Antireflection with Moth Eye Structures

A film of moth-eye-like nanostructures can improve the sunlight visibility of screens on mobile phones and tablets. The images show the nanostructures from above and from the side. (Source: S.-T. Wu, Creol)

Screens on even the newest phones and tablets can be hard to read outside in bright sunlight. Inspired by the nano­structures found on moth eyes, researchers led by Shin-Tson Wu of the Uni­versity of Central Florida have developed a new anti­reflection film that could keep people from having to run to the shade to look at their mobile devices. The anti­reflection film exhibits a surface reflec­tion of just 0.23 percent, much lower than the surface reflection of usual phones of 4.4 percent. Reflection is the major reason it’s diffi­cult to read a phone screen in bright sunlight, as the strong light reflecting off the screen’s surface washes out the display.

“Using our flexible anti­reflection film on smart­phones and tablets will make the screen bright and sharp, even when viewed outside,” said Wu. “In addition to exhi­biting low reflec­tion, our nature-inspired film is also scratch resistant and self-cleaning, which would protect touch screens from dust and finger­prints.” The new film contains tiny uniform dimples, each about 100 nano­meters in diameter. The coating can also be used with flexible display appli­cations such as phones with screens that fold like a book, which are expected to hit the market as soon as next year.

Many of today’s smart­phones use a sensor to detect bright ambient light and then boost the screen’s bright­ness level enough to overcome the strong surface reflection. Although this type of adaptive bright­ness control can help improve readabi­lity, it also drains battery power. Other methods for solving the sunlight visi­bility problem have proved difficult to implement. Looking for a simpler approach to improve screen readability outside, the researchers turned to nature. The eyes of moths are covered with a pattern of anti­reflective nano­structures that allow moths to see in the dark and prevent eye reflec­tions that might be seen by predators. Because other research groups have experi­mented with using moth-eye-like nano­structures to reduce the sunlight reflected off the surface of solar cells, Wu and his team thought the same technique might also work on mobile screens.

“Although it is known that moth-eye structures can reduce surface reflec­tion, it is rela­tively diffi­cult to fabricate an anti­reflection film with this nano­structure that is large enough to use on a mobile phone or tablet,” said Guanjan Tan. “Because the structures are so small, a high-resolution and high-precision fabri­cation technique is necessary.” The researchers developed a fabri­cation technique that uses self-assembled nano­spheres to form a precise template that can be used to create the moth-eye-like structure on a coating. The simpli­city and precision of this process allowed fabri­cation of the intricate structure in a film large enough to apply to a mobile screen.

The researchers also created a computa­tional model to simulate the optical behavior of the coatings. After showing that the model accurately represented experi­mental results, the researchers used it to optimize the size of the moth-eye nano­structures to achieve the best performance. Tests of the film after optimi­zation showed that when viewed in sunlight, glass covered with the new film exhibited a more than four-fold improve­ment in contrast ratio. When viewed in the shade, glass with the new film showed about a ten-fold improve­ment in contrast ratio. The researchers also used standard industrial procedures to test its flexi­bility as well as its anti-scratch and self-cleaning capa­bilities.

“Our measured results indicate the moth-eye-like anti­reflection film shows excellent optical behavior and mechanical strength,” said Jun-Haw Lee of National Taiwan University, a key member of the research team. “Our film provides an efficient and low-cost method to reduce the surface reflec­tion and improve the sunlight readability of mobile devices.” The researchers are now working to further improve the anti­reflection film’s mechanical properties, including finding the best balance of surface hardness and flexi­bility, to make the film surface rugged enough for long-term use on touch screens. They are also using the simu­lation model to further optimize the moth-eye structure’s shape and size to obtain even better optical performance than ever thought possible. (Source: Creol)

Reference: G. Tan et al.: Broadband antireflection film with moth-eye-like structure for flexible display applications, Optica 4, 678 (2017); DOI: 10.1364/OPTICA.4.000678

Link: College of Optics and Photonics CREOL, University of Central Florida, Orlando, USA

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