A new kind of laser captures light just like some colorful bird feathers. The device mimics the nanoscale structure of colorful feathers to make high-intensity laser light with almost any color.
Lasers work by trapping light in or near a material that can emit more photons with the same wavelength, or color. Incoming photons excite the atoms in the material, and make them spit out more identical photons. But to get enough photons for a bright beam of laser light, the photons need to hang around in the material for a long time.
One way to buy time for photons is by forcing them to bounce back and forth. Traditional lasers do this by bouncing the photons between two mirrors. In recent years, physicists have built lasers from slabs of specialized glass with air holes drilled in them. Light can get trapped on a particular path between the holes, and bounce around long enough to make laser light.
Physicists have tried arranging the holes in both tightly ordered and completely random patterns. But both of those options had drawbacks ? ordered lasers only work at one wavelength and are expensive to build, and random lasers aren?t very efficient.
Physicist Hui Cao of Yale and colleagues tried something in between: an arrangement of holes that looks random from afar but has pockets of order up close. This is similar to the setup of air pockets in bird feathers. Their results are published May 6 in Physical Review Letters.
Certain brightly colored birds, like kingfishers or parrots, have feathers embedded with a not-quite-random arrangement of air pockets. Wavelengths of light that are related to the distance between the air pockets get scattered and built up more than others, giving the feathers their characteristic colors.
?After we learned this, we said, ?Oh, that?s a smart idea!?? Cao said. ?Can we use this to improve our lasers? Maybe we can use short-range order to enhance light confinement and make lasing more efficient.?
Cao?s team drilled holes in a 190-nanometer-thin sheet of gallium arsenide, a special sort of plastic that transmits light efficiently and is commonly used in optics. The holes were spaced between 235 and 275 nanometers apart. The plastic included a layer of equally spaced quantum dots, which emit lots of light when struck with one photon. When light entered the plastic, the physicists reasoned, it should bounce around between the holes long enough to make the quantum dots produce enough photons to start lasing.
When the researchers lit up the tiny wafer, it produced laser light with wavelengths of about 1,000 nanometers, in the near-infrared range of the electromagnetic spectrum. It was much more efficient than random lasers. The researchers also found that they could change the wavelength of the laser light by changing the spacing between the holes.
?Just like the birds, who can tune their short-range order to get different color from their feathers. We can do the same thing,? Cao said.
Cao doesn?t have any particular applications in mind for this tunable, efficient laser. But she points out that by giving up on long-range order, her laser is much cheaper and easier to build than previous models.
?We can have control, and it doesn?t have to be perfect,? she said. ?That?s what we learned from nature.?
Cao and colleagues are now trying to use actual bird feathers as a template. They hope to embed tiny semiconductors in the air holes and dissolve away the keratin that holds them together. This might be an easier way to make lasers with extremely short wavelengths, in the blue or ultraviolet range.
It might be even more interesting to figure out how the birds build their feathers in the first place, said biologist Matt Shawkey of the University of Akron in Ohio.
?Birds seem to do it very cheaply. They have thousands of these feathers,? he said. ?If you can get these things to build themselves, taking the painstaking process out, then you?d barely have to put any energy and time into it. It would be really cool to see which parameters the birds are changing to get these feathers to self-assemble.?
Images: 1. A kingfisher?s bright colors come from light scattering off a not-quite-random arrangement of air pockets. (Pkhun/Wikipedia) 2. A scanning electron microscope image of the air pockets in a bird?s feather. (Hui Cao) 3. The semirandom arrangement of holes in the laser mimics the arrangement in birds? feathers. (Hui Cao)
?Control of Lasing in Biomimetic Structures with Short-Range Order.? Heeso Noh, Jin-Kyu Yang, Seng Fatt Liew, Michael J. Rooks, Glenn S. Solmon and Hui Cao. Physical Review Letters, 106, 183901. May 6, 2011. DOI: 10.1103/PhysRevLett.106.183901.
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