A potential problem in achieving lasing using nanoparticles was that light may not exist long enough in small dimensions to be helpful. This challenge was accepted by a research team from Aalto, who found a smart way by producing lasing in dark modes.

By using electron beam lithography, the researchers built 100 x 100μm² arrays of silver nanoparticles (60nm x 30nm) on a borosilicate glass, which are about 400nm apart. They added Rhodamine 6G dye molecules to modify the refractive index of the surrounding 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 was 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 team then observed a low beam divergence of 0.3 degree and spatial coherence lengths of 100μm, which would be enough to design out-coupling mechanisms for integration into photonic devices.

Dark modes are attractive for applications where low power consumption is needed. But without any tricks, dark mode lasing would be quite useless because the light is trapped at the nanoparticle array and cannot leave. But by utilising the small size of the array, there would be an escape route for the light.

"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.