The “moth-eye” effect: how tiny surface textures suppress reflections and capture light

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

Structural Color: How peacocks, Morpho butterflies, and stained glass get their colors

An object’s color arises from how it reflects light. Both pigments and microscale surface structures affect how an object reflects light, but they create color in very different ways. Color therefore has two components: pigmented color and structural color.

You’re likely familiar with how pigments create color by absorbing certain wavelengths (or colors) of light. For example, chlorophyll is a pigment that gives plants their color by absorbing all colors except green (Figure 1A). The remaining green light reflects off the plant into our eyes.

Structural color arises from the selective reflection or absorption of certain colors due to light’s interaction with microfeatures like diffraction gratings, photonic crystals, and plasmonic metamaterials (Figure 1B). Structural color is responsible for the vibrant colors of peacock feathers, butterfly wings, opals, and stained glass. Some animals like octopuses and chameleons achieve adaptive camouflage by altering both the pigments and microtextures of their skin. Scientists hope to harness this phenomenon to design microstructures that enhance, shield, or manipulate light for advanced technologies such as camouflage, cloaking, and anti-counterfeiting technologies.

Figure 1: Illustrations of pigmented color (A) and structural color (B). Plants are green because the pigment chlorophyll absorbs all colors except green. Morpho butterflies appear blue despite being pigmented brown because the microstructures on their wings selectively reflect blue light. Note that light from the sun includes a mixture of all visible colors.

Diffractive Color

Peacocks and Morpho butterflies get their vibrant colors from a type of structural color known as diffractive color. Diffraction occurs when light interacts with regularly repeating structures, often called diffraction gratings or photonic crystals. The repeating features cause some wavelengths of light to cancel out (destructive interference) while other wavelengths add together (constructive interference), making those colors vibrant. The enhanced color can change with viewing angle, so diffractive materials are often iridescent.

To better understand this phenomenon, we used a scanning electron microscope (SEM) at NC State’s Analytical Instrumentation Facility (AIF) to take a closer look at peacock feathers, Morpho butterfly wings, and one of Smart Material Solution’s roll-to-roll nanoimprint lithography (R2R NIL) molds. Figure 2 shows a photo of a peacock feather and an SEM image of the structures responsible for its bright colors. The butterfly wings in Figure 3 have tree-shaped microstructures that diffract a vibrant blue color that changes little with viewing angle, whereas the micropatterns on the curved surface of the R2R NIL mold in Figure 4 result in a rainbow due to the angle dependence of the diffracted color.

Also at NC State, Professor Michael Dickey’s research group has created switchable diffractive gratings that can be turned on by applying pressure that buckles the surface into regularly repeating microstructures.

Figure 2: Photo and SEM image of a peacock feather.

Figure 3: Photo and SEM images of a Morpho butterfly wing.

Figure 4: Photo and SEM image of a R2R NIL mold created by Smart Materials Solutions.

Plasmonic Color

Another type of structural color, known as plasmonic color, is responsible for the brilliant colors of stained glass. But what are plasmons? And how do they give rise to color?

Metals contain electrons that are free to move around the material. These free electrons are known as an electron cloud, a sea of electrons, or an electron plasma. When light waves strike a nanostructured metal, they can create plasmons — or waves in the electron cloud with many electrons vibrating back and forth together. Energy is conserved because some of the light’s energy is transferred into the plasmon. In this way, the nanostructured metal absorbs a certain color of light.

The wavelength of light that is absorbed depends strongly on geometry of the nanostructured metal, so the resulting color can be tuned by changing the size and shape of the metal. The nanoscale metal can take many forms including metal nanoparticles, metal films separated by a nanoscale dielectric cavity, and nanopatterned metal films. In stained glass, suspended lead nanoparticles cause plasmonic absorption and therefore create bright colors.

