Researchers from the University of Maryland’s Joint Quantum Institute (JQI) combined the qualities of two methods for light-trapping — whispering-gallery mode (WGM) and photonic crystals — in a hybrid device called the microgear photonic crystal ring. The device holds implications for nonlinear optics, where light interacts with the matter it travels through to produce new colors and directions, and is in the area of cavity quantum electrodynamics. In cavity quantum electrodynamics, which explores the interactions between atoms and light, single atoms or quantum dots could be trapped near a localized, intense beam of light, which would allow their behavior to be studied. The application could potentially lead to the control of quantum matter with light. As is true of WGMs, the microgear photonic crystal ring can trap multiple colors of light. Like photonic crystals, it can capture specific colors of light in tightly confined, high-intensity bundles. It is easy to guide light into the device, which could serve as a platform for applications in sensing, metrology, light-matter interactions, and nonlinear optics. WGMs trap light in a ring made of silica or another material that is transparent to light. The light travels around the ring many thousands of times before leaking out, producing high-intensity light in a small volume. Photonic crystals are periodically structured, with a grid that reflects light of a specific color. To accumulate the light and trap it in a tiny space, a defect is introduced into the crystal’s grid. Compared to WGMs, photonic crystals create more light intensity per photon; but they require detailed electromagnetic design and precise manufacturing to implement. In addition, creating photonic crystals that are able to trap multiple colors has been an ongoing challenge. The researchers applied a periodic modulation to the inside boundary of a micro-ring resonator to open a large bandgap, while maintaining the ring’s circularly symmetric outside boundary and high optical quality (Q) factor. The device is designed to target a specific WGM to open a large photonic crystal bandgap up to tens of free spectral ranges, compressing the mode spectrum. At the same time, the device maintains the high-Q, angular momenta, and waveguide coupling properties of the WGM modes. Even though the hybrid device enables a higher optical intensity than conventional WGMs, the process of getting light in and out of the device remains straightforward. To add the photonic crystal element to the hybrid device, the researchers cut notches — the microgears of the device — into the inside wall of the ring. Defects were added by modifying the size of some of the notches. The microgears confine just a few colors of light into tight bundles, while allowing other colors to circle around the micro-ring freely. “People have been saying for a long time that micro-rings and photonic crystals have complementary strengths, and so it would be great to put them together to get the best of both worlds,” Kartik Srinivasan, a fellow of JQI and of the National Institute of Standards and Technology (NIST), said. “But in general, when people put them together this didn’t happen — sometimes you could even get the worst of both worlds. The notion that you can stick a photonic crystal into a micro-ring with this kind of strength and modulation, while retaining a high quality factor (low loss), has actually been rather surprising for a lot of people, myself included.” In a further study, the researchers showed that incorporating a photonic crystal pattern in an integrated micro-ring can result in WGMs with fractional optical angular momentum, and that the modes with fractional angular momenta exhibit high optical quality factors with good cavity-waveguide coupling and an order of magnitude reduced group velocity. By introducing multiple artificial defects, the researchers showed that multiple modes can be localized to small volumes within the ring, while the relative orientation of the delocalized band-edge states can be well controlled. The researchers added multiple defects into the notch pattern. Each defect confined the light to a small fraction of the circumference of the micro-ring (much like in a photonic crystal). The researchers found that up to four defects could be added into the same micro-ring to confine light in four places and build high intensity in a tightly confined space. The team further found that it could use the microgears to control different colors of light in different ways, at the same time. Some colors were trapped in the defects and confined to a volume that was much smaller than the ring. Other colors circulated freely around the micro-ring. They were not confined by the defects, but were still under the influence of the gear structure, giving the researchers greater control over the light beam. In a conventional WGM, the electromagnetic field of a light beam or pulse wraps around the micro-ring, forming a standing wave that transitions through a number of peaks and troughs along the edge of the ring. Although it’s possible to predict the number of peaks and troughs the wave will undergo, the way they will line up in the ring is random and cannot be predicted. Through the microgears and defects in the hybrid device, the researchers can control exactly where the peaks and troughs of the free-floating color land. They can even create something like a Möbius strip made out of light. “If everything is symmetric, light can stand anywhere it likes. But now we can control it,” Xiyuan Lu, a researcher at JQI and NIST, said. According to Srinivasan, the device shows a streamlined optimal use for properties of photonic crystals. “In photonic crystals, you can kind of engineer one mode pretty well, but it’s difficult to engineer multiple modes simultaneously,” Srinivasan said. “With this device, we can envision mixing between different colors of light that we can really engineer the modes of, while having these additional resources of strong confinement and high intensity.” For cavity quantum electrodynamics, Srinivasan said the device creates an important platform. “We have a platform now where it’s straightforward for us to have multiple sites within one of these resonators that can host single quantum emitters,” he said. “There are potential applications, like single-photon sources and quantum gates. But a part of it is also fun electromagnetism and fun optical phenomena in these devices.” The researchers are confident that many uses will be found for the microgear photonic crystal ring, in part because it is easy to design, fabricate, and operate. “In our case, the platform seems to be quite forgiving,” Lu said. “If you do anything new, chances are it can work well.” The research was published in Nature Photonics (www.doi.org/10.1038/s41566-021-00912-w) and Physical Review Letters (www.doi.org/10.1103/PhysRevLett.129.186101).