Perfect Mirrors

Robin Hanson (
Wed, 30 Dec 1998 10:10:10 -0800

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>Date: Tue, 29 Dec 1998 23:56:18 EST
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>Subject: [polymath] So where can I get some of this perfect window paint?
>December 15, 1998
>M.I.T. Scientists Turn Simple Idea Into 'Perfect Mirror'
> 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
>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
>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
>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
>"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
>"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
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Robin Hanson   
RWJF Health Policy Scholar             FAX: 510-643-8614 
140 Warren Hall, UC Berkeley, CA 94720-7360 510-643-1884