Topology Protects Light Propagation in Photonic Crystal

A team from the research institute AMOLF and Delft University of Technology (TU Delft) observed light propagation within a photonic crystal without the usual distortion caused by reflections. Because the crystal consists of two parts, each with a slightly different pattern of perforations, light can propagate along the boundary between the two parts in such a way that it is “topologically protected,” and therefore does not bounce back at imperfections. Even when the boundary forms a sharp corner, the light follows it without a problem.

AMOLF's Ewold Verhagen and TU Delft's Kobus Kuipers are two of the researchers investigating a new class of electronic materials known as topological insulators. While most materials are either conductive for electrons or not, topological insulators exhibit a different kind of conduction altogether.

“The inside of a topological insulator does not allow electron propagation, but along the edge, electrons can move freely,” Verhagen said. “Importantly, the conduction is topologically protected; the electrons are not impacted by disorder or imperfections that would typically reflect them. So the conduction is profoundly robust.”

Scientists have long sought to replicate this behavior for light conduction. “We really wanted to accomplish topological protection of light propagation at the nanoscale, and thus open the door to guiding light on optical chips without its being hindered by scattering at imperfections and sharp corners,” Verhagen said.

The researchers used two-dimensional crystals with two slightly different hole patterns. The “edge” that enables light conduction is the interface between the two hole patterns.

“Light conduction at the edge is possible because the mathematical description of light in these photonic crystals can be described by specific shapes, or more accurately, by topology,” Kuipers said. “The topology of the two different hole patterns differs, and precisely this property allows light conduction at the boundary, similar to electrons in topological insulators. Because the topology of both hole patterns is locked, light conduction cannot be revoked; it is topologically protected.”

When the researchers imaged light propagation with a microscope, they saw that it behaved as predicted. Moreover, they witnessed the topology, or mathematical description, in the observed light.

"For these light waves, the polarization of light rotates in a certain direction, analogous to the spin of electrons in topological insulators,” Kuipers said. “The spinning direction of light determines the direction in which this light propagates. Because polarization cannot easily change, the light wave can even flow around sharp corners without reflecting or getting scattered, as would happen in a regular waveguide.”

By using silicon chips and light of a similar wavelength as used in telecommunication, Verhagen expects to increase the application prospects.

“We are now going to investigate if there are any practical or fundamental boundaries to topological protection and which functionalities on an optical chip we could improve with these principles,” Verhagen said. “The first thing we are thinking of is to make the integrated light sources on a photonic chip more reliable. This is important in view of energy-efficient data processing or green ICT [information and communications technology]. Also, to efficiently transfer small packages of quantum information, the topological protection of light can be useful.”

The research was published in Scientific Advances (www.doi.org/10.1126/sciadv.aaw4137).