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Semi-random scattering of light

What is the exact path of light inside a highly scattering material like white paint? This is a question that is impossible to answer, as the particles inside the paint are distributed randomly. This, at the same time, is a very attractive property for applying photonics in non-hackable security applications. Still, you would like to have a look inside, to see what is happening. For this reason, researchers of the University of Twente (MESA+ Institute), built a light scattering microcube that is both random and controlled. Contradictory as it seems, this is a way to exactly know what is happening inside. The research results are in Advanced Optical Materials.

Earlier research by UT researchers demonstrated the way light can be controlled, even when it travels through randomly scattering media like white paint. This might lead to a credit card that can’t be hacked, and to new medical imaging applications as well. In brief: we know how light falls on the surface, we can even predict how it gets out. But the path it travels inbetween, is unknown. Why don’t we reverse the question, the UT scientists thought: let’s create a structure that we exactly know and that is random at the same time. In practice: let’s make a tiny cube with hundreds of nanorods inside. Although they seem organised in full randomness, you exactly know what you did and where these rods are. And thus: where the light is, at any given moment.

Micro-sized Turkish sweet

This is done using a precision 3D printing technology called Direct Laser Writing, available at UT’s MESA+ NanoLab. The nanorods are written using a laser and a special gel material. After hardening, the material inbetween is washed away. A spunge-like cube remains, bearing some resemblance to a tiny version of the Turkish sweet called pismanye that is made out of spun sugar. The size of the cube is 15 micron by 15 micron by 15 micron, for example, with 400 to 2000 nanorods inside. What part of the incident light comes out, is a major question, and in what way is this influenced by the number of rods? For lower number of rods - less randomness -, more light travels straight through the material and exits at the location you would expect. For higher numbers, light also exits at other locations, the research shows.

In their earlier publication, using a classic mathematics paradox, the UT researchers demonstrated how these rods should be organized for getting a homogeneous distribution across the whole cube. This is a manufacturing challenge as well: even if the structure looks great from the outside, there maybe a lump of hardened polymer at the center of the cube that fully overrides the desired effects. Images using special X-Ray microscopy, available in Grenoble, show that the entire cube consists of the expected rods.

This research gives more insight into scattering light inside randomly organized materials. It helps defining the boundary conditions for applications in information security or imaging. Including ‘scattering cubes’ like this in LED light sources for special light properties, could be an attractive application as well, according to research leader Pepijn Pinkse of the Complex Photonic Systems group, part of UT's MESA+ Institute for Nanotechnology.

The paper Deterministic and Controllable Photonic Scattering Media via Direct Laser Writing, by Evangelos Marakis, Ravitej Uppu, Maryna Meretska, Klaas-Jan Gorter, Willem Vos and Pepijn Pinkse, is published online in Advanced Optical Materials.

ir. W.R. van der Veen (Wiebe)
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