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US nanotech boffins track evanescent light

By following the Poynting vector, of course

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Researchers in the States have found a way of predicting how evanescent light waves might behave. The breakthrough could clear the way for a new generation of nanoscale optical devices, including solar thermal energy technologies.

When things get very small, nanoscale small, the rules all change and almost every assumption based on knowledge of the big world has to be checked and checked again. For example, from our experience with lasers we know that light travels in a nice straight line, right?

Wrong. Make the path the light has to travel down small enough, and the poor photons weave across it like so many drunken fools wheeling home from a pub. Sort of.

Specifically, we're talking about evanescent light, the light that is emitted when photons are radiated between two surfaces separated by a gap that is less than the wavelength of light. The light wave is interrupted and - until now - unpredictable evanescent waves are produced.

"Understanding the behaviour of light at this scale is the key to designing technologies to take advantage of the unique capabilities of this phenomenon," said Zhuomin Zhang, a lead researcher on the project and a professor in the Woodruff School of Mechanical Engineering.

"This discovery gives us the fundamental information to determine things like how far apart plates should be and what size they should be when designing a technology that uses nanoscale radiation heat transfer."

The team set out to track the evanescent waves by tracking the direction of the electromagnetic energy flow (a Poynting vector), rather then the direction of the photons themselves (which is unknowable, in physics terms). Even electrodynamics is different at the nanoscale, and the cornerstone of the research was working out what those differences are.

"We’re using classic electrodynamics to explain the behaviour of the waves, not quantum mechanics," Zhang said. "We’re predicting the energy propagation - and not the actual movement - of the photons."

Zhang explains that the team observed the light bending as the photons tunnelled through the vacuum separating the surfaces, just nanometres apart. The evanescent waves separated during this process, allowing the team to predict the energy path of the waves.

This information is vital to the construction of near-field thermophotovoltaic systems, nanoscale imaging based on thermal radiation scanning tunnelling microscopy and scanning photon-tunnelling microscopy, the researchers said. ®

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