December 15, 1998

M.I.T. Scientists Turn Simple Idea Into 'Perfect Mirror'


A team of scientists at the Massachusetts Institute of Technology has recently announced what may be the most significant advance in mirror technology since Narcissus became entranced by his image reflected on the surface of a still pool of water.

Their invention, which they are calling the "perfect mirror" combines the best features of the two previously known types of mirrors by reflecting light at any angle with virtually no loss of energy. It promises to have significant applications in many fields, including fiber optics, cellular telephones, energy conservation, medicine, spectroscopy and even, perhaps, cake decoration.

Using layers to make the impossible possible.

"This is very significant," said Dr. Eli Yablonovitch, a physicist at the University of California at Los Angeles. "There are going to be some important applications."

The announcement by the M.I.T.

team was initially greeted with disbelief by scientists, who for generations had been taught that mirrors with the properties the team claimed were impossible.

John D. Joannopoulos, a leader of the team that invented the mirrors, had even published a "proof" of their impossibility in his widely read textbook on the field. "Goes to show how much I know," Dr. Joannopoulos, an M.I.T. physics professor, said with a grin, conceding his mistake.

But the basic idea behind the mirrors is so simple, depending on no new physical insight or mathematical theory, physicists say, that anyone who reads the M.I.T. paper is quickly convinced of its correctness. Writing about the discovery in Science magazine, Jon Dowling, a physicist at the National Aeronautics and Space Administration's Jet Propulsion Laboratory, at the California Institute of Technology said, "Every once in a while someone comes along with a great idea that in hindsight seems so trivial you could kick yourself for not having thought of it first."

Mirrors come in two basic varieties. The most common are metallic mirrors like those found on the walls of Versailles or on medicine cabinets. Metallic mirrors work pretty well, but they have limitations. The most important is that they waste energy, absorbing a small fraction of the light that falls on them. That is because when light, which, like radio waves, is a form of electromagnetic radiation, strikes a metallic mirror the electrons in the metal move just as they do when a radio signal strikes an antenna. Pushing electrons around takes energy, which dims the reflected image. So metallic mirrors cannot be used in applications like communications and high-powered lasers, where minimizing energy loss is important.

For applications in which energy loss is important scientists depend on a more sophisticated device known as a dielectric mirror. A dielectric is a material like glass or plastic, that does not conduct electricity. Narcissus was actually enamored of his image in a crude sort of dielectric mirror, because water is a dielectric.

But dielectrics like water or glass do not reflect light well, so practical dielectric mirrors are made by stacking alternating thin layers of two dielectrics. Every time light passes from one layer to the next a little bit of it is reflected. If the thicknesses of the layers are chosen carefully these reflected light waves combine and reinforce one another, strengthening the intensity of the reflected light. By stacking many layers scientists can make mirrors that are nearly perfect reflectors.

Another useful property of dielectric mirrors is that they can be designed to reflect only a small range of frequencies and let the rest pass unmolested. For example, dielectric mirrors can be designed to reflect infrared light but transmit visible light. Because infrared light is heat, dielectric mirror windows would insulate a room from the heat of day without impeding the view. But there is a problem.

The main drawback is that standard dielectric mirrors, unlike metallic mirrors, reflect only light that strikes them from a limited range of angles. A dielectric window that blocked heat from radiating from the sidewalk might only let in the oblique rays of the noon sun. This limitation of dielectric mirrors has restricted their use to specialized devices like lasers in which the light can be constrained to strike at a known angle. Until the M.I.T. team reported its findings, scientists believed that this limitation of dielectric constants was an inconvenient law of nature, regrettable but unavoidable.

Dr. Joannopoulos said the M.I.T. team members realized by accident that they might have overlooked something. Joshua Winn, a graduate student, was playing with a computer model of a dielectric mirror when he noticed that it seemed to be reflecting light at a much larger angle than he had thought possible. Puzzled, he turned to Shanhui Fan, a post-doctoral fellow in physics who came up with an explanation. Satisfied, the two promptly filed it away as a theoretical novelty and forgot it.

"That's the problem with being a theorist," Dr. Joannopoulos said. "Being theorists, we tend to think in a different way."

Meanwhile, Yoel Fink, a graduate student at M.I.T.'s Plasma Fusion Science Center who was proficient in experiment and theory, was wrestling with a project his lab was doing for the Defense Advanced Research Agency. Maybe, he thought, a multilayered dielectric mirror could be made to do the trick. He made the suggestion at a large meeting.

And the minute he did, Fink said, he saw Joannopoulos light up.

Within three months, Dr. Fink had made the first mirror, completed in February, from nine alternating layers of polystyrene -- a plastic -- and tellurium. Measurements confirmed what theory had predicted. The mirror reflected infrared light equally well from all angles and as efficiently as the best metallic mirrors.

For months the researchers lived in fear that something so obvious had to be well-known.

"How could something about mirrors not be known?" asked Dr. Edwin L. Thomas, the other leader of the team and an M.I.T. professor of physical science and engineering.

"We had this feeling that sooner or later somebody's going to walk up to us, tap us on the shoulder and say, 'Yeah, we knew this a hundred years ago.' But apparently not."

"I think there's going to be a lot of activity, with people saying, 'This is simple! It's not hard to make,' " Dr. Thomas said. In one early application the M.I.T. group has rolled the mirrors into spaghetti-thin tubes called "omniguides." A beam of laser light can be guided by such tubes far more efficiently than by fiber optics because glass fibers absorb light. And, unlike fiber optics, the omniguides can guide light around corners. In the operating room such omniguides could precisely guide the light of the powerful lasers surgeons use.

Even more promising is the possibility of replacing conventional fiber optics used in communications with omniguides. The absorption of light by conventional glass fibers means that the signal must be boosted every 20 kilometers or so. This requires amplifiers, which only work in a narrow band of frequencies. Omniguides would carry light with far less loss of energy, meaning they could stretch for thousands of miles without amplifiers. Engineers would not be limited to a small band of wavelengths by the abilities of amplifiers. "You could have a thousand times the bandwidth. That's a very big deal," Dr. Fan said.

The M.I.T. scientists also envision coating windows with infrared reflecting mirrors to keep heat in or out of rooms.

The mirrors could be chopped into tiny flakes and mixed with transparent paint to allow them to be applied directly to walls or windows.

The M.I.T. mirrors could also be useful in improving thermophotovoltaic cells, devices that trap waste heat and convert it to energy. Dr. Dowling suggested that, because the new mirrors could be made to reflect radio waves, they could be used to boost the performance of cellular telephones. Even the apparel industry could benefit. "You could use this type of stuff to make fiber and very light weight clothing to keep the heat in," Dr. Joannopoulos speculated. "I think this could be really big," he said. "We're limited only by our imaginations." For the M.I.T. team that is not a severe limitation: Dr. Fink suggested, half seriously, that mirrors could be made of edible materials to make reflective cake icing. "Really, what food do you know of that's highly reflective?"


Copyright 1998 The New York Times Company