Researchers at MIT have created the first perfect mirror using a new method of trapping light that could lead to breakthroughs in the efficiency of fiber optic networks and optical instruments.
“For many optical devices you want to build, you need a way to confine light,” Marin Soljačic, an assistant professor in the Department of Physics and a member of MIT’s photonics and electromagnetics group, said. Traditional methods of trapping light often employ mirrors, other reflective surfaces or high-tech materials such as photonic crystals that rely on a general wave phenomenon known as Anderson localization. In all of these cases, Soljačić says, the passage of light is blocked and it is forced into a reflection. The reflection is never perfect because some of the energy is absorbed by the reflective material or scattered at different angles.
“For a human checking their hair or makeup, this lack of perfection doesn’t matter; but when you’re talking about reflecting lasers down a hundred miles of optic fiber, or solar power installations, these tiny imperfections can cause a huge drop in efficiency,” Extreme Tech explained. In contrast, a perfect reflection could help retain this energy.
The new phenomenon was discovered almost by accident while Soljačić and his colleagues were studying the behavior of a photonic crystal that had holes drilled into it to form a lattice. Developed using computer modeling and then demonstrated experimentally, the “perfect mirror” concept pits two light waves with the same wavelength, but exactly opposite phases, against one another. While these two waves with similar wavelengths effectively cancel each other out in a phenomenon known as destructive interference, light of other wavelengths can still pass through freely, preserving the original image or signal.
In the case of the work completed by the MIT research team, Soljačić and his colleagues determined that while at most angles, light was partially absorbed by the photonic crystal as they expected, with a specific wavelength of red light at an angle of 35 degrees, the light was perfectly reflected with no absorption or scattering.
The discovery is published in the journal Nature by Soljačić and professor of physics John Joannopoulos, associate professor of applied mathematics Steven Johnson, and graduate students Chia Wei Hsu, Bo Zhen, Jeongwon Lee and Song-Liang Chua.
“It’s a very different way of confining light,” Soljačić said. He and his colleagues believe the phenomenon is also capable of reflecting other types of waves—including sound waves, radio waves, electrons and even waves in water—with absolutely zero distortion. As Extreme Tech points out, the new technique could also be useful in the development of invisibility cloaks.
A. Douglas Stone, a professor of physics at Yale University who was not involved in the research, said that the demonstration is “very significant, because it represents a new kind of mirror which, in principle, has perfect reflectivity.” The finding, he added, “is surprising because it was believed that photonic crystal surfaces still obeyed the usual laws of refraction and reflection,” but in this case, they do not.
The research team remains in the process of discerning why this deviation from known scientific laws took place, but, according to graduate student Chia Wei Hsu, potential applications in a variety of industries in the near future are a possibility.
