Researchers demonstrated a mechanism for concentrating light at an extremely small scale, which can be used for a broad spectrum of wavelengths. The team, comprising scientists from Atomic and Molecular Physics (AMOLF), Delft University of Technology, and Cornell University, leveraged the topological properties of photonic crystals to concentrate light on a chip and achieve broadband localization of light. The new approach to concentrating and localizing light could be used in photonic chip-based applications for quantum communication, optical sensing, and lasing, for example. The mechanism for the local enhancement of optical fields is based on the strong suppression of backscattering. Another important aspect of the method is the topology of the physical system. Artist’s impression of the light concentration at the ‘wall’ at the end of the waveguide. Courtesy of AMOLF. “What makes this design special is that the conduction of light is topologically protected, meaning that scattering or reflection of light by imperfections in the crystal is suppressed,” researcher Daniel Muis said. Using valley photonic crystals as a topological photonic platform, the researchers demonstrated that light could be localized at the termination of a reciprocal topological waveguide. Only for terminations that approximately conserved the valley degree of freedom was reflection suppressed strongly enough to result in localization. The researchers explored what might happen if the waveguide was terminated with a wall of material that light could not pass through. “Since the light has nowhere to go and reflections are suppressed, it should accumulate in front of that wall,” Muis said. “The light does eventually bounce back through the waveguide, but only after a delay. This results in a local amplification of the light field.” To verify their predictions about the accumulation of light within the photonic crystal, the researchers used microscopy to scan the light fields with an ultrathin needle positioned above the surface of the crystal. The results confirmed that the team’s approach to concentrating light led to strong confinement of light at the termination of the topological photonic waveguide. “We indeed saw a clear amplification of the light field at the end of the topological waveguide,” Muis said. “Interestingly, this only happened when the ‘wall’ terminating the waveguide was placed at a certain angle. This was exactly what our partners at Cornell had predicted. It proves that the light amplification is related to the topological suppression of back reflection.” An electron microscopy image of the silicon photonic crystal. The topological waveguide is formed at the boundary between the green and blue regions, and is terminated by the crystal with round holes on the right side (left). A measurement of the optical intensity in the photonic crystal. Light enters through the topological waveguide from the left and accumulates at the end of the waveguide due to suppressed back reflection (right). Courtesy of AMOLF. The researchers compared different waveguide termination geometries, confirming that the origin of suppressed backscattering came from the near conservation of the valley degree of freedom. Muis said that the light amplification is concentrated in a very small volume — as small as the wavelength of the light itself. The method is inherently broadband and therefore works for many different wavelengths. AMOLF group leader Ewold Verhagen said that, until now, the only ways to concentrate light were through optical cavities or by using waveguides to compress the light like a funnel. “The first method uses resonance, which limits the focusing or concentration of light to a specific wavelength,” he said. “The second method works similar to a traditional lens, only in a device much larger than the wavelength of the light used.” According to the researchers, the mechanism should apply to any type of wave in a structured medium, including sound waves or even electrons in specific crystals. This approach to localized optical field enhancement could provide a new way to enhance electromagnetic fields at the nanoscale, for use in nanophotonics and quantum applications, and could enable the strong light-matter interactions necessary for the efficient manipulation and sensing of light. More broadly, the demonstrated paradigm of optical energy localization, in combination with robust guiding and manipulation of light, could open opportunities for on-chip photonic technology. “For a next step, it would be interesting to use a pulsed laser to look at the time interval in which the light continues to accumulate, to see how much the field amplification can be maximized, and to use it for applications in light manipulation on optical chips,” Muis said. The research was published in Science Advances (www.doi.org/10.1126/sciadv.adr9569).