Normally, light cannot be focused to a spot smaller than half its wavelength, something known as the diffraction limit. However, in recent years, scientists have managed to compress light down to the nanoscale by coupling it to the conduction electrons that oscillate collectively at the surface of metals, or surface plasmons. In the case of the metal nanoparticle arrays, the resulting excitations of light and electrons are referred to as "lattice plasmons."

Most plasmon-based lasers are difficult to tune easily – and especially in real time – because the gain is made from solid materials, such as inorganic semiconducting nanowires or organic dyes in a solid matrix.

Liquid gain materials

Now, researchers led by Teri Odom are saying that they may have found a way to make a tunable plasmon laser by using liquid gain materials with the plasmonic nanocavity arrays. Indeed, we have found that we can tune the nano-device’s emission wavelength as a function of the refractive index of the solvent, explains Odom.

“Using liquid gain materials has two main advantages,” she tells nanotechweb.org. “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 the 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 allows us to dynamically tune the lasing emission by simply flowing in liquids with different refractive indices.”

Ultrasensitive sensors and lab-on-a-chip devices

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

Xiang Zhang of the University of California at Berkeley says that making a tunable plasmon laser using microfluidics is interesting. "Such a configuration could be very useful in biomedical dignostics 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 these nanoscopic 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 device is detailed in Nature Communications doi:10.1038/ncomms7939.