Illustration (top) and simulation (bottom) of imaging slow nanolight in a thin boron nitride slab. Incident light pulses are converted by a gold (Au) film into slow hyperbolic polariton (HP) pulses propagating in the boron nitride (h-BN) slab. The HPs are traced in space and time by first scattering them with a nanoscale sharp scanning tip and then measuring the time delay between the scattered HPs and the incident light pulse as a function of tip position. Image: CIC nanoGUNE.Researchers at CIC nanoGUNE and ICFO – The Institute of Photonic Sciences, both in Spain, have imaged how 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 slabs of boron nitride, a natural hyperbolic material for infrared light. This work has been funded by the EC Graphene Flagship and was recently reported in Nature Photonics.
Hyperbolic materials are very special because they behave like a metal in one direction, but like an insulator in the other. Up to now, these materials have been used to fabricate complex nanostructures for conducting subwavelength-scale imaging, and for focusing and controlling light at the nanoscale. However, in order to exploit the full potential of these materials, it is necessary to study and understand how light behaves inside them.
This work lays the foundation 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 that future nanophotonic devices, perhaps with biosensing or optical computing applications, are functioning as expected.
"The difficulty in performing the reported experiments is the extremely short wavelength of light when it is inside a hyperbolic material," explains Rainer Hillenbrand, leader of the nano-optics group at nanoGUNE. "When light moves inside the material – in our case mid-infrared light in a 135nm boron nitride slab – it travels in the form of what we call a polariton, where the light is actually coupled to the vibrations of the matter itself."
These polaritons can be considered a double-edged sword for scientists. On the one hand, they squeeze light into much smaller volumes than is normally possible. This is helpful for a wide range of applications that require the manipulation of light in tiny spaces, such as detecting and identifying individual molecules. On the other hand, this ultra-high confinement means that special techniques have to be developed to study their behavior.
Edward Yoxall, who performed the experiments at nanoGUNE along with Martin Schnell, elaborates: "Because the wavelength of a polariton is so small, we cannot use 'conventional' optical equipment, such as lenses and cameras, to image it. Instead, we have to use a special type of microscope." This microscope – a scattering-type scanning near-field infrared microscope – can see details 1000 times smaller than a standard infrared microscope, able to visualize ‘objects’ of just 10nm in size.
"But it's not just the spatial resolution that makes tracking polaritons tricky," continues Yoxall. "If we want to see how a polariton moves, we need to detect and track it in both space and time. This can be accomplished by using extremely short flashes of light – or pulses – that are just 100 femtoseconds long." That is less than one millionth of a millionth of a second. By using these very short flashes in combination with their near-field microscope, the researchers were able to watch the polaritons pass different locations along the boron nitride slab, allowing them to measure their speed.
By using both the space and time information gathered during the experiment, the scientists have been able to determine exactly how the polaritons were traveling. The resultant time- and space-resolved maps revealed a range of intriguing behaviors for the polaritons. These included a dramatic slowing down of the pulse velocity – below 1% of the velocity of light in a vacuum – and a reversal of the direction in which the polariton waves were propagating in relation to the direction of the energy flow.
"An exciting result is the speed at which the polariton moves", says Yoxall. "There's a lot of interest in slow light, and what we've shown here is a novel way of achieving this." Slow light in conventional photonic structures has great potential for numerous applications in sensing and communication technologies, owing to enhanced light-matter interactions. The deep subwavelength-scale confinement of slow polaritons in hyperbolic materials could lead to the miniaturization of these photonic structures.
This story is adapted from material from Elhuyar Fundazioa, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.