Threading Light Through the Opaque
Freshly fallen snow is blinding white because the jumble of flakes scatter light in all directions. Such scattering also implies that little light passes through snow, so that if you're ever buried deep in it, you'll find yourself in the dark. But according to theoretical physicists, it should always be possible to fiddle with light waves to make them wend their way through such a disordered material, no matter how thick. And now a duo of experimenters has demonstrated that feat.
A typical light wave is a regular rippling of electromagnetic fields traveling through space, much like evenly spaced ripples lapping at a straight lakeshore. When such waves of light hit a disordered material, different parts of the wave reflect off the myriad surfaces in material in a random way; most of the light scatters from the surface and only a little of the light "diffuses" through. Less and less light gets through as the material is made thicker.
However, it should be possible to wedge light through the disordered solid. The idea is to shape the waves of light, pulling some parts of the wave forward and pushing others back so that when the different parts of the wave scatter in the material they reinforce each other in a process called constructive interference and bounce through an "open channel" (see figure). In the 1980s, theorists used a mathematical scheme called random matrix theory to show that, in principle, there should always be an open channel through a disordered material: Increasing the thickness reduces the number of these open channels, but some always remain. "However thick a material is, it should be possible to create a wave that can be transmitted," says Allard Mosk of the University of Twente in the Netherlands.
Now, Mosk and Ivo Vellekoop, also of the University of Twente, have shown how to find such a channel. They focused a laser beam onto an opaque layer of granular zinc oxide and used a digital camera to measure the light emerging from the other side. The researchers then used signals from the camera to control the shape of the incoming wave through a computerized feedback loop. A liquid crystal display shapes the wave by delaying single segments of the laser beam independently. By optimizing these delays, the researchers boosted the light reaching the camera by as much as 44% over the initial unshaped case, as they report in an upcoming Physical Review Letters article. The authors also doubled the opaque layer thickness from 5.7 to 11.3 microns and still obtained about the same increase in transmission. They can extrapolate from their results and show that the maximum transmission possible is two-thirds of the incoming light, which matches the prediction from random matrix theory.
"This is a profound experimental result," says John Pendry of Imperial College London, who was one of the theorists who helped develop random matrix theory. He says this 20-year-old idea has been well-accepted, but direct evidence for the channels has not been provided until now. Being able to enhance the light transmission through scattering materials could have potential applications in biological imaging and therapy, as well as improving cellular phone reception among tall buildings. "Disorder and diffusion crop up all the time and are mainly a nuisance," Pendry says. “But by understanding them, we can maybe use them to our advantage."