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Researchers combine a polymer coating with silicon dioxide nanostructures to create a coating to prevent fogging and unwanted reflections. The technique solves a common problem for sensors such as lidar used in autonomous cars.

Researchers have developed an optical coating system that combines antifogging and antireflective properties. The new technology could help boost the performance of lidar systems and cameras.

"Walking into a warm room from the cold outside can cause glasses to fog up, blinding the user," said research team leader Anne Gärtner from Fraunhofer Institute for Applied Optics and Precision Engineering and Friedrich Schiller University Jena, both in Jena, Germany. "The same can happen to sensors such as the lidar systems used in autonomous cars. It is important that surfaces remain highly transparent, even if fogging occurs, so that functionality is maintained."

In the Optica Publishing Group journal Applied Optics, Gärtner and colleagues describe how they combined a polymer coating that prevents fogging with porous silicon dioxide nanostructures that reduce reflections. Although the coatings described in the paper were designed specifically for lidar systems, the technology can be tailored for many different applications.

"In our coating system the anti-fogging and anti-reflective properties are excellently combined, something which has not been previously feasible," said Gärtner. "Samples manufactured with this new coating technology have already been used successfully for a year in several airborne lidar prototypes operating in various climatic conditions around the world."

Mimicking moth eyes to produce transparent anti-reflective coatings

Scientists develop a simple and scalable strategy to produce resin films with anti-reflective nanostructures--inspired by eyes of moths

There is a huge number of human problems that scientists and engineers have solved by drawing ideas directly from mechanisms found in other lifeforms, from Velcro to Japan's famous bullet trains, the Shinkansen. Thus, it should not come as a surprise to know that many remarkable advances in anti-reflective coating were inspired by the peculiar biostructures found in moth eyes.

As mainly nocturnal animals that wish to stay hidden from surrounding predators, moths have evolved to develop eyes that are non-reflective. Their eyes have a periodic nanometric structure that makes the eye surface graded, as opposed to polished. This causes most incident light to bend at the surface and therefore, be transmitted through the eye instead of being reflected off it. This nanoscale arrayed structure is so effective that researchers have tried to mimic it using other materials to create anti-reflective coatings with varying degrees of success.

However, in spite of the recent progress in nanoscience that allows the adoption of this idea for various practical applications, there are still barriers to overcome in terms of scalability and cost of manufacturing. To tackle these problems, scientists from Tokyo University of Science and Geomatec Co., Ltd., Japan, have been working on a novel strategy to produce moth-eye nanostructures and transparent films. In their latest study, published in Micro and Nano Engineering, they present a promising method to fabricate moth-eye molds and films at large scales.

Antireflection coating makes plastic invisible

Antireflection (AR) coatings on plastics have a multitude of practical applications, including glare reduction on eyeglasses, computer monitors and the display on your smart-phone when outdoors. Now, researchers at Penn State have developed an AR coating that improves on existing coatings to the extent that it can make transparent plastics, such as Plexiglas, virtually invisible.

"This discovery came about as we were trying to make higher-efficiency solar panels," said Chris Giebink, associate professor of electrical engineering, Penn State. "Our approach involved concentrating light onto small, high-efficiency solar cells using plastic lenses, and we needed to minimize their reflection loss."

They needed an antireflection coating that worked well over the entire solar spectrum and at multiple angles as the sun crossed the sky. They also needed a coating that could stand up to weather over long periods of time outdoors.

"We would have liked to find an off-the-shelf solution, but there wasn't one that met our performance requirements," he said. "So, we started looking for our own solution."

Out of the fog: Honeycomb films

Researchers are producing honeycomb-shaped films using an approach that mimics what happens when we breathe on a glass surface. The films have potential use in numerous applications, from tissue regeneration and bio-sensing to solar cells, explains an article in the journal Science and Technology of Advanced Materials.

Tohoku University materials scientist Hiroshi Yabu reviewed the latest research on the fabrication of honeycomb films using the breath-figure technique. This technique builds on years of research into understanding what happens when a breath fog forms on a glass surface, and how dew droplets form.

Water droplets condense when humid air is passed over certain substances mixed with solvents. They form a highly uniform, closely packed array and sink into the substance. When the solvent then evaporates, a porous, honeycomb-shaped film forms.

These honeycomb films have a wide range of potential applications depending on their individually unique properties. For example, peeling off a honeycomb film from a silicon wafer leads to the formation of nano-sized spikes on the wafer, making it 'anti-reflective'; a property that could be used to improve the efficiency of solar cells.

Making glass invisible: A nanoscience-based disappearing act

If you have ever watched television in anything but total darkness, used a computer while sitting underneath overhead lighting or near a window, or taken a photo outside on a sunny day with your smartphone, you have experienced a major nuisance of modern display screens: glare. Most of today's electronics devices are equipped with glass or plastic covers for protection against dust, moisture, and other environmental contaminants, but light reflection from these surfaces can make information displayed on the screens difficult to see.

Now, scientists at the Center for Functional Nanomaterials (CFN) -- a U.S. Department of Energy Office of Science User Facility at Brookhaven National Laboratory -- have demonstrated a method for reducing the surface reflections from glass surfaces to nearly zero by etching tiny nanoscale features into them.

Whenever light encounters an abrupt change in refractive index (how much a ray of light bends as it crosses from one material to another, such as between air and glass), a portion of the light is reflected. The nanoscale features have the effect of making the refractive index change gradually from that of air to that of glass, thereby avoiding reflections. The ultra-transparent nanotextured glass is antireflective over a broad wavelength range (the entire visible and near-infrared spectrum) and across a wide range of viewing angles. Reflections are reduced so much that the glass essentially becomes invisible.

Synthetic material acts like an insect cloaking device

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, called 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 and the Wormely Family Early Career Professor in Engineering. "We didn't know, until my former postdoc and lead author of the study 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."

Reducing reflectivity in solar cells and optics with micro- and nanoscale structures

When it comes to solar cells, less is more—the less their surfaces reflect a sun's rays, the more energy can be generated. A typical fix to the problem of reflectivity is an anti-reflective coating, but that might not always be the best solution, depending on the application.

Lawrence Livermore National Laboratory (LLNL) researchers have come up with guidelines for an alternative to anti-reflective coatings on optical devices such as solar cells, glasses and cameras, finding that reflectivity of silicon optics can be reduced to as little as 1 percent by engineering their surfaces with layers of hierarchical micro- and nanometer length structures.

A team of LLNL researchers, led by chemical engineer Anna Hiszpanski and UC Santa Cruz graduate student Juan Diaz Leon, described the parameters in a recent paper published by the journal Advanced Optical Materials . The technology has its roots in nature, mimicking the hierarchical structures found in the eye of a moth, allowing them to absorb more light and better navigate in darkness.

"It's a different anti-reflective approach," said Hiszpanski, who performed the experiments and was the co-lead author on the paper. "The design rules for these hierarchical anti-reflective structures haven't been explicitly laid out in these size scales. I'm hopeful they will enable others to more quickly design and fabricate optimal structures with the anti-reflective properties needed by their applications."

New screen coating makes reading in sunlight a lot easier—the secret? Moth eyes

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 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 .23 percent, much lower than the iPhone's surface reflection of 4.4 percent, for example. 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.

Researchers led by Shin-Tson Wu of the College of Optics and Photonics, University of Central Florida (CREOL), report on their new antireflection coating in Optica, The Optical Society's journal for high impact research.

"Using our flexible anti-reflection 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."