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‘Perfect’ mirror could lead to concentrated solar breakthrough

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Researchers at MIT in the US have stumbled upon a new method to trap light that could lead to a wide variety of applications, not least of all a vast improvement in the efficiency of concentrated solar power generation.

The breakthrough involves what is being described as a kind of “perfect” mirror, which works in a way that deviates from known scientific laws, pitting light waves against light waves, and setting up two waves that have the same wavelength, but exactly opposite phases — where one wave has a peak, the other has a trough — so that the waves cancel each other out. Meanwhile, light of other wavelengths (or colors) can pass through freely.

The discovery – which could apply to any type of wave: from sound, radio, electrons (whose behavior can be described by wave equations), and even the ocean – was reported this week in the journal Nature by professors of physics Marin Soljačić and John Joannopoulos, associate professor of applied mathematics Steven Johnson, and graduate students Chia Wei Hsu, Bo Zhen, Jeongwon Lee and Song-Liang Chua.

As Soljačić explains, for many optical devices you want to build – including lasers, solar cells and fiber optics – you need a way to confine light. This has most often been accomplished using mirrors, as well as exotic photonic crystals and devices that rely on a phenomenon called Anderson localisation. In all of these cases, light’s passage is blocked: In physics terminology, there are no “permitted” states for the light to continue on its path, so it is forced into a reflection.

The new system, however, is “a very different way of confining light,” says Soljačić. “Light of a particular wavelength is blocked by destructive interference from other waves that are precisely out of phase. “

The discovery is “very significant,” says A. Douglas Stone – a professor of physics at Yale University who was not involved in this research, “because it represents a new kind of mirror which, in principle, has perfect reflectivity.” The finding, he said in a press release, “is surprising because it was believed that photonic crystal surfaces still obeyed the usual laws of refraction and reflection,” but in this case they do not.

The researchers are still trying to figure out why this deviation from known scientific laws took place. However, there is some excitement about what a perfect mirror could mean for various industries. The most obvious application is more powerful and efficient lasers, but concentrated solar power and fiber optics could also be improved.

Stone adds, “This is in fact a realization of the famous ‘bound state in the continuum’ proposed by von Neumann and [theoretical physicist and mathematician Eugene] Wigner at the dawn of quantum theory, but in a practical, realizable form. The potential applications the authors mention, to high-power single-mode lasers and to large-area chemical [and] biological sensing, are very intriguing and exciting if they pan out.”  

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