Researchers at National Taiwan University are expanding on the anti-reflective properties of moth eyes to make better protective skins for U.S. Air Force unmanned aerial vehicles.
With funding from the Asian Office of Aerospace Research and Development (AOARD) -- an international detachment of the Air Force Office of Scientific Research -- Prof. Li-Chyong Chen has produced an anti-reflective nanostructure surface using arrayed silicon nanotips.
Chen's silicon nanotip surface mimics the naturally formed surfaces found on many plants and animals. The color on butterfly wings, camouflage for cicadas and night vision for moths are all made possible by tiny surface structures that absorb certain wavelengths of light.
A key aspect of Chen's research has been finding an efficient and easy-to-integrate approach for preparing the nanotips on a single surface. Her patented electron-cyclotron-resonance (ECR) method offers the opportunity to do so in only one step.
This plasma-assisted method selectively dry etches a single silicon wafer by masking and unmasking certain portions. The un-etched portions form an array of nanotips.
Because of the high density of plasma in the ECR method, Chen has been able to create a thick forest of silicon nanotips. Of equal importance, she can produce them uniformly over a large area, recently demonstrating the approach on a 6-inch wafer.
The resulting nanostructure surface not only mimics the moth eye, but also surpasses its function in anti-reflection by absorbing almost all of the direct light that falls on it.
"The new silicon nanotip surface demonstrates a low hemispherical reflectance of <1% from the ultraviolet to the infrared region and shows significant suppression of specular reflection in the far-infrared to terahertz region," explains Dr. Thomas Erstfeld, AOARD senior advisor.
In addition, these findings are nearly unaffected by the angle of the light source shined on the surface, allowing Chen to add broadband anti-reflection and quasi-omni-directional anti-reflection to the list of properties that show promise for Air Force optical defensive applications.
Molly LaChance
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