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 radiative cooling 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 Material 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. 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.