Getting Started: Researchers Create Slow Light Device With High Optical Quality

AMHERST, Mass. – Researchers, including a postdoc at the University of Massachusetts Amherst, have created a gear-shaped photonic crystal micro-ring that increases the strength of light-matter interactions without sacrificing optical quality. The result is an on-chip microresonator with an optical quality factor 50 times better than the previous record in slow-light devices that could improve microresonators used in a range of photonics applications, including sensing and metrology, nonlinear optics and cavity quantum electrodynamics.

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Andrew McClung

Optical microresonators are structures that enhance light-matter interactions through a combination of long temporal confinement (i.e. high quality factor) and strong spatial confinement of an electromagnetic wave. The device the authors developed in many ways integrates the best attributes of two types of optical microresonators – a photonic crystal and a whispering gallery mode resonator – into a single device. While combining the two has been attempted in the past, previous micro-ring devices that were successful in slowing light to increase interactions (a consequence of the photonic crystal) had to sacrifice quality factor. In this new “microgear” photonic crystal ring, the researchers observed modes with a 10-fold slower group speed compared to conventional microring modes without any quality factor degradation.

The study, led by first author Xiyuan Lu and principal investigator Kartik Srinivasan, both of the National Institute of Standards and Technology (NIST) and the University of Maryland, appears in the January 2022 issue of “Nature Photonics.” . UMass Amherst’s Andrew McClung, a postdoc in Amir Arbabi’s photonics lab and Lu’s former NIST colleague, provided computer modeling and simulations for the work.

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fault
The figure on the left illustrates the defect zone of photonic crystal bumps introduced in the lower middle zone of the microring structure. The picture on the right shows what the defect looks like in practice. (Image: NIST/Lu)

“We show that optical modes in these structures can exhibit much lower group velocity than modes in standard integrated photonic (“slow light”) waveguides while maintaining low loss (high quality factor), and that we can further localize these modes spatially by introducing a ‘defective’ region into the resonator,” says Srinivasan. “Due to its unique combination of features, the overall system is attractive for many applications of microresonators, which are typically used to enhance light-matter interactions in a wide range of contexts, from single-photon sources to single-photon gates to nonlinear ones. optical.”

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The electromagnetic field simulation looking up the micro-ring shows how the fault is localized in a region smaller than the entire ring. (Image: NIST/Lu)

What sets this work apart, McClung says, is the geometry of their micro-ring.

“In the past, people made holes in the center of these rings to introduce the photonic crystal,” he says. “Instead of punching a hole, we created little bumps inside the ring. This introduces the modulation you need and it disrupts the mode less aggressively.

In the faulty version of the device, instead of the bumps being perfectly periodic along the circumference of the ring, a number of the bumps have a slightly different amplitude, forming a means of localizing the light in only a small fraction of the ring.

“Devices like ours can be used to enhance the light-matter interaction, and we are currently working on using our microgear photonic crystal ring to increase the strength of the interaction between light and a vapor of atoms. of rubidium for applications in quantum networks,” added Lu.

This research is supported by the DARPA Science of Atomic Vapors for New Technologies (SAVaNT) and NIST on a chip programs.

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