14 September 2015 Tracking slow nanolight in boron nitride slabs in Nature Photonics

Image of polariton in an inverse space

ICFO and nanoGune develop nanoimaging technique for visualizing the exotic propagation of light in “hyperbolic materials” in time and space Researchers in the Nano-Optoelectronics group led by Prof. Frank Koppens at ICFO, working with colleagues at CIC nanoGUNE on a Graphene Flagship project, have collaborated to image the ways in which light moves inside an exotic class of matter known as hyperbolic materials. They observed for the first time ultraslow pulse propagation and backward propagating waves in deep subwavelength-scale thick slabs of boron nitride – a natural hyperbolic material for infrared light. This work has been published today in Nature Photonics.

Hyperbolic materials are solids that behave like both a metal and an insulator. Until now, these materials have been used to fabricate complex nanostructures that permit subwavelength imaging, as well as the focusing and controlling of light at the nanoscale. However, in order to maximize the potential of these new kinds of materials in the future, it is necessary to study and understand how light behaves inside them.

By using both the space and time information that is gathered during the experiment, the scientists have been able to deduce exactly how the polariton was travelling. The time- and space-resolved maps revealed a range of intriguing behaviors of the polaritons, including a dramatic slowing down of the pulse velocity - below 1 percent of the light velocity in vacuum- and a reversal of the direction in which the polariton waves were propagating in relation to the direction of their energy flow.

This recently published work lays the foundations for studying the precise manner in which light travels through complex optical systems at the subwavelength scale in extremely high levels of detail. Such a capability will be vital for verifying the proper functioning of future nanophotonic devices, such as those with biosensing or optical computing applications.