Filtering Reflections by Angle
The ability to selectively filter light is invaluable for many systems, and we have ways of doing so based on the light's frequency or polarization. Filtering light based on its direction however, has been proving difficult. Researchers at MIT though have successfully achieved just that with a precisely engineered mirror.
This mirror is different than what you may find in your home though because it is made up of many ultrathin layers of two materials, alternating between them. At the interfaces between the layers, some light will be reflected, but at what is called the Brewster angle, there are no reflections and light is able to pass through. Previous research has demonstrated selectively reflecting light based on angle, but that was limited to a smaller range of frequencies, while this approach is able to reflect the entire visible spectrum.
With the current design, the selectivity is limited to a range of about ten degrees, but by adding more layers it should be possible to shrink that. Potentially this research could be used to improve solar thermophotovoltaics, where controlling reflections is important, and to create displays with viewing angles limited to those directly in front of them.