The new photonic chip creates an overlapping topological frequency comb

The new Photonic chip creates overlapping topological frequency combs

A new chip with hundreds of microscopic rings generated the first topological frequency comb. Credit: E. Edwards

Scientists in search of compact and powerful multicolor laser light sources have generated the first topological frequency comb. Their result, which relies on a tiny silicon nitride chip patterned with hundreds of microscopic rings, appears in the journal. science.

Light from an ordinary laser shines with a single, clearly defined color – or, equivalently, a single frequency. A frequency comb is like a souped-up laser, but instead of emitting a single frequency of light, a frequency comb shines with many dots of pristine, even frequencies. The equal spacing between the spikes resembles the teeth of a comb, which gives the frequency comb its name.

The earliest frequency combs required large equipment to create. Recently, researchers have focused on their miniaturization into integrated, chip-based platforms. Despite major improvements in shrinking the equipment needed to generate frequency combs, the basic ideas have not changed. Creating a useful frequency comb requires a stable light source and a way to distribute that light across the comb teeth taking advantage of optical gain, loss, and other effects that occur as the light source becomes more intense.

In the new work, JQI collaborator Mohammad Hafezi, who is also the Minta Martin Professor of Electrical and Computer Engineering and Physics at the University of Maryland (UMD), JQI collaborator Kartik Srinivasan, who is also a member of the National Institute of Standards and Technology and some colleagues have combined two lines of research into a new method for generating frequency combs.

One line is trying to miniaturize the creation of frequency combs using microscopic resonator rings fabricated from semiconductors. The second involves topological photonics, which uses patterns of repeating structures to create paths for light that are immune to small imperfections in fabrication.

“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 lead author of the new paper. “Our idea was basically, could similar physics be realized in a special mesh of hundreds of connected rings? It was quite a large scale up in the complexity of the system.”

By designing a chip with hundreds of resonator rings arranged in a two-dimensional grid, Flower and his colleagues created a complex interference pattern that takes incoming laser light and loops it around the edge of the chip while the chip material itself separates it. up to many frequencies.

In the experiment, the researchers took pictures of the light from above the chip and showed that it was, in fact, circling around the edge. They also took a portion of the light to perform a high-resolution analysis of its frequencies, demonstrating that the circulating light had the structure of a double-frequency comb. They found a comb with relatively wide teeth, and tucked inside each tooth, they found a smaller hidden comb.

Although this comb is only a proof of concept for now—its teeth aren’t evenly spaced and are a little too noisy to be called pristine—the new device could eventually lead to more smaller and more efficient for frequency combs that can be used in atomic clocks, beam-finding detectors, quantum sensors, and many other tasks that require precise measurements of light.

The well-defined spacing between spikes on an ideal frequency comb makes them excellent tools for these measurements. Just as the equal lines on a ruler provide a way to measure distance, the equal spikes of a frequency comb allow the measurement of unknown frequencies of light. Mixing a frequency comb with another light source produces a new signal that can detect the frequencies present in the second source.

Repetition breeds repetition

Qualitatively at least, the repeating pattern of microscopic ring resonators on the new chip gives rise to the pattern of frequency dots that circle around its edge.

Individually, the microrings form tiny little cells that allow photons—quantum particles of light—to bounce from ring to ring. The shape and size of the microrings were carefully chosen to create the right kind of interference between the different hopping paths, and taken together, the individual rings form a super-ring. Together, all the rings spread the incoming light to many comb teeth and direct them along the edge of the mesh.

The microrings and the larger superring provide the system with two different time and length scales, since light takes longer to travel around the larger superring than any smaller microring. This eventually leads to the generation of two overlapping frequency combs: One is a thick comb produced by smaller microrings, with frequency spikes spaced far apart. Inside each of those coarsely spaced spikes lives a finer comb, produced from super-rings.

The authors say that this interlaced comb-within-a-comb structure, reminiscent of Russian nesting dolls, could be useful in applications that require accurate measurements of two different frequencies that happen to be separated by a wide gap.

The new Photonic chip creates overlapping topological frequency combs

A scheme of the new experiment. The incoming pulsed laser light (pump laser) enters a chip that hosts hundreds of microrings. The researchers used an on-chip IR camera to capture images of light circulating around the edge of the chip, and they used a spectrum analyzer to detect a frequency comb placed on the circulating light. Credit: C. Flower

Doing things right

It took more than four years for the experiment to come together, a problem made worse by the fact that only one company in the world could manufacture the chips the team had designed.

Early chip samples had very thick microrings with very sharp bends. Once incoming light passed through these rings, it would scatter in all sorts of unwanted ways, killing any hope of generating a frequency comb.

“The first generation of chips didn’t work at all because of this,” says Flower. Returning to the design, he shortened the width of the ring and rounded the corners, eventually landing on a third generation of chips, delivered in mid-2022.

While iterating on the chip design, Flower and his colleagues also discovered that it would be difficult to deliver enough laser power to the chip. In order for their chip to work, the intensity of the incoming light had to exceed a threshold – otherwise no frequency comb would form.

Normally, the team would have reached for a commercial CW laser, which delivers a continuous beam of light. But those lasers spread too much heat to the chips, causing them to burn or swell and not match the light source. The team needed to concentrate the energy in bursts to deal with these thermal issues, so they turned to a pulsed laser that delivers its energy in a fraction of a second.

But this presented its own problems: off-the-shelf pulsed lasