Polymer-Based Mirror Outshines All Others

These thin, flexible polymer mirrors should be a boon to fields such as optoelectronics, whose lasers, electronic displays, optical fibers, and other light-related technologies benefit everything from communications, to medicine, to astronomy.

Unlike other mirrors, these can reflect visible light from all angles with great efficiency, report Andrew J. Ouderkirk and colleagues in their Science paper.

The bathroom mirror you face bleary-eyed each morning is made by coating a hard surface with a thin layer of metal. These mirrors reflect light no matter what direction it's coming from. They aren't very useful for high-tech purposes, though, because they absorb some of the light instead of reflecting it.

For applications that require a more efficient reflection, dielectric mirrors are the tools of choice. These mirrors are made of alternating layers of transparent materials that each reflects a small fraction of the light that hits them. When the layers are just the right thickness, the reflected light waves merge and amplify, intensifying the reflection. (Think of a perfectly timed merge between two ocean waves that produces a whitecap.)

For all their brightness, however, conventional dielectric mirrors have an important flaw. Although they work fine when light approaches them straight on, they have trouble reflecting light that hits at certain angles. In fact, the more sharply angled the light beam's path into the mirror, the more poorly the light is reflected. Ultimately, when the path exceeds a certain angle, the mirror stops reflecting altogether. This phenomenon, called Brewster's Law for the scientist who discovered it almost 200 years ago, has long been thought to be an insurmountable drawback to mirrors made of multiple layers of dielectric materials.

Recently, a team of researchers demonstrated that it's possible to make multilayer mirrors that are less choosy about the direction of incoming light (see Science, 27 November, 1998, p. 1679). They used a mixture of organic and inorganic materials to make mirrors that reflected infrared wavelengths. However, Ouderkirk and his colleagues at 3M wanted to use purely organic polymer materials to make mirrors that reflect visible light, which required a different approach.

"Polymers can do things inorganic systems can't. They are commercially available and when made into a multilayer mirror, can have good reflectivity at all angles," Dr. Ouderkirk said.

The mirrors created by Dr. Ouderkirk and his research team, which includes Michael F. Weber, Carl A. Stover, Larry R. Gilbert, and Timothy J. Nevitt, reflect visible light no matter what direction it's coming from. Because they consist of thin, flexible stacks of layered polymers such as polyester, these mirrors are cheap, versatile, and easy to make in large volume.

In a similar fashion to the light hitting a dielectric mirror, some light waves are reflected from the polymer mirrors each time a light beam hits a new layer of material. However, every other layer of these new mirrors is a "birefringent" material, meaning that its crystal structure causes an incoming light beam to split into two rays that travel through at different speeds. By alternating thin films of birefringent and non-birefringent ("isotropic") polymers, Ouderkirk and his colleagues gained much more control over the interactions of the reflected light beams. Ultimately, the researchers were able to skirt Brewster's law and induce light to efficiently reflect off their polymer mirror no matter what its initial angle of approach.

While the researchers are reporting their results for the first time in the upcoming Science paper, 3M has begun to commercialize several applications that use these nifty new mirrors. For example, these materials have proven to be extremely effective at "piping" visible light over great distances without affecting its color or intensity. Researchers have also used a similar approach to create mirrors that only reflect certain wavelengths and that improve reflective polarizers, making the screens on hand-held computers and laptops much brighter and easier to read.