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Researchers create 'perfect mirror'

MIT researchers reported in the November 27 issue of Science that they have made an important new advance in an age-old device -- the mirror.

One of the most common optical components, mirrors are so ubiquitous that it is hard to imagine that they have unknown properties. But MIT researchers expect that their new approach to making a mirror will take much of the scientific community by surprise. "We've discovered an important aspect of (dielectric) mirrors that was apparently overlooked," said Yoel Fink, graduate student at the Plasma Science and Fusion Center (PSFC) and one of the authors of the Science article.

"This is an overlooked new aspect of a well-known device with possibly far-reaching technological implications," Mr. Fink said. Among the potential uses for the device, dubbed the "perfect mirror" by the researchers, is trapping light for longer than ever before possible. This would open up a myriad of technological and research possibilities.

A team of researchers from materials science and engineering, physics and the PSFC reported that their "perfect mirror" combines the best characteristics of two existing kinds of mirrors -- metallic and dielectric.

The familiar metallic mirror is omnidirectional, which means it reflects light from every angle. It also absorbs a significant portion of the incident light.

Dielectric mirrors, unlike metals, do not conduct electricity and therefore can reflect light more efficiently. Light travels in dielectric materials at speeds that are lower than in air. When light traveling in a particular direction through one type of dielectric material encounters another type, part of the light is reflected while the other part is transmitted at a different angle.

Dielectric mirrors are made of multiple layers of transparent dielectric materials. Such materials, which can be made to be extremely low loss compared to their metallic counterparts, are used to reflect a prescribed range of frequencies coming from within a limited set of angles. Dielectric mirrors are used in devices such as lasers, which need very high reflectivity.

The new kind of mirror developed at MIT can reflect light from all angles and polarizations, just like metallic mirrors, but also can be as low-loss as dielectric mirrors. In addition, it can be "tuned" to reflect certain wavelength ranges and transmit the rest of the spectrum. A device such as this, operating in visible light, would appear to be one color -- red, for instance -- while also being transparent.

In the materials currently used by the researchers, the "perfect mirror" looks like a dark gray film. It is designed to reflect a portion of the infrared part of the spectrum.

The mirror's omnidirectional reflectivity occurs because all the light waves travelling forward through the mirror cancel each other out or interfere. Because they are made from relatively common materials, these mirrors could be made at a low cost and used for applications covering large areas.

"Potential uses depend on the geometry of the system. For example, coating an enclosure [with 'perfect mirrors'] will result in an optical cavity, a hollow tube will produce a low-loss broad band waveguide, while a planar film could be used as an efficient heat barrier or collector for thermoelectric devices," the authors wrote of the device, which has three patents pending. In other words, said author Edwin L. Thomas, the Morris Cohen Professor of Physical Science and Engineering at MIT, "walls, windows or even car interiors coated with the 'perfect mirror' could very efficiently reflect heat while appearing transparent."

The mirrors are made using polymer processing techniques, which make it possible to manufacture very high-quality optical devices for relatively low cost without a lot of specialized equipment. A close collaboration between theoretical physicists and material scientists at MIT and a National Science Foundation-funded shared analytical facility enabled Mr. Fink and his colleagues "to progress rapidly from the initial idea to a working prototype. It gave us the ability to combine theoretical insight, design flexibility and a quick turnaround time," he said.

"For me, what's really interesting is the ability to trap light and manipulate the way it flows in matter," Mr. Fink said. "This is going to revolutionize the way people think about confining light." Trapping light invites all sorts of intriguing questions, Fink points out. For instance, if you light a candle in a room lined with perfect mirrors, would the room stay illuminated even after the flame is extinguished?

In addition to Mr. Fink and Professor Thomas, the MIT team includes physics graduate student Joshua N. Winn, physics postdoctoral associate Shanhui Fan, PSFC research scientist Chiping Chen, Materials Processing Center research associate Jurgen Michel and John D. Joannopoulos, the Francis Wright Davis Professor of Physics.

This work is funded in part by the Defense Advanced Research Agency through the US Army Research Office and by the Air Force Office of Scientific Research.

A version of this article appeared in MIT Tech Talk on December 9, 1998.

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