Light changes when going from one transparent medium, like air, into another, like water or glass. This is evident from the distorted images we see while looking through the water in a pool or from the magnifying effect of a glass telescope, microscope, or camera lens.
Figure 1: Fresnel reflections are distracting on a glass window (above) or display screen and wasteful on the cover of a solar panel.
These effects occur because different materials have different refractive indices. Refractive index is a measure of how fast light moves through a material. The larger a material’s refractive index, the slower light travels through it. When the light moves from one material into another, e.g. air into glass, there is an abrupt change in refractive index. The light suddenly slows down as it enters the glass, causing some of the light to be reflected. This reflection is called a Fresnel reflection (Figure 1). We observe Fresnel reflections everyday when we see our own reflection in a window at night or glare on the screen of our smartphone. Moth-eye structures can help eliminate these reflections.
Nature’s inspiration - a matter of life and death
Moths are pretty defenseless creatures, and as creatures of the night, they rely on camouflage and stealth to survive. For many animals, the only way we can spot them at night is by the Fresnel reflections off their beady eyes. However, moths have evolved their own solution to prevent these unwanted reflections - billions of tiny nanofeatures on their eyes that not only suppress Fresnel reflection, but also draw that light into their eyes, improving the moth’s vision in the dark.
To get a better look at these anti-reflective nanofeatures, we took the Atlas moth shown in Figure 2 to a scanning electron microscope (SEM) at NCSU’s Analytical Instrumentation Facility (AIF). The SEM images revealed microscale hexagons covered with the nanofeatures that make the eyes anti-reflective. Scientists have attempted to mimic these nanofeatures to create so-called moth-eye anti-reflective coatings for products ranging from televisions and displays to solar panels. For solar panels, moth-eye coatings can increase light absorption and efficiency.
Figure 2: Photo of an Atlas moth with SEM images of one of its eyes.
How Nanopatterns Reduce Reflections
The nanoscale bumps of a moth’s eye make them anti-reflective by altering the effective refractive index at the interface between air and the eye. Let’s consider the analogous case of light traveling from air into glass as illustrated in Figure 3. Air has a refractive index of 1, whereas glass has a refractive index of 1.5. This means that light travels faster in air than in glass, and the abrupt change in speed across a smooth interface causes the Fresnel reflection, Figure 3A.
The key to the anti-reflective behavior of moth eyes is the nanofeatures that are extremely small – much smaller than the wavelength of visible light. Thus, the light cannot resolve the nanofeatures just as we cannot see the tiny water droplets that make up a cloud. Instead of observing the individual nanofeatures, the light observes an average of the nanofeatures and the air. The light therefore experiences a gradual change in the refractive index instead of the abrupt change at a smooth interface, Figure 3B. As a result, the light gradually slows down and the amount of light reflected is reduced. Not surprisingly, the size and shape of the features matter! We studied the effects with partners at the University of Delaware during an Army-funded SBIR project and published the results in JOSA-B.
Figure 3: Illustration of the effective refractive index as a function of position for a smooth interface (A) and a moth-eye structure (B).
Replicating Nature’s Design
Figure 4: Metal mold shim created by SMS with textured and smooth sections (left) alongside a polymer replica showing reduced glare in the textured region (right).
Figure 4 shows how we have been able replicate this effect on a piece of clear plastic at Smart Material Solutions (SMS). The metal mold on the left was partially textured using SMS’s patented nanocoining process. In the polymer replica on the right, the textured region appears transparent, while the smooth region, although completely clear, has so much reflective glare that we can hardly see through it. The man-made moth-eye texture suppresses reflection, and in doing so, actually increases how much light goes through the plastic film.
Moth-eye coatings have several benefits over traditional anti-reflective coatings such as quarter-wave films or multiple graded index films. Moth-eye coatings exhibit broadband, wide-angle anti-reflectivity, meaning that they work for a wide range of wavelengths (colors) and angles of the incoming light. In addition, the moth-eye nanofeatures can also make a surface self-cleaning or superhydrophobic, critical properties for applications including solar cells, lenses, and windows, where the buildup of dirt, dust, pollen, and grime can substantially degrade performance.
Problems and Solutions
Unfortunately, in addition to their promises to enhance solar panel efficiency and create self-cleaning, glare-free display screens, nanofeatures also inspire trepidation in engineers, who time and again find themselves facing the same problems: these textures are easily damaged by touch, easily contaminated by fingerprints, and extremely difficult to fabricate in a cost-effective way.
However, these problems are gradually being solved. Durability and contamination issues can be somewhat mitigated by new polymer formulations with low surface energy and exceptional hardness. In the meantime, SMS is focused on applications that are low touch or can tolerate some defects, such as solar cells.
And the problem of fabrication at scale is SMS’s specialty. We recently worked with MicroContinuum, which used a seamless mold created by SMS’s core technology nanocoining, along with roll-to-roll nanoimprint lithography (R2R NIL) to nanopattern over 500 feet of motheye film, Figure 5. It’s a multifaceted engineering challenge, but over time Smart Material Solutions will help bring these enormous benefits into the consumer world.
Figure 5: Photo (A) and SEM (B) of a piece of the over 500 feet of moth-eye film embossed at MicroContinuum, Inc. This moth-eye film has nanofeatures with a pitch of 300 nm that induce blue and green diffraction at low viewing angles and anti-reflective properties at normal angles.
Author: Stephen Furst
Contributor: Brenna Tryon