Using electron beam lithography, they've built 100x100μm 2 arrays of silver nanoparticles (60nm in diameter and 30nm high) on a borosilicate glass, with the nanoparticles spaced about 400nm from each other. The added solution of Rhodamine 6G dye molecules modifies the refractive index of the surrounding the structure, shifting the surface lattice resonances of the metal nanoparticles. The dye molecules were then excited with a 100fs laser pulse (at a 500nm centre wavelength) and lasing of the nanoparticles was observed at visible wavelengths and at room temperature.
The nanoparticle array lased both in dark and bright modes, with narrow linewidths of 0.2nm and an increase of four orders of magnitude in emission intensity above threshold. The researchers observed a low beam divergence of 0.3° and spatial coherence lengths of 100μm, which would be enough to devise out-coupling mechanisms for integration into photonic devices.
"The dark-mode out-coupling mechanism that we introduce is not simple scattering or leakage from sharp edges of the system, rather, it is gradual, coherent build-up of dipole moments and radiation intensity. This inspires ideas for the design of not only out-coupling schemes but also beam guiding, trap potentials, topologically non-trivial lattices and edge modes, for instance by gradually changing the pitch and by particle shapes supporting higher order multipoles", the researchers concluded.
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