A single living cell has been coaxed into producing laser light, researchers report in Nature Photonics.
The technique starts by engineering a cell that can produce a light-emitting protein that was first obtained from glowing jellyfish.
Flooding the resulting cells with weak blue light causes them to emit directed, green laser light.
The work may have applications in improved microscope imaging and light-based therapies.
Laser light differs from normal light in that it is of a narrow band of colours, with the light waves all oscillating together in synchrony.
Most modern forms use carefully engineered solid materials to produce lasers in everything from supermarket scanners to DVD players to industrial robots.
The new work, by Malte Gather and Seok Hyun Yun at the Wellman Center for Photomedicine at Massachusetts General Hospital in the US, marks the first time the phenomenon has been seen in a living system.
The pair used green fluorescent protein (GFP) as the laser's "gain medium", where light amplification takes place.
GFP is a well-studied molecule, first isolated from jellyfish, that has revolutionised biology by acting as a custom-made "torch" that can light up living systems on command.
In the new work, cells derived from human kidney cells were genetically engineered to produce GFP.
Bathed in light
The cells were then placed one at a time between two tiny mirrors, just 20 millionths of a metre across, which acted as the "laser cavity" in which light could bounce many times through the cell.
Upon bathing the cell with blue light, it could be seen to emit directed and intense green laser light.
The cells remained alive throughout and after the process. The authors note in an accompanying interview in the journal that the living system is a "self-healing" laser; if the light-emitting proteins are destroyed in the process, the cell will simply produce more.
"In cellular sensing, we may be able to detect intracellular processes with unprecedented sensitivity," they said.
"For light-based therapeutics, diagnosis and imaging, people think about how to deliver emission from an external laser source deep into tissue. Now we can approach this problem in another way: by amplifying light in the tissue (itself)."