Insect Eyes Inspire Antireflective Coating

Biologically inspired antireflective surface that emulates the intricate surface architectures of leafhopper-produced brochosomes, soccer ball-like microscale granules with nanoscale indentations. (Source: S. Yang, B. Boschitsch, PSU)

Synthetic micro­spheres with nanoscale holes can absorb light from all direc­tions across a wide range of frequencies, making them a candidate for anti­reflective coatings, according to a team of Penn State engineers. The synthetic spheres also explain how the leaf hopper insect uses similar particles to hide from predators in its environ­ment. Scientists have long been aware that leaf hoppers extrude micro­particles – brocho­somes –  and wipe them on their wings. Because the particles are super­hydrophobic, the leaf hopper’s wings stay dry in wet conditions.

What was not under­stood before the current work is that the brocho­somes also allow leaf hoppers and their eggs to blend in with their backgrounds at the wave­lengths of light visible to their main predators, such as the ladybird beetle. “We knew our synthetic particles might be interes­ting optically because of their structure,” said Tak-Sing Wong, assistant professor of mechanical engi­neering. “We didn’t know, until Shikuan Yang brought it up in a group meeting, that the leaf hopper made these non-sticky coatings with a natural structure very similar to our synthetic ones. That led us to wonder how the leaf hopper used these particles in nature.”

Doing a search of the scien­tific literature turned up nothing about the leaf-hopper brocho­somes’ use as camouflage. But the pits’ sizes in the synthetic micro­spheres are very close to the wave­length of light, and can capture up to 99 percent of light, ranging from ultraviolet through visible and into the near infrared. The particle surface acts like a meta­material, the type of material used in cloaking devices. “The problem is that in the field, these leaf hoppers produce very little of this product, and it is very hard to collect,” Wong said. “But we had already produced large quan­tities of these structures in the lab, enough to put inside a machine to look at their optical proper­ties.”

Now, the researchers simulated insect vision and found that the brocho­somes are very likely camou­flage coatings against leaf hopper predators. Camouflage is common in nature, but there are very few examples of natural anti­reflective coatings, moth eyes being a prominent exception. Moth eyes are covered in anti-reflective nano­structures that prevent light from reflecting off them at night when predators might see them. The synthetic micro­spheres are produced via a rather complex five-step process using electro­chemical deposition. However, the process can be scaled up and many different materials can be used to make the synthetic brocho­somes, such as gold, silver, manganese oxide or even a conduc­tive polymer.

“Different materials will have their own applications,” Wong said. “For example, manganese oxide is a very popular material used in super­capacitors and batteries. Because of its high surface area, this particle could make a good battery electrode and allow a higher rate of chemical reaction to take place.” As an anti­reflective coating, this material could have appli­cations in sensors and cameras, where capturing unwanted light reflec­tion could increase the signal-to-noise ratio. This also could be parti­cularly useful in tele­scopes. For solar cell appli­cations, a coating of synthetic brocho­somes could increase light capture at multiple wave­lengths and from every angle due to the 3D soccer-ball-shaped structure of the spheres, making it unnecessary to build devices to track the sun. “In the future, we may try to extend the structure to longer wave­lengths. If we made the structure a little larger, could it absorb longer electro­magnetic waves such as mid-in­frared and open up further appli­cations in sensing and energy har­vesting,” Wong said. (Source: PSU)

Reference: S. Yang et al.: Ultra-antireflective Synthetic Brochosomes, Nat. Commun. 8, 1285 (2017); DOI: 10.1038/s41467-017-01404-8

Link: Wong Laboratory for Nature Inspired Engineering, Penn State University, University Park, USA

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