Researchers have found a way to confine light so that it is insensitive to imperfections in the material that confines it. The new approach is based on “topological protection,” a concept used extensively in solid-state electronic physics. It could help lower the cost while improving the speed of photonic devices. Using a waveguide lattice structure, a team of researchers from Penn State, the University of Pittsburgh, and the University of Illinois was able to protect the mode frequency at midgap and minimize the volume of a photonic defect mode. Light entered at one end of the waveguide array and was trapped and confined as it propagated through the waveguides. The trapped light became immune to imperfections in the waveguides and was able to tolerate significant imperfections in the lattice structure. Microscope image of a cross section of waveguide array in a topological crystalline insulator lattice geometry. New research shows that this configuration allows light to be confined in a way that is insensitive to imperfections in the material. This advance could lead to cheaper and more efficient photonic devices, such as lasers and optical fibers. Courtesy of Rechtsman Laboratory, Penn State University. According to researchers, the light became insensitive because of a phenomenon known as topological protection. “The waveguide structure is a photonic analog of so-called topological crystalline insulators, and this form of topological protection can potentially be used across a range of photonic devices, including in nanoscale lasers, specialized nonlinear optical fibers, and for robustly and precisely coupling between photons and electrons for manipulating quantum information,” said professor Mikael Rechtsman. Researchers demonstrated this approach in a femtosecond-laser-written waveguide array by observing the presence of a topological zero mode confined to the corner of the array. The robustness of this mode was guaranteed by a topological invariant that protects zero-dimensional states embedded in a two-dimensional environment. According to researchers, the experiment illustrates a form of topological protection that has not been previously demonstrated. This approach to confining light could make photonic devices less expensive to produce and, at the same time, more efficient. The experiment offers an example of a potential cross-disciplinary use of topological protection, uniting photonics and solid-state electronics, and demonstrates the broad applicability of the phenomenon beyond electronic solid-state physics. Rechtsman emphasized the importance, and the challenges, of trapping light and confining it to very small spaces. “It compresses the maximum amount of optical power into the smallest area or volume inside a material, making it interact more strongly with the material, and thus it is more efficient at whatever it is meant to do," Rechtsman said. “A major difficulty with doing this has been that strong confinement brings with it extreme sensitivity to any imperfections in the material, which can often either inhibit efficiency or make the device very expensive to fabricate. Our results suggest that we can overcome this difficulty.” The research was published in Nature Photonics (doi:10.1038/s41566-018-0179-3).