Researchers in the US say they have succeeded in tuning the wavelength of light emitted by a tiny laser made from plasmonic nanocavity arrays. The plasmon laser, which consists of arrays of gold nanoparticles surrounded by dye molecules dispersed in a liquid, is robust and can operate at room temperature. An important use for the new device could be detecting cancer biomarkers at very low concentrations, say the scientists.

Normally, the diffraction limit means that light cannot be focused to a spot smaller than half its wavelength – about 250 nm for green light. This puts a limit on how small a laser or other optical device can be made. But in recent years, scientists have managed to compress light down to much smaller length scales by coupling it to surface plasmons – conduction electrons that oscillate collectively at the surface of a metal. A similar effect occurs in arrays of tiny metal nanoparticles, where the resulting excitations of light and electrons are called "lattice plasmons".

Most plasmon-based lasers operate at fixed wavelengths, and it is very difficult to adjust the wavelength, especially in real time. This is because the wavelength of a plasmon laser is defined by its gain material, which is normally a solid such as an inorganic semiconducting nanowire or an organic dye in a solid matrix.

Liquid gain materials

Now, researchers led by Teri Odom of Northwestern University say that they may have found a way to make a tunable plasmon laser by filling a plasmonic nanocavity array with a gain material that is a liquid solvent. This allows them to change the emission wavelength of the laser by adjusting the refractive index of the solvent used.

"Using liquid gain materials has two main advantages," Odom explains. "The first is that organic dye molecules can readily be dissolved in solvents with different refractive indices. So, the dielectric environment around the nanoparticles can be tuned, which also enables us to tune the lasing wavelength in real time. Second, the fact that the gain materials are in liquid form allows us to manipulate the gain fluid within a microfluidic channel, which means that we can dynamically tune the lasing emission by simply using liquids with different refractive indices."

And that is not all: the researchers say that their tiny lasers are easy to fabricate and can emit light over the entire gain bandwidth of the dye employed. "This means that with the same nanocavity structure, that is the same nanoparticle arrays, we can tune the lasing wavelength over 50 nm (from 860 to 910 nm) by simply changing the solvent the dye is dissolved in," says Odom.

Molecular sensing

Xiang Zhang of the University of California, Berkeley, who was not involved in the project, says that making a tunable plasmon laser using microfluidics could lead to interesting practical applications. "Such a configuration could be very useful in biomedical diagnostics and molecular sensing in liquid environments. We could mix a cancer biomarker in the liquid gain, for example, and detect it with very high sensitivity, provided that heating is not an issue near the plasmon particles."

Odom says that the tiny light sources could be used in ultrasensitive sensors to detect weak physical and chemical processes on the nanoscale. They might also be integrated in lab-on-a-chip devices, she adds.

The tunable plasmonic laser is described in Nature Communications.