More precise control of light could be on the horizon, prompting advances in solar photovoltaics, detectors for telescopes and microscopes, and privacy filters for display screens. Researchers from MIT have developed the first system that allows light of any wavelength to pass through from a specific direction and that reflects all light coming from other angles. “It is a very fundamental building block in our ability to control light,” said researcher Marin Soljacic, a professor of physics at MIT. In this angular-selective sample, a beam of white light passes as if through transparent glass. The red beam comes at a different angle and is reflected away, like a mirror. Courtesy of MIT. Alternating a stack of about 80 ultrathin layers of two alternating materials, the investigators found that the thickness of each layer could be precisely controlled. The interface between the two materials typically causes reflections. However, at the Brewster angle, which provides appropriate polarization, there is no reflection at all. Researcher Yichen Shen, a graduate student at MIT, said, “We are able to reflect light at most of the angles, over a very broad band [of colors] — the entire visible range of frequencies.” The angular selectivity could be made narrower by adding more layers to the stack. Experiments conducted so far have seen the angle of selectivity at about 10°; about 90 percent of the light coming within that angle was allowed to pass through. The study was performed with layers of glass and tantalum oxide, but the researchers say that any two materials with different refractive indices could be used. Optical systems, such as microscopes and telescopes, could benefit from the researchers’ findings, allowing for better viewing of faint objects that are close to brighter ones. The filtering also could be applied to display screens on phones or computers, allowing only those viewing from directly in front to see them. Shen said, “This could have great applications in energy, and especially in solar thermophotovoltaics.” The work was funded by the Army Research Office, MIT's Institute for Soldier Nanotechnologies, the US Department of Energy and the MIT S3TEC Energy Research Frontier Center. The research is published in Science (doi: 10.1126/science.1249799). For more information, visit: www.mit.edu
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