Color is an amazing and wondrous thing, yet the creation of color is a challenge.

It is well known that white light is a combination of components with different wavelength in the visible spectrum. When white light is reflected on surface of materials, it can appear to be a different color due to wavelength dependent reflectivity of white light. In the last few centuries, colored glass has been tinted through the use of metal particles. Light with different wavelengths can be scattered under the principles of plasmonic resonances. The reason that leaves are green is that wavelengths of light that correspond to green are reflected, while the others are absorbed.

In ancient times, people used natural products to generate different colors, but nature restricted their color palette. They were unable to create more complex colors. With the advent of chemical technology, colorful pigments have been developed for various decorative, printing and packaging industries.

At the same time, nature has more subtle wisdom. There are vivid colors on the wings of a butterfly, the skin of a chameleon, the feathers of a bird and in the opal. These colors are derived from a periodically arranged nanostructure that matches the wavelength of the visible light. These nanostructures are known as photonic crystals.

1904 Nobel Laureate in Physics Lord Rayleigh (John William Strutt) experimented with one-dimensional photonic crystals in periodic multi-layer dielectric stacks in 1887. This concept was later developed by Eli Yablonovitch and Sajeev John in 1987. They were able to develop the original theory into one-dimensional, two-dimensional and three-dimensional photonic crystals.

Different materials have different refractive indices, and partial reflection occurs when light passes through the interface between two materials. In certain circumstances, light of a specific wavelength can be reflected back. From an optical point of view, partial reflection occurs whenever light passes between two layers, and when there are multiple layers, the numerous reflected beams interfere with each other. The use of low loss dielectric multilayer films, usually incorporating alternating layers of silica and titanium dioxide, while some wavelengths are reflected back.

On this basis, colored lenses and crystal jewelry can be designed, along with devices that reflect X-rays, infrared light and even microwaves. These photonic crystal structures produce more stable and durable colors than chemically-generated pigments. Photonic crystal structures can provide brighter and more durable colors than their chemical counterparts.

Crystals are very common in nature and a constant direction of research in solid physics. Using diamonds as an example, they are forced by atoms forming a specific structure. Salt forms crystals of sodium and chlorine around the eight vertices of a cube. Similarly, diamonds are densely packed carbon atoms. Crystals follow specific rules based on their atomic make-up, their structure and their interactions. As a result, each crystal has unique properties in areas such as electrical conductivity, thermal conductivity, ductility (measure of how pliable a material is before it becomes brittle and breaks) and mechanical strength.

Although photonic crystals are man-made, their periodic arrangement is inspired by natural crystals. Major breakthroughs in science and technology have developed from our deepening understanding of nature and its materials.

The control of visible light has been the source of considerable scientific research since the 17^th century, and it was not until the 20^th century that semiconductor and transistor technologies allowed scientists to truly understand light.

Modern nano-fabrication technology allows the creation of photonic crystals specific for individual purposes. Such specificity ensures the quality of the crystals. They are widely used in many interdisciplinary fields such as bioimaging, spectroscopy, optical holography, facial recognition, laser radar technology, virtual reality and augmented reality.

Photonic crystals allow users to precisely control beams of light that are transmitted and modulated on a photonic chip. In theory, this could provide vital solutions in fields like lab-on-a-chip computing and optical information transmission.

Reference: John D. Joannopoulos, Steven G. Johnson, Joshua N. Winn, Robert D. Meade, Photonic Crystals: Molding the Flow of Light. 2nd edition, Princeton University Press, 2008.

Acknowledgement: Professor Qingsong Liu of Southern University of Science and Technology read the full text and put forward valuable suggestions. Professor Yang Liu, an ornithologist at Sun Yat-sen University, provided guidance on butterfly classification.

About the author: Li Guixin, Associate Professor, Department of Materials Science and Engineering, Southern University of Science and Technology,  Shenzhen Institute of Quantum Science and Engineering. He graduated from the Physics Department of Beijing Normal University with a bachelor’s and master’s degree from 1999-2006. In 2009, he obtained his Ph.D. degree from the Physics Department of Hong Kong Baptist University. He has served as postdoctoral research assistant professor at Hong Kong Baptist University, Imperial College London, University of Birmingham, and Paderborn University. Research interests include optical metasurface and nonlinear optics.