Researchers at 3M (St. Paul, MN) claim that they have not actually broken Brewster's law, but it would probably be fair to say that they have refracted it significantly. Brewster's Law predicts a decreasing reflectivity for p-polarized light from high reflectance at normal angles of incidence to zero reflectance when the light grazes the surface at Brewster's angle. By using highly birefringent polymers in thin-film, multilayer mirrors, however, the 3M group appears to have exceeded Brewster's fundamental and two-centuries-old optical limitation. For example, they can now build a mirror that works just the opposite: reflectivity of p-polarized light could increase from zero at normal incidence to a maximum at grazing incidence. They can also make a filter that reflects s- and p-polarizations equally.
The researchers claim that in placing Brewster's angle at normal incidence or wherever might seem convenient for a particular application (or by getting rid of Brewster's angle entirely) they aren't really breaking Brewster's law, just generalizing it. "Brewster's law actually describes a subset of all of the possibilities," says 3M corporate scientist Andrew Ouderkirk (see Fig. 1).
The 3M group is well on the way to developing and manufacturing products for a range of applications.1 Consumer applications include solar control films to passively cool automobile interiors on hot summer days. Architectural applications include highly efficient light pipes to illuminate windowless building interiors with natural light. High-technology applications include optical filters and possibly displays.
"We've been test marketing some of the products in smaller volumes," Ouderkirk said. "We're trying to grow manufacturing technology, application technology, and the fundamental optics of these films all at the same time." 3M has had a reflective polarizer, based on similar technology, on the market for some time, he said. Full release of mirror products may begin this year.This burgeoning commercial activity is based on the use of highly birefringent materials in multilayer mirrors, where the magnitude of birefringence is comparable to the change in refractive index between adjacent material layers. Combining the high reflectivity and wavelength selectivity of multilayer mirrors with the fact that birefringent materials present a different refractive index at different angles of incidence has yielded "surprising and useful optical effects" that the 3M group refers to as giant birefringent optics (see Fig. 2).
"In the early 1990s, we were looking at the interaction of excimer lasers with polymers and found that we could melt a very thin surface of the polymer without degrading the polymer," Ouderkirk said. "Having a thin amorphous layer on a high-index polybirefringent material created some very interesting optical properties that started us thinking about applications of very-high-index-difference multilayer polymers."
The 3M group then pursued a multilayering technology for generating hundreds of layers of polymers simultaneously while meeting thickness tolerances on the order of a few nanometers. Subsequent work led to technology for developing birefringence in polymers, all of which eventually led to the giant birefringent optics.
- M. F. Weber et al. Science 287, 2451 (March 31, 2000).