Now there's a new way to trap light
11 July 2013
MIT researchers have discovered a new physical phenomenon that could lead to new types of lasers and sensors.
Light is found to be confined within a planar slab with periodic array of holes, although the light is theoretically "allowed" to escape. Blue and red colors indicate surfaces of equal electric field (image: Chia Wei Hsu)
There are several ways to 'trap' a beam of light — usually with mirrors, other reflective surfaces, or high-tech materials such as photonic crystals. But now researchers at MIT have discovered a new method that could find a wide variety of applications.
The new system, developed via computer modelling and then demonstrated experimentally, pits light waves against light waves; it sets up two waves that have the same wavelength, but exactly opposite phases so that they cancel each other out. Meanwhile, light of other wavelengths can pass through freely.
The researchers say that this phenomenon could apply to any type of wave: sound waves, radio waves, electrons (whose behaviour can be described by wave equations), and even waves in water.
“For many optical devices you want to build [including lasers, solar cells and fibre optics] you need a way to confine light,” says MIT's Professor Marin Soljacic. "This has most often been accomplished using mirrors of various kinds, including both traditional mirrors and more advanced dielectric 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."
In the new system, however, that is not the case. Instead, light of a particular wavelength is blocked by destructive interference from other waves that are precisely out of phase. “It’s a very different way of confining light,” Soljacic adds.
While there may ultimately be practical applications, such as high-power single-mode lasers and large-area chemical and biological sensing, at this point the team is focused on its discovery of a new, unexpected phenomenon.
The discovery is reported this week in the journal Nature by professors of physics Marin Soljacic and John Joannopoulos, associate professor of applied mathematics Steven Johnson, and graduate students Chia Wei Hsu, Bo Zhen, Jeongwon Lee and Song-Liang Chua.