Angle-sensitive device could improve photography and solar energy harvesting
Weishun Xu and Yuhao Zhang
Like a traffic cop directing cars, a layered stack of transparent materials permits light arriving from only a single direction to pass through. Such angle-sensitive filters could improve cameras and telescopes or allow solar cells to convert sunlight into energy more efficiently.
People have created effective light filters for thousands of years. Stained glass, for instance, filters light by color, allowing only a single shade to shine through while reflecting light at other wavelengths. But scientists have had trouble filtering light based on the angle from which it arrives.
In creating a directional light filter, MIT physicist Yichen Shen and his team knew that for the interface between any pair of materials, light arriving from one specific angle, known as the Brewster angle, can simply pass through unimpeded — it won’t get reflected or bent.
Knowing an interface’s Brewster angle provided a way to allow light from a desired angle to pass through, but Shen’s team still had to figure out how to block out the light from all other directions. They solved the problem by creating a stack of 84 alternating layers of two transparent materials: glass and tantalum oxide. Whenever light struck the stack at any angle other than the Brewster angle, it underwent some combination of reflection and bending as it encountered each boundary between layers. By the time the light reached the 83rd and final boundary, it got totally reflected away. “It’s a very clever approach,” says Peter Bermel, an electrical engineer at Purdue University in West Lafayette, Ind.
To test their device, Shen and his team pointed a camera at a picture of a rainbow and placed their 2-by-4-centimeter rectangular stack in between. (The stack was placed in a tank of colorless liquid that has the same optical properties as glass so that no light would bend or reflect when passing from the liquid to the glass part of the stack.)
At most angles, the stack acted like a mirror, showing an image of the camera. It kept its reflective properties and remained a mirror when the researchers rotated the sample. Only at the magic angle of 55 degrees — the Brewster angle — did the glass-tantalum oxide stack become as transparent as a windowpane and the rainbow came into view, the researchers report in the March 28 Science. “It works over the entire range of visible wavelengths,” Shen says, and it should also work with nearly any pair of transparent materials. Bermel hopes to see future research combine angular control with filters of color and polarization.
Shen envisions someday fitting a camera with an angle-sensitive filter that would address the tricky proposition of photographing people outside on a bright day. As long as the sun isn’t directly behind the person being photographed, the filter would eliminate background glare. Similarly, the technology could work in telescopes so that a backyard astronomer can spy on a faint star or galaxy that would otherwise be overwhelmed by the brightness of a neighboring object.
Shen says a variation of the device could also improve solar thermal energy systems, which focus sunlight and generate energy from its heat. A directional filter could allow the sun’s rays, which come in at a predictable angle, to pass through but prevent the escape of heat in the form of infrared light.
Y. Shen et al. Optical broadband angular selectivity. Science. Vol. 343, March 28, 2014, p. 1499. doi: 10.1126/science.1249799.