Physics

A new way to trap light

Print edition : August 23, 2013

Light is found to be confined within a planar slab with a periodic array of holes although the light is theoretically "allowed" to escape. The blue and red colours indicate surfaces of equal electric field. Photo: Chia Wei Hsu, MIT

Known ways of “trapping” light use mirrors, other reflective surfaces or high-tech materials such as photonic crystals. Scientists at the Massachusetts Institute of Technology have now come up with a new method to do the same that could find a wide variety of applications and lead to new types of lasers and sensors.

The new system is based on interference of light waves. Two waves of light with exactly the same wavelength (or colour) but with exactly opposite phases—where one wave has a peak, the other has a trough—are made to interfere with each other so that the waves cancel each other out. Meanwhile, light of other wavelengths (or colours) can pass through freely. According to the researchers, this phenomenon could apply to any type of wave: sound waves, radio waves, electrons (which can also behave like waves), and even waves in water. The work was published recently in the journal Nature by the physicists Marin Soljacic and John Joannopoulos, the mathematician Steven Johnson, and graduate students Chia Wei Hsu, Bo Zhen, Jeongwon Lee and Song-Liang Chua.

“For many optical devices you want to build,” Soljacic says —including lasers, solar cells and fibre optics—“you need a way to confine light.” This has been usually accomplished using mirrors of various kinds: traditional mirrors and more advanced dielectric mirrors, exotic photonic crystals and devices that rely on a phenomenon called Anderson localisation. In all of these cases, the passage of light gets blocked. In the language of physics, there are no “permitted” states for the light to carry on in its path and so it is reflected. In the new system, however, only light of a particular wavelength is blocked by destructive interference. “It’s a very different way of confining light,” Soljacic says.

The researchers first saw the possibility of this phenomenon through numerical simulations; the prediction was then verified experimentally. The researchers do see many potential applications of this new phenomenon, including large-area lasers and chemical or biological sensors. The theoretical possibility of such an effect was shown by the mathematician and computational pioneer John von Neumann as far back as 1929, but it has not been seen experimentally before, except for special cases involving symmetry.

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