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17 Jun 2002
Researchers at 3M in St Paul, MN have made polymer-based dielectric mirrors that reflect visible light from all angles.
Dielectric mirrors, those based on alternate layers on transparent materials that each reflect a small fraction of the light that hits them, are better than polished metal mirrors for optoelectronics applications because 'normal' mirrors tend to absorb light. However, standard dielectric mirrors are not usually good at reflecting light that does not hit them perpendicularly. This phenomenon, called Brewster's Law, has been known for almost 200 years and was thought to be a barrier to applications of mirrors that consist of multiple layers of dielectric materials.
The latest research avoids problems with Brewster's Law by designing polymer layers utilizing a technique known as giant birefringement optics, which relies on tailoring the way in which the refractive index of a material varies with angle. The researchers have made mirrors from simple plastics that maintain or increase their reflectance as the incident angle of the light increases.
Two years ago, scientists at the Massachusetts Institute of Technology succeeded in reflecting infrared light in all directions with a dielectric mirror made from a mixture of organic and inorganic materials (see Science, 27 November 1998, page 1679).
These results are published in the 31 March issue of the journal Science but 3M has already begun commercializing applications for this technology. The mirrors help to pipe visible light over long distances without affecting its color or intensity. The technique can also be adapted to create mirrors that only reflect certain wavelengths of light. This offers applications for making brighter and more readable displays.
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