Mother Nature probably wasn’t thinking about lasers when she invented the jellyfish, but it turns out the sea creature evolved a substance well suited to letting a new type of laser work at room temperature.
Scientists in Scotland and Germany have used green fluorescent protein (GFP), a common tool for biological imaging first derived from a species of jellyfish, as the active material in a polariton laser. Such a laser might be used as a tag for tracking how cancer cells spread through the body, or could lead to optical logic on computer chips, faster data transmission, or even quantum computing. “Nature has provided us with a material that has properties that are very useful for this kind of device,” says Malte Gather, a professor of physics at the University of St. Andrews, Scotland. With Sven Höfling, a physicist at St. Andrews and the University of Würzburg, Germany, he and his fellow researchers published a description of their device in the current issue of Science Advances.
Polariton lasers work differently than traditional lasers. They rely on polaritons, quasi-particles that consist of an electron-hole pair (also known as an exciton) mixed with a photon. When energy is pumped into the system, the density of polaritons can become high enough for them to synchronize with each other and form a condensate, which then quickly releases the energy as a beam of coherent photons. This release of photons can take place at energies much lower than in a traditional laser, as no population inversion is required for this to happen. So, less energy is required to run them, which would be a particular advantage for communications on a computer chip.
But the problem with polariton lasers has been that the excitons tend to be unstable and can also bump into and annihilate each other too quickly. One way scientists had gotten around this was by supercooling the devices to cryogenic temperatures to stabilize the excitons, but that makes the lasers more complex and expensive.
The team tackled that issue by using enhanced GFP, a version of the protein that had been genetically manipulated to shine more brightly than the natural substance. They created a laser cavity by sandwiching a thin film of the GFP—500 nanometers thick—between two mirrors. The GFP molecule is naturally arranged into a bunch of tiny cylinders, each a few nanometers in size and consisting of 11 sheets of atoms. The part of the molecule that fluoresces is nestled in the center of the cylinder, out of contact with the others of its kind, so the excitons can’t annihilate one another as easily. “The cylinder helps protect the light-emitting component from its environment,” Gather explains.
Though this is not the first room-temperature polariton laser, Gather says that it should be more compatible with biological applications than a semiconductor version. He envisions using these lasers as tags for cancer cells spreading throughout the body. Cells with minute genetic differences could be tagged with lasers emitting different wavelengths of light, allowing scientists to study how the cells change as they spread through the body. Current fluorescent tags only come in a handful of different colors; Gather says it might be possible to produce at least 5,000 slightly different wavelengths with lasers.
He hopes to study other biological materials capable of emitting light in other colors besides green. There are proteins that work in every part of the visible spectrum and into the near-infrared, he says, including a red protein from coral.