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Shaping light with groundbreaking biologically inspired materials

The Proceedings of the National Academy of Sciences published work by Physics Professor Max Bi’s group which unveils designs for a new photonic material inspired by patterns in tissue layers like human skin.

So, what is photonics? You can think about it as using light in the same way we use electricity, as electrons are to electricity, photons (light particles) are to photonics. Fiber optic cables and lasers are some everyday photonic technologies. Researchers in photonics look for ways to manipulate light in the same way we manipulate electricity: wires, switches, especially related to computing. Computers based on light have the potential to speed up computers significantly while drastically improving energy efficiency.

An expanding and exciting area of research is the study of materials with a photonic band gap. These materials allow some frequencies of light to pass through them, but they have a specific range of frequencies that don’t make it through. “Because of the specific structure, the light at this frequency runs into itself and cancels out,” says Bi. The structure he mentions is key. Traditional photonic materials are crystalline, made up of patterns that repeat themselves over long distances.

Assistant Professor in College of Science Dapeng Bi. Photo by Matthew Modoono/Northeastern University

A one-dimensional photonic crystal looks like a layered cake and can filter light coming from one direction. A two-dimensional photonic crystal looks like a chessboard on its top, as it is made up of columns of material stacked together. A three-dimensional photonic material can be thought of as cubes stacked to create a large cube that looks like a chessboard from all sides. This three-dimensional structure can filter light from each direction, but light must always strike the surface along certain directions, with very little margin for error. These structures are very fragile, and creating a photonic crystal at nanoscale, one that could be useful in a handheld or portable device, is extremely difficult. “You cannot make a mistake with a crystal. If one of the checkerboard sites is missing, that may give rise to disruptions of this photonic property” says Bi.

Professor Max Bi and his graduate student Xinzhi Li along with postdoctoral researcher Amit Das sought to create a new material that would solve some of the shortcomings with crystals and make some breakthroughs along the way. Photonics, however, was not in either of their wheelhouses. In fact, Bi said that his student, Li was more familiar with the topic in the beginning. “It’s been a lot of fun working with Xinzhi learning together. Usually you work with a professor and it’s like: here’s a project, go do it. This is something we had to work together, make mistakes together, and debug things together, so it’s been really fun.” Bi’s background is in soft matter physics, a field that focuses on materials that have interesting physical properties that depend on changes in temperature near room temperature and can be easily deformed due to external stress. Bi has worked with granular materials as well as biological materials by simulating the mechanics and behavior of cells and tissues, often working alongside biologists.

Recent work in photonic designs found a concept of hyperuniformity, which was believed to be essential to photonic materials. Li described hyperuniformity as “something between totally random points and crystalline order, it has hidden order but you can’t easily see it.” Hyperuniformity inspired Bi to explore using biological designs to create a photonic material, starting with cells that form skin layers in animals, where Bi says “no one would have guessed that there’s hidden order going on, but apparently there is.” By placing rods of material at the centers of these cells, the material becomes photonically active, with a significant photonic band gap, or filtering ability. Bi is currently working with 3D printers to prototype these structures.

A comparison of typical cell shapes in epithelial tissue, adapted from (Kasza et al, PNAS 111, 32 (2014), and 2 dimensional photonic materials based on cell packings in epithelial tissues.

As they developed the material, other interesting properties came into view. Each cell within the pattern of the material has a shape that it would prefer to be, specifically a certain perimeter. Bi describes the phenomenon with a rubber band model, which shows that the tension of a stretched rubber band is pulling against its limits, desperate to return to its slack state. The cells in the material all have an ideal perimeter that it tries to be as close to as possible. Adjusting this ideal perimeter changes the material significantly.

There’s a magic number where the perimeter is large enough that the material starts to behave strangely: it becomes a fluid. Remarkably, the fluid can still function as a photonic material after it transitions, though not as well as its solid form. At very high perimeters, the photonic ability disappears, though hyperuniformity persists. This contradicts earlier research in the field, Bi remarked. “According to prominent researchers in previous studies, they claim that [hyperuniformity] is absolutely necessary and sufficient to get band gaps, and we found that it’s not.”

An animation of a 3D aggregate (left) and its corresponding photonic network (right).

On top of functioning as fluids, the designs by Li, Das, and Bi are not limited to two dimensions. In fact, they can be extrapolated to fit a three-dimensional model. This allows them to create a self-assembling, 3D photonic material that can filter light from any angle, a huge advantage over traditional photonic crystals. “3D is traditionally much harder to engineer band gaps for. So even though it’s quite small, this is somewhat of an exciting result for people who are working on photonics,” Bi said.

Li, Das, and Bi are excited about the possibility of self-assembling photonic liquids, and they’re working with researchers to make it a reality. “You want to mix something with something else and shake it in a test tube,” Bi says, “and it comes out with a structure like this.” A process like this would incredibly simplify manufacturing of photonic materials, opening the door to their use in wider applications.

In the future, Li, Das and Bi will continue to explore photonic materials and look to biology for more inspiration. They hope that their materials can soon become a reality through collaborations with experimentalists and help advance the study of photonic materials.

Learn more on the Theoretical Soft Matter and Physics group website.

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