An entirely new way to filter light, one that has long been sought but until now remained elusive, has been achieved by researchers at MIT.
For the first time, it’s become possible to selectively filter light according to its direction of propagation — in other words, to filter it based on where it’s coming from.
The new system from MIT “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 technical breakthrough should lead to advances in solar photovoltaics, telescopes, microscopes, and a number of other consumer products.
“We are excited about this,” states professor of physics Marin Soljačić, “because it is a very fundamental building block in our ability to control light.”
The new system relies on a structure that consists of a stack of ultrathin layers, composed of two alternating materials, where the thickness of each layer is controlled to a precise degree.
“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.”
MIT adds more:
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, states MIT graduate student Yichen Shen, “we are able to reflect light at most of the angles, over a very broad band (of colors) the entire visible range of frequencies.”
Previously, the researchers had already demonstrated approaches which allowed for the selective reflecting of light, but these were all very limited with regards to colors. The new approach is considerably different — the breadth will allow for a considerably greater number of applications.
Shen explains: “This could have great applications in energy, and especially in solar thermophotovoltaics — harnessing solar energy by using it to heat a material, which in turn radiates light of a particular color. That light emission can then be harnessed using a photovoltaic cell tuned to make maximum use of that color of light. But for this approach to work, it is essential to limit the heat and light lost to reflections, and re-emission, so the ability to selectively control those reflections could improve efficiency.”
Other potential applications are possible in the fields of microscope and telescope technology. For example, “by using a system that receives light only from a certain angle, such devices could have an improved ability to detect faint targets.”
Another interesting possibility is as a means of improving the privacy of display screens on phones, tablets, computers, etc — with this system applied, products could be made so that only those directly in front of the device would be able to see anything.
The new findings are detailed in a paper published this week in the journal Science.