by Waree Sethapun ’20
Have you ever seen the colorful wings of a butterfly or the bubble-like shine of an iridescent opal? Did you ever wonder what kind of exotic pigments were the cause of their brilliant colors? However, it is in fact not pigments at all but instead physical structures! They both have repeating structures that are on the scale of the wavelengths of visible light called photonic crystals. Photonic crystals let some wavelengths of light constructively interfere with each other, causing it to reflect back into the eye of the viewer, but cause some wavelengths to destructively interfere and thus prevent viewers from seeing that color. The bands of wavelengths that destructively interfere are called photonic band gaps.
There are more examples of photonic bandgaps present in nature, such as those from a diamond crystalline structure. In present day, Princeton University’s research team published a paper revealing the first photonic band gap in a new kind of material: foam. You might picture this foam as the chaotic arrangement of bubbles from pouring a soda, but the foam they investigated in the study is birthed from years of physical and mathematical investigation. In 1887, Lord Kelvin, Irish-Scottish mathematical physicist and engineer, asked how space could be partitioned into cells of equal volume with the least area of surface between them—today, this problem is fittingly referred to as “The Kelvin Problem.” In order to solve his thought experiment, Kelvin proposed a 14-faced polyhedron with 6 square faces and 8 hexagonal faces, now known as a Kelvin structure. However, a shape with 0.3% smaller surface area was found in 1993 by Trinity College Dublin physicist Denis Weaire and his student Robert Phelan, called the Weaire–Phelan structure. The pair found Kevin’s conjecture too simple, and wanted to provide a counterexample.
Though you might not know it, you have probably seen it used for artistic purposes before, such as in the Beijing Water Cube building of the 2008 Olympics which was inspired by the Weaire-Phelan structure. The resulting contruction is inherently strong but lightweight as the structure can fill large spaces with minimal material.
The Princeton Researchers ran a rigorous computer simulation in a supercomputing facility on the interactions of various foams—including the Weaire-Phelan structure and the Kelvin structure—and other materials with light to find the photonic band gaps present.
They found that Weaire-Phelan structure had significantly wider band gaps than Kelvin structures, but narrower band gaps than diamond structures. While band-gap sizes are important for controlling electromagnetic waves, a material must have other good qualities for it to be widely used, which is why researchers are so excited about foam! Foams can be used in situations which require lightweight and flexible materials. The conductivity, heat transfer capabilities, and various mechanical properties of foam has been extensively studied and used in existing technology, making it easy to adapt for further use.
Photonic band gap materials are already improving fibre-optic telecommunications by increasing the efficiency of waveguides—devices which guides electromagnetic waves. Waveguides have the potential to do more in medical and renewable energy field as well. If researchers were to use foam for these innovations, they could become more versatile, lightweight and durable, and even possibly cheaper to produce than current materials.
“Always look out for what’s at the wayside of research,” Michael Klatt, a co-author of this research and postdoctoral researcher at Princeton, said because investigating foams was not initially the main objective of his research into controlling light using hyperuniform materials. In terms of future research, we must be aware of this emerging field of “phoamtonics” and other world-changing innovations that come with it in the future.
Klatt, M. A., Steinhardt, P. J., & Torquato, S. (2019). Phoamtonic designs yield sizeable 3D photonic band gaps. Proceedings of the National Academy of Sciences, 116(47), 23480-23486.
Schultz, S. (2019, November 19). Foam offers way to manipulate light. Retrieved January 3, 2020, from https://www.princeton.edu/news/2019/11/19/foam-offers-way-manipulate-light.