Light-splitting crystals from inexpensive ingredients

Photonic crystals

Researchers from Princeton and Columbia universities have proposed a method for growing specialized materials called photonic crystals by ensuring that tiny particles settle into a single uniform crystal structure. Previously, the particles assumed a variety of structures, which made the resulting crystals unsuitable for high-performance uses. At left, particles form the initial two layers of a crystal, labeled A and B. With the addition of the third layer, the crystal forms one of two possible shapes, shown in side views at right. The vertical blue string to the right of each crystal shows a chemical chain that the researchers add to the mix to force one structure to form versus the other.

HIGHLY PURIFIED CRYSTALS that split light with uncanny precision are key parts of high-powered lenses, specialized optics and, potentially, computers that manipulate light instead of electricity. But producing these crystals by current techniques, such as etching them with a precise beam of electrons, is often extremely difficult and expensive.

Now, researchers at Princeton and Columbia universities have proposed a method that could allow scientists to customize and grow these specialized materials, known as photonic crystals, with relative ease.

“Our results point to a previously unexplored path for making defect-free crystals using inexpensive ingredients,” said Athanassios Panagiotopoulos, the Susan Dod Brown 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.”

In an article published online July 21, 2014, in the journal Nature Communications, Panagiotopoulos and colleagues propose that photonic crystals could be created from a mixture in which particles of one type are dispersed throughout another material. Called colloidal suspensions, these mixtures include things like milk or fog. Under certain conditions, these dispersed particles can combine into crystals.

Creating solids from colloidal suspensions is not a new idea. In fact, humans have been doing it since the invention of cheese and the butter churn. But there is a big difference between making a wheel of cheddar and a crystal pure enough to split light for an optical circuit.

One of the main challenges for creating these optical crystals is finding a way to create uniform shapes from a given colloidal mixture. By definition, crystals’ internal structures are arranged in a pattern. The geometry of these patterns determines how a crystal will affect light. Unfortunately for optical engineers, a typical colloidal mixture will produce crystals with different internal structures.

In their paper, the researchers demonstrate a method for using a colloidal suspension to create crystals with the uniform structures needed for high-end technologies. Essentially, the researchers show that adding precisely sized chains of molecules — called polymers — to the colloid mixture allows them to impose order on the crystal as it forms.

“The polymers control what structures are allowed to form,” said Nathan Mahynski, a graduate student in chemical and biological engineering at Princeton and the paper’s lead author. “If you understand how the polymer interacts with the colloids in the mixture, you can use that to create a desired crystal.”

The researchers created a computer model that simulated the formation of crystals based on principles of thermodynamics, which state that any system will settle into whatever structure requires the least energy. They found that when the crystals formed, tiny amounts of polymer were trapped between the colloids as they came together. These polymer-filled spaces, called interstices, play a key role in determining the energy state of a crystal. “Changing the polymer affects which crystal form is most stable,” Mahynski said.

Besides Panagiotopoulos and Mahynski, the paper’s authors include Sanat Kumar, a professor and chair of chemical engineering, and Dong Meng, a postdoctoral researcher, both at Columbia. Support for the project was provided in part by the National Science Foundation.

–By John Sullivan