Large-Area Plasmonic Absorbers

Smart Material Solutions is collaborating with MicroContinuum, Inc. and Professor Mark Mirotznik’s lab at the University of Delaware to fabricate large-area tunable infrared plasmonic absorbers. This project, which is funded by a Phase II Army SBIR grant, uses plasmonic metal-dielectric-metal (MDM) stacks to create tuned absorption for infrared surface mimicry. These plasmonic MDM stacks combine the resonance of the dielectric cavity with that of the plasmonic top layer to create strong, tunable absorption. In the phase I project, we fabricated the plasmonic absorber shown in Figure 5 using scalable processes such as nanocoining and nanoimprint lithography (NIL). The phase II project focuses on the roll-to-roll (R2R) nanofabrication of at least one square meter of a plasmonic metamaterial.

Figure 5: Photo and SEM image of a 150 mm x 150 mm plasmonic absorber with a micropatterned metal mesh fabricated during the phase I project.

Keeping clean on the Moon: Nanostructured surfaces to reduce lunar dust adhesion

Smart Material Solutions has been working for five years on scaling so-called “functional surface textures.” In May 2021, we partnered with Prof. Chih-Hao Chang of the University of Texas, Austin NASCENT Center and won a NASA-funded Small Business Innovation Research (SBIR) contract to adapt our technology for a new functionality: dust mitigation on spacecraft components on the moon.

If you’re not a fan of dust, the Moon is not the place for you. The moon has no liquid water, wind, weather, or even atmosphere. Everything we see on the surface is the result of meteor impacts, which all at once melt, turn to glass, and then shatter the minerals on the surface. The result is an extremely abrasive, microscopic, electrically charged dust that sticks to everything and is never worn down by the weather - even after billions of years! Lunar dust wreaked havoc during the Apollo Missions, with so much being tracked into the Lunar Module that it created haze in the cabin. With return to the moon via Artemis inevitable, the problem has resurfaced as a NASA priority.

Nanostructures on a surface dramatically affect how that surface interacts with the environment. For instance, texturing a surface with nanoscale features can make the surface self-cleaning via the lotus effect or decrease the adhesion of dust to a surface. This provides an opportunity for a passive solution to the dust problem. Passive solutions require no power or increase in payload mass, unlike, for example, an air spray, windshield wiper, vibrating panel, electrodynamic dust shield, or other proposed “active” solutions. 

Nanostructures essentially place the tiny micro-dust particles on a bed of nails, as shown in Figure 1. This reduces the contact area and the microscale adhesion forces, such as Van der Waals forces. The spacing between the nanostructures is important. It determines whether a dust particle will sit on top of multiple features, as desired, or whether it will stick between them, effectively making the problem worse.

Figure 1 - Large dust particles (left) sit on top of the features of a textured surface (desirable), while small dust particles (right) can fall in between the features (undesirable).

Lunar dust consists of a wide array of particles ranging from 1 μm to 100 μm in size. Single-digit micron-scale surface features may help reduce the adhesion of 10 μm particles or larger but become clogged over time with the smaller 1 μm particles. So the first goal of surface design is to reduce the spacing between the surface features as much as possible.

Another factor that impacts adhesion is the radius of curvature and material properties of the two bodies being attracted. In general, softer surface materials with larger features will have a larger contact area and thus larger adhesion force, as shown in Figure 2. So given the option, the goal is to make the individual features as sharp as possible to limit contact area.

Figure 2 - Sharper features (right) have a smaller contact area and thus reduced adhesion.

These geometric properties combine with material properties like hardness, surface energy, and electrical conductivity to determine a surface's passive affinity for dust. Unfortunately, due to the harsh realities of the lunar environment, including unfettered UV exposure and 200 degree temperature swings, the materials available for texturing are limited and typically so robust that they’re hard to pattern through the typical thermal embossing or UV curing processes. Solving this problem, at scale, is the goal of SMS’s NASA-sponsored SBIR Project. Figure 3 shows exciting preliminary results from this project.

Figure 3 - Video comparing the dust-mitigating properties of a smooth and a nanopatterned film. The two films were covered in Lunar dust simulant and tilted so that gravity could remove the bulk of the dust. After tilting, the smooth film remains covered in dust, whereas the nanopatterned film is largely free of dust.

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