Microresonator Increases Strength of Light-matter Interactions

Researchers at the University of Massachusetts Amherst, the University of Maryland, and the National Institute of Standards and Technology (NIST) built an on-chip microresonator that strengthens light-matter interactions without losing optical quality. The microresonator could be used for light-matter interactions in sensing and metrology, nonlinear optics, cavity quantum electrodynamics, and other photonics applications.

The device is based on a photonic crystal ring that resembles an internal gear. It combines the most useful characteristics of a photonic crystal resonator and a whispering-gallery mode (WGM) resonator. As a result of this combination, the new “microgear” device maintains a high optical quality factor (Q-factor) when slowing light to increase interactions.

Figure at left illustrates the defect area of photonic crystal bumps introduced into the lower middle area of the microring structure. Image at right shows what the defect looks like in practice. Courtesy of NIST/Lu.

The researchers applied a periodic modulation to the inside boundary of a microring resonator, opening a large bandgap, as in a photonic crystal cavity, while preserving the ring’s circular-shaped outside boundary and high Q-factor, as in a WGM cavity. The microgear photonic crystal ring targets a specific WGM to open a large photonic bandgap to tens of free spectral ranges. The mode spectrum is compressed while the Q-factor, angular momentum, and waveguide coupling properties of the WGM mode are maintained.

“What differentiates this work is the geometry of the microring,” researcher Andrew McClung said. “In the past, people have put holes in the center of these rings to introduce the photonic crystal. Instead of punching a hole, we created little bumps along the inside of the ring. This introduces the modulation you need and it perturbs the mode less aggressively.”

In experiments, the researchers observed modes with group velocity slowed by 10× relative to conventional microring modes, while supporting Q = (1.1 ± 0.1) × 10⁶. This Q-factor is about 50× the previous record for slow-light devices, the researchers said.

Electromagnetic field simulation looking down at the top of the microring shows how the defect is localized to a region smaller than the entirety of the ring. Courtesy of NIST/Lu.

In a defect version of the device, instead of making the bumps perfectly periodic along the circumference of the ring, the researchers placed some bumps at a slightly different amplitude, forming a way to localize light within just a small fraction of the ring.

The researchers demonstrated that they could localize into photonic crystal defect modes, reducing mode volume by more than 10× compared to conventional WGMs, while maintaining a high Q-factor value of up to (5.6 ± 0.1) × 10⁵. They were able to achieve the additional photonic crystal defect localization without the need for detailed electromagnetic design.

“We show that the optical modes in these structures can show a much lower group velocity than the modes in standard integrated photonic waveguides (slow-light), while maintaining low loss (high quality factor), and that we can further localize these modes spatially by introducing a ‘defect’ region within the resonator,” principal investigator Kartik Srinivasan said.

According to the researchers, controlling the resonance frequencies and waveguide coupling in the microgear photonic crystal ring is straightforward, owing to its WGM heritage. “Due to its unique combination of features, the overall system is appealing for many applications of microresonators, which in general are used to enhance light-matter interactions in a wide range of contexts, from single-photon sources to single-photon gates to nonlinear optics,” Srinivasan said.

By using a photonic crystal to modify fundamental properties of WGM, such as group velocity and localization, the microgear photonic crystal ring device has combined the best of both resonator types to offer an innovative platform for a range of photonics applications. “We are currently working on using our microgear photonic crystal ring to increase the strength of interaction between light and a vapor of rubidium atoms for applications in quantum networking,” researcher Xiyuan Lu said.

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-021-00912-w).