Researchers create first topological frequency comb on a silicon nitride chip

Scientists in the quest for compact and robust sources of multicolored have achieved a groundbreaking milestone by generating the first topological frequency comb. This innovative result, reliant on a small nitride chip patterned with hundreds of microscopic rings, has been detailed in the journal Science.

Traditionally, light from an ordinary laser emits a single, sharply defined color, or frequency. A frequency comb, however, is a sophisticated laser that emits light with many pristine, evenly spaced frequency spikes, resembling the teeth of a comb—hence its name. The early iterations of required bulky equipment. Recent advancements have focused on miniaturizing these combs into integrated, chip-based platforms. Despite these advancements, the fundamental principles of generating frequency combs—requiring a stable light source and a method to disperse this light into comb-like teeth through optical gain, loss, and other intensity-dependent effects—remain unchanged.

In their pioneering work, JQI Fellow Mohammad Hafezi, also a Minta Martin professor of electrical and computer and physics at the University of Maryland (UMD), JQI Fellow Kartik Srinivasan, a Fellow of the National Institute of Standards and Technology, and their colleagues have merged two significant research avenues to devise a novel method for generating frequency combs.

One research avenue aims to miniaturize frequency comb creation using microscopic resonator rings fabricated from semiconductors. The other involves topological , leveraging patterns of repeating structures to create light pathways that are resilient to small fabrication imperfections.

“The world of frequency combs is exploding in single-ring integrated systems,” says Chris Flower, a graduate student at JQI and the UMD Department of Physics and the lead author of the new paper. “Our idea was essentially, could similar physics be realized in a special lattice of hundreds of coupled rings? It was a pretty major escalation in the complexity of the system.”

The researchers designed a chip with hundreds of resonator rings arranged in a two-dimensional grid. This configuration engineered a complex interference pattern that circulates input laser light around the chip's edge, splitting it into many frequencies. In their experiment, snapshots from above the chip confirmed the circulation of light along the edge. They also extracted some light for high-resolution frequency analysis, demonstrating that the circulating light exhibited a dual structure of frequency combs. One comb had relatively broad teeth, while within each tooth, a smaller, secondary comb was present.

Although this nested comb is currently a proof of concept—with its teeth not yet evenly spaced and somewhat noisy—the new device has the potential to lead to smaller, more efficient frequency comb equipment. Such advancements could significantly enhance atomic clocks, rangefinding detectors, , and various other requiring precise light measurements.

The precise spacing between spikes in an ideal frequency comb makes them excellent tools for accurate measurements. Just as evenly spaced lines on a ruler measure distance, the evenly spaced spikes of a frequency comb allow for the measurement of unknown light frequencies. Mixing a frequency comb with another light source generates a new signal that can disclose the frequencies in the second source, demonstrating the wide-ranging potential of this breakthrough.

Source: Joint Quantum Institute