High-Reflection Mirror Is All-Polymer Film
ST. PAUL, Minn. -- A new generation of multilayer polymer film developed by 3M Corp. shows advanced control of optical reflection at interfaces. In its commercial release last year, the film attracted attention for its high reflectance of both incident polarizations at all incident angles over a wide wavelength band in the visible range. In the most recent iteration, 3M's 448-layer film demonstrates reflectivity greater than 99 percent for both polarizations and extends its reflectivity into the infrared between 800 and 1200 nm.
"We were surprised by the incredible reflectivity of these films," said Andrew Ouderkirk, a corporate scientist at the company's Film/Light Management Technology Center. "They provide lower cost and weight along with the best reflective characteristics of interference mirrors, then further add the ability to independently treat the polarization components of light."
A light lunch? No, an all-polymer film lining the bag's interior is reflecting light from an exterior white-light source. The film demonstrates reflectivity >99 percent at both incident polarizations at all incident angles over a wide wavelength band in the visible and infrared ranges. Courtesy of 3M.
Rather than depending upon large index differences between layers of isotropic media, 3M tailors the refractive indices along all three axes of the film's layered media. By adjoining birefringent polymer layers to isotropic polymer layers with uniform indices of refraction, the company constructed a material with an imaginary Brewster's angle; i.e., the interfacial reflection for p-polarized light is constant or increasing with incident angle, as opposed to the usual situation where the reflection decreases to zero at Brewster's angle.
The X- and Y-indices in the birefringent material differ from the index in the isotropic layer, while the Z-index is the same as in the isotropic layer. Light with incidence normal to the interface will see a change of index in both polarizations. As the angle of incidence increases, the s-polarized light continues to see the same index difference, as does the in-plane component of the p-polarized light. However, the projection of the p-polarized light along the Z-axis sees no index difference between the two layers, so it is unaffected by the transition between layers. Therefore, there is no angle at which the reflectance for p-polarized light goes to zero.
The result is a material that can exhibit the high reflectivity of dielectric mirrors without the high variation of reflectance with incident angle. Manipulating the material indices allows the p-polarized reflectance to be tailored to either match the reflectance for s-polarized light or to differ from it in well-controlled ways.
The company's research team further describes the propagation effects of the 448-layer extruded polymer films in the Nov. 27, 1999, issue of Science. Ouderkirk calls this class of interference mirrors "giant birefringent optics." Invented in the early 1990s, the films found use in LCDs and handheld computers. In the future, the technology could be applied in components for long-distance light transmission, inexpensive laser resonators and optoelectronic filters or as thermoformed dichroic films.
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