Physicists Unveil Tiny New Cancer-Busting Nanolasers

​Physicists at Northwestern University have created perhaps the smallest tunable laser beam ever, according to ​a new report in Nature Communications. The work, which exploits the peculiar tendencies of particles known as surface plasmons, paves the way toward new and better ways of detecting cancer biomarkers in very low concentrations, e.g. rooting out cancer while it's still easily treatable—or treatable at all.

In more precise terms, the group has beaten the ​diffraction limit, which is basically the maximum resolution imposed on an optical system. In laser terms, it's the smallest possible "spot" that can be achieved by a beam, which, in the very best case scenario, can only be as small as about half the wavelength of the light in question. This puts the lower limit on beam resolution at a couple hundred nanometers, at best.

Yet optical physicists have been beating this limit thanks to surface plasmons, which are the electrons found at the surface of a conducting metal known to exhibit a strange sort of collective behavior. Basically, the whole mess of electrons oscillate together as if they're just one single electron. In a plasmonic laser, where light is coupled to these oscillating electrons, some of the light's energy gets stored among them.

The general effect is of more laser/light in a smaller beam, allowing a violation of the diffraction limit. The result: nanoscale lasers.

There's a pretty big catch, however. For the most part, physicists have been beating the diffraction limit thanks to plasmons lining a solid metal conductor. And using solid materials, "precludes the possibility of dynamic tuning," the paper explains. In other words, the resulting tiny laser can only really operate at one wavelength, which is a significant barrier. The Northwestern group swaps this solid conductor for a "tunable lattice," which is where the conducting material is dissolved in a liquid.

"Using liquid gain materials has two main advantages," Terri Odom, a Northwestern University materials scientist and the lead author behind the new paper, ​told Physics World. "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."

The possibilities of such a technology include the sensing of weak physical and chemical processes that would otherwise be unlikely to register using (relatively) macroscale techniques. Early detection of cancer and other biomarkers is one potential aspect, but, as Odom and co. note in the current paper, this could also enable new possibilities for ​"lab-on-a-chip" technologies.