News | April 13, 2000

3M Creates Highly Efficient Polymer-Based Mirror

Source: 3M

3Mctric multilayer mirrors formed of polymeric materials provide high efficiency reflectance over a broad angle of incidence

A new type of reflective film made from polyester and other polymers reflects light with a brightness and versatility superior to other mirrors, according to a team of researchers at <%=company%> (St. Paul, MN) who recently reported their results in Science magazine.1 Sources at 3M say that these thin, flexible polymer mirrors should be a boon to fields such as optoelectronics; unlike other mirrors, the polymeric assemblies can reflect visible light from all angles with great efficiency.

Multilayer dielectric stacks
Ordinary mirrors of silver-coated glass reflect light from different directions, but aren't very efficient because they absorb some of the light as well as reflect it. Multilayer dielectric stacks use positive interference to increase reflectance, decreasing the amount of light lost to absorption.

For all their brightness, however, dielectric mirrors have limitations. 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, 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 selective about the direction of incoming light.2 They used a mixture of organic and inorganic materials to make mirrors that reflected infrared wavelengths. However, Ouderkirk and his colleagues 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," Ouderkirk said in a 3M statement.

Alternating birefringent layers
The mirrors created by Ouderkirk and his team, which includes Michael Weber, Carl Stover, Larry Gilbert, and Timothy Nevitt, reflect visible light no matter what direction it's coming from. 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, so that 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.

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, according to 3M. The company has begun to commercialize several applications that use the new polymeric mirrors. For example, the materials have proven to be extremely effective at propagating visible light over great distances without affecting its wavelength 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.

References
1. Andrew Ouderkirk et. al., "Giant Birefringent Optics in Multilayer Polymer Mirrors," Science 287, pp. 2451-2456, (2000).

2. Yoel Fink et. al., "A Dielectric Omnidirectional Reflector," Science 282 pp. 1679-1682, (1998).

For more information contact: Andrew J. Ouderkirk, 3M, 3M Center, St. Paul, MN 55144-1000. Tel: 651-736-6361. Fax: 651-737-6546. Email: ajouderkirk@mmm.com.

Edited by Gordon Graff, Laboratory Network