How can we deliver internet to some of the most remote areas on earth? One option currently under exploration is to use unmanned aircrafts that communicate with invisible infrared laser beams. But how do you detect this light efficiently when it comes in under a broad range of angles? In an upcoming article in ACS Photonics, Eindhoven University of Technology’s IPI-researchers from the group Photonics and Semiconductor Nanophysics and DIFFER prove that nanoantennas can do the job.

‘The problem with efficient light collection is the fact that the so-called etendue is conserved,’ project leader Jaime Gómez Rivas explains the main challenge that had to be overcome. Etendue is a measure for how spread out a beam of light is in terms of area and angle. The fact that this property cannot be reduced by optics basically means that light that enters a piece of optics under large angles cannot be focused onto a small spot. ‘Since the speed of a detector scales with its size, you need to use small detectors to enable gigahertz or even terahertz communication applications. With current-day technology that implies you are restricted to detecting only a small part of the emitted light power.’

To overcome this problem, postdoc Shaojun Wang from Gómez Rivas’ group developed a novel geometry consisting of arrays of so-called plasmonic nanoantennas covered in fluorescent dye molecules. The dye molecules absorb the incident light over a broad range of angles. The light is coupled into the nanoantennas, which transform it into a collimated beam of light. This beam is subsequently directed towards a lens, which focuses the now narrow-angled, large area beam onto a small focal spot. This way, the etendue is effectively reduced.

In the paper, the researchers prove that with this geometry it is possible to absorb and reemit light coming from all directions and under all angles equally well, with a performance that matches that of a perfect absorber and perfect emitter. ‘And this is achieved with a set-up that is far from optimal,’ Gómez Rivas proudly comments. ‘For instance: we choose to make the absorbent layer out of dye molecules, since from previous projects we had ample experience with these types of organic molecules. When we replace this layer by other materials such as two-dimensional semiconductors, we will be able to achieve much higher absorption and emission efficiencies, leading to an even further improvement of the performance.’

The researchers tested the performance of their system at three different wavelengths in the visible part of the spectrum. But by changing the shape, spacing and material of the nanoantennas, also broad-angle beams consisting of other wavelengths can be transformed into their narrow-angled counterparts. ‘For actual applications in wireless communications, we will have to move into the infrared, which is something we hope to work on in the future,’ Gómez Rivas observes.

Nevertheless, the biggest hurdle has been taken with this proof-of-principle, he concludes. ‘We have shown that a relatively simple nanophotonics system can have major advantages for wireless communication. Since focusing broad-angled light onto a small area is also relevant for other applications like lithography machines and solar panels, this paper might be relevant for a wide range of new possible applications.’

Read the preprint of the article here: https://pubs.acs.org/doi/10.1021/acsphotonics.8b00298

Shaojun Wang, Quynh Le Van, Thibault Peyronel, Mohammad Ramezani, Niels Van Hoof, Tobias G. Tiecke, and Jaime Gómez Rivas, Plasmonic nanoantenna arrays as efficient etendue reducers for optical detection, ACS Photonics, DOI: 10.1021/acsphotonics.8b00298