Inspired by moth eyes, scientists have developed a new technology that manipulates surfaces to make them ‘invisible’ across a wider wavelength range. Most lenses, objectives, eyeglass lenses and lasers come with an anti-reflective coating. However, this coating works optimally only within a narrow wavelength range. Now, scientists at Max Planck Institute for Intelligent Systems in Germany have introduced an alternative technology.
Instead of coating a surface, they manipulate the surface itself. By comparison with conventional procedures, this provides the desired anti-reflective effect across a wider wavelength range. It largely increases the light transmittance through surfaces. In the future, the nanostructured surfaces may improve high-energy lasers as well as touch-screens and the output of solar modules, researchers said.
They took a page out of the design book for moth cornea. The corneas of these mostly nocturnal insects reflect almost no incoming light. There is no glow of light bouncing off the moth’s eyes to betray their presence to potential predators. Less reflected light also means that moths are able to use practically all the scarce night-time light to see.
This magic from the world of insects inspired scientists to try the same tactics for the design of optical components. Like the corneas of moths, the components must allow light to pass through while light reflection is of little use. “The eye surface is densely covered with column-like structures. They are only a few hundred nanometres high and taper conically towards the tip,” physicist Zhaolu Diao said.
The columns look like regularly spaced stalagmites on a cavern floor. As the light passes through this boundary layer, its refractive index changes continuously, starting from the ambient air to the materials of the outer moth eye layers. This gradual refractive index change has the effect that the layer hardly reflects any of the incoming light. To imitate the moth eye principle, scientists developed a two-step process.
In the first step, they deposited gold particles in a regular honeycomb pattern on a large surface. In this pattern, the gold particles settle in the points of crossroad. In the second step, the gold-studded crossroads serve as mask in a chemical etching process. As a result, no material is etched away underneath the gold-studded crossroads, and the desired upright column-like structures remain. The structured surfaces covered as much as two by two centimetres.
While this technique registered first successes in the past, it has so far only worked for short wave UV radiation and visible light, researchers said. Until then, the columns etched out of the surface were at most 500 nanometres high. The columns are not high enough to reach the 99.5 per cent or higher light transmittance for the wavelengths in the near infrared light (NIR) range. The group fine-tuned their procedures and found a way to increase the size of the deposited gold particles, etching out columns as high as 2,000 nanometres.
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