Optical Angular Selectivity Achieved
Some of you may have noticed that looking at water with polarized sunglasses can cause the water to change appearance, depending on the angle you look at it. The reason for this is that at a certain angle, the Brewster angle, the light reflecting off of the surface is polarized. Researchers at MIT have recently developed a way to manipulate the Brewster angle to create a material that will reflect light at every angle, but one.
Normally when light comes to the boundary of two materials with different refractive indices, it will be mostly reflected, but at the Brewster angle, only some of the light is reflected, with the rest passing through. What the MIT researchers have done is created a photonic crystal that has a very narrow band of angles that will allow light to pass through it. Basically it is a mirror at all but a single angle, its Brewster angle, when it becomes transparent. To create this crystal, the researchers actually stacked one hundred photonic crystals with alternating refractive indices.
There are a number of potential uses for this angular selectivity, including privacy filters, light detectors, telescopes, and even solar power. As sunlight strikes a solar panel, it will heat up and radiate out some of the energy, but with an angular selective layer on top, the sunlight could be let through while the radiated energy is reflected back to the absorber.