, and the numerical work put femtosecond-scale correlations at the plasmonic field hotspots around the nanoparticles. "We found that correlations formed at the plasmonic electric field hotspots around the nanoparticles enabled phase locking between multiple lasing modes," said postdoctoral researcher Roman Calpe [7][4]. Doctoral researcher Janne Heikkinen, lead author of that study, said the instrument was the hard part: "A major challenge was developing a measurement setup capable of resolving these ultrafast dynamics" [5].
The second sample was built so two different mode families would lase at once. Varying the diameters of the gold nanoparticles across the array let topologically trivial dipolar modes and topologically nontrivial quasi-bound-state-in-the-continuum modes form together [8]. In photonics, that topological label sorts modes into trivial and nontrivial classes, and has also been used to send light in one direction only, in the manner of an electrical diode [14].
"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 polarizations," said assistant professor Antti Moilanen [11].
The two mode families in that sample differed in two ways at once. They belonged to distinct topological classes, and close to the nanoparticles their polarizations were orthogonal at the sample plane [10]. Either difference could account for the missing correlations, and this measurement does not separate them [15]. A structure whose trivial and nontrivial modes share a polarization would test the topology half on its own.
The pulsed result is a laboratory arrangement: a nanoparticle lattice combined with a liquid dye gain medium and optically pumped [3]. The phys.org account does not report pulse durations, modulation rates or lasing wavelengths, so the output cannot yet be set against commercial mode-locked lasers [16]. The Eastern Finland group names chip-scale ultrafast light sources and low-crosstalk photonic technologies as the applications the work opens [13], and describes the two papers together as design principles for nanoscale photonic devices [12]. As a design rule, that is one demonstration on each side of the coherence question [2][9].
What to watch
- A structure whose trivial and nontrivial modes share a polarization, which would test topological class as the deciding property on its own.
- Whether the phase-locked pulse modulation survives in a solid or electrically pumped gain medium instead of a liquid dye.
- Pulse durations and modulation rates in the papers themselves, needed before this source can be compared with existing mode-locked lasers.