New Coatings Break Reflectivity Barriers

Richard Gaughan

Researchers at the Massachusetts Institute of Technology have discovered a class of dielectric coatings that can reflect a broad wavelength regime over a wide angular range.
Professor John D. Joannopoulos and his colleagues discovered the reflective coatings during their research into photonic crystals -- structures that use photonic bandgaps to manipulate light in much the same way semiconductors manipulate current with their electronic band structure.
This perspective gave the researchers a new way of looking at the behavior of a propagating electromagnetic field at a dielectric boundary. "People have been developing dielectric coatings for a long time, but certain solutions had been missed," said Joannopoulos, who holds the Francis Wright Davis physics professorship at MIT. "We've opened up the solution space."
The solution space they've opened up is dramatic. Traditionally, dielectric coatings are developed for a specific wavelength and a specific incident angle. The MIT team has broken both barriers. As long ago as March 1997 its analytical results and numerical computations showed the possibility of a multilayer coating that would reflect all angles with high efficiency, maintaining that high reflectance over a bandwidth of several microns in the infrared. In the Nov. 27, 1998, issue of Science, Joannopoulos and his group reported those theoretical calculations along with experimental results from the first coating they designed to illustrate their breakthrough.
Yoel Fink, a graduate student in MIT's material science and engineering department, deposited a nine-layer stack of alternating layers of polystyrene and tellurium on a sodium chloride substrate. Calculations predicted a reflectivity of nearly 100 percent for all angles from a wavelength of 10 to 13 µm, and experiments confirmed the prediction.
That wasn't a surprise to Joannopoulos. "We had no doubt it would work. We understand the mathematics, and the analytical and computational results were clear," he said.
The only thing that was unclear to the research team was whether it had really found something new. The scientists spoke with coating experts around the world. All were skeptical. "The more we heard, 'That's impossible,' the more confident we were we had discovered a new class of solutions," Joannopoulos said.
That class of solutions could have a very large impact. For everything from wavelength filters to waveguides, the efficiency of these coatings could add an unprecedented performance improvement to a wide variety of optical systems. Under consideration are flexible delivery systems for CO₂ surgical lasers, home and car windows that selectively transmit light, and covers to enhance the efficiency of solar cells.
Still, Joannopoulos warned, the coatings aren't perfect. "We lose something to absorption within the materials, and we're limited by fabrication imperfections as well." But the bottom line is reflectivity "to a whole lot of nines."