Filtering light based on direction
MIT researchers have developed a system that allows light of any color to pass through only if it is coming from one specific angle; the technique reflects all light coming from other directions. This new approach could ultimately lead to advances in solar photovoltaics, detectors for telescopes and microscopes, and privacy filters for display screens.
The work is described in a paper appearing in the journal Science, written by MIT graduate student Yichen Shen, professor of physics Marin Soljačić, and four others. The new structure 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,” Soljačić explains.
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.
Using a stack of about 80 alternating layers of precise thickness, Shen says, “We are able to reflect light at most of the angles, over a very broad band [of colors]: the entire visible range of frequencies.”