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Adding a smattering of bacteria can help a laser beam travel further through murky water before it peters out. The finding could help us perform non-invasive medical diagnostics or image deep into tissue without causing any damage.
The presence of particles in a liquid normally causes light to scatter – that’s why your car headlights don’t penetrate far in dense fog. But when Zhigang Chen at San Francisco State University and colleagues shone a high-intensity green laser through seawater containing a cyanobacteria called Synechococcus, they found the light travelled further than they expected.
This boost is the result of a previously known effect: light exerts a force on the cells because their refractive index differs from that of the seawater they are sitting in. In this case, the force pulls the cells toward the centre of the light beam. Once there, another force causes the cells to align along the direction of the beam while also pushing them away from the centre.
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This forms a bacteria “fibre” surrounding the beam, says Chen, which can act as a waveguide – constricting and guiding light travelling inside it. The effect lets the laser travel an extra few centimetres through the water.
Medical applications
The idea could be used to create environmentally friendly optical components that can be tailored to a range of applications, says Chen. “It may be possible to grow and fix biological waveguides and produce biological microchip technology.”
Despite being hit by a high-power laser, most of the cyanobacteria survived. The team is now studying whether human red blood cells can produce a similar waveguide effect and survive the process. If so, it would open the technique up to medical applications, such as imaging through biological fluids.
However, it remains to be seen whether this works in practice.
“A drawback is that the effect only occurs at very high laser power,” says Christof Gebhardt, a biophysicist at Ulm University in Germany. This power level generally destroys mammalian cells, he says.
Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.119.058101
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