Researchers develop simple method for light control
That finding that could ultimately lead to advances in solar photovoltaics, detectors for telescopes and microscopes, and privacy filters for display screens.
Light waves can be defined by three fundamental characteristics: their color, or wavelength, polarization, and direction. While it has long been possible to selectively filter light according to its color or polarization, selectivity based on the direction of propagation has remained elusive.
In a paper published in the U.S. journal Science, researchers at the Massachusetts Institute of Technology (MIT) and Zhejiang University, however, reported a system that allows light of any color to pass through only if it is coming from one specific angle while reflecting all light coming from other directions.
The researchers said they "are excited" because it represents " a very fundamental building block" in the efforts to control light.
According to Yichen Shen of the MIT, lead author of the study, the structure of the system consists of a stack of ultrathin layers of two alternating materials where the thickness of each layer is precisely controlled.
"When you have two materials, then generally at the interface between them you will have some reflections," Shen said. "But at these interfaces, there is this magical angle called the Brewster angle, and when you come in at exactly that angle and the appropriate polarization, there is no reflection at all."
While the amount of light reflected at each of these interfaces is small, by combining many layers with the same properties, most of the light can be reflected away except for that coming in at precisely the right angle and polarization, he explained.
Using a stack of about 80 alternating layers of precise thickness, Shen said they are able to reflect light at most of the angles over a broad range of wavelengths.
The new findings could have great applications in energy, and especially in solar thermophotovoltaics, by ways of selectively controlling light reflections to improve its efficiency, Shen said.
The findings could also prove useful in optical systems, such as microscopes and telescopes, for viewing faint objects that are close to brighter objects, for example, a faint planet next to a bright star.
"By using a system that receives light only from a certain angle, such devices could have an improved ability to detect faint targets," he said.
The filtering could also be applied to display screens on phones or computers, so only those viewing from directly in front could see them.
While these experiments were done using layers of glass and tantalum oxide, Shen said that in principle any two materials with different refractive indices could be used. ■
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