Lasers are coherent light sources routinely employed in optical communications, supermarket checkout tills and in computer printing. To make sure that the laser stably outputs at single wavelengths, conventional lasers exploit specific mode-selection rules to produce a central colour with a narrow bandwidth.

Multimodal lasers are different in that they can simultaneously emit light at different wavelengths. However, for these devices to work, single-colour lasers need to be operated as an array of lasers. This significantly adds to unit costs and, importantly, means that they cannot be integrated in compact photonic devices.

A team led by Teri Odom has now developed the first multimodal laser able to emit light of different colours in a single device. The laser is made of a nanoparticle superlattice integrated with a liquid gain in a platform that offers access to different colours with tuneable intensities depending on the geometric parameters of the lattice.

Nanolasing cavities

The nanoparticles in the device act as nanolasing cavities that support localized optical fields extending tens of nanometres from their surface. When these nanoparticles are arranged in a 2D array, they can then interact with each other to form a new type of photon-plasmon excitation called a lattice plasmon. Plasmons are collective oscillations of electrons on a metal surface.

The cavities are surrounded by a gain medium made of an organic solvent with dye molecules. At certain photonic states (the band edge states), the lattice plasmons stop moving and they can be amplified by stimulated energy transfer from photoexcited dye molecules in the gain to generate coherent laser light. When the amplified light constructively interferes, light can be emitted perpendicular to the nanoparticle array surface at well-defined wavelengths.

Distinct electromagnetic field distributions

The new multimodal devices are better than traditional devices in many ways, says Odom. “In a nanoparticle superlattice, each lasing mode has a distinct electromagnetic field distribution,” she explains. “For example, the field maxima of one mode could be located close to the nanoparticles, while another mode could have its field more concentrated in the region between the particles.”

This means that mode competition for the available gain at any overlapping region is minimized,” she tells nanotechweb.org. “This is typically a problem for multimodal lasers, since all the potential colours can end up collapsing into a single, dominant colour.”

Revolutionizing future laser cavity designs

The lasing beams in the new device can also be finely controlled, she adds. “Varying the nanoparticle superlattice geometry provides us with a robust way to manipulate the emission wavelengths, the angles at which light is emitted from the surface of the device and the numbers of multiple lasing beams. What is more, tuning the nanoparticle size allows us to modulate the output intensity of each lasing peak, something that is not possible in conventional lasers.”

And that is not all: the nanoparticle superlattice can be scaled and integrated with commercial optical devices. “This new-generation single architecture boasting multiple stable and tuneable lasing wavelengths will greatly improve optoelectronic device efficiencies and could revolutionize future laser cavity designs,” says Odom.

According to the Northwestern team, the new multimodal laser could be useful for the laser community interested in reducing the size of multi-wavelength-emitting devices to the nanoscale. It could also benefit researchers in photonics who would like to integrate coherent light sources on optical circuits and study strong light–matter interactions at the nanoscale.

The laser is detailed in Nature Nanotechnology doi:10.1038/nnano.2017.126.