A new method could allow scientists to custom-grow defect-free photonic crystals. A team from Princeton University and Columbia University created computer models demonstrating the technique, adding precisely sized chains of polymers to a colloidal suspension. While the creation of solids via colloidal suspension is not a new idea, the researchers said this was the first time the technique has been shown to be viable for creating a crystal pure enough to split light for an optical circuit. Colloids form the initial two layers of a crystal (left). When a third layer is added, the crystal forms one of two possible shapes (right). The vertical blue lines show a representative shape of the polymer when confined in the voids between colloids in each crystal. Courtesy of Nathan Mahynski/Princeton University. “Our results point to a previously unexplored path for making defect-free crystals using inexpensive ingredients,” said Dr. Athanassios Panagiotopoulos, a professor of chemical and biological engineering at Princeton. “Current methods for making such systems rely on using difficult-to-synthesize particles with narrowly tailored directional interactions.” The researchers’ computer model simulated the formation of crystals based on principles of thermodynamics, and allowed them to analyze the equilibrium state of different possible crystal shapes to explore how they would be affected by the presence of different polymers. They found that when the crystals formed, tiny amounts of polymer were trapped between the colloids. These polymer-filled interstices are key to determining the energy state of a crystal, the researchers said. “If you understand how the polymer interacts with the colloids in the mixture, you can use that to create a desired crystal,” said lead researcher Nathan Mahynski, a graduate student in chemical and biological engineering at Princeton. “Changing the polymer affects which crystal form is most stable. As the crystal forms, the polymer helps set the crystal’s shape.” The work was funded in part by the National Science Foundation. The research was published in Nature Communications (doi: 10.1038/ncomms5472). For more information, visit www.princeton.edu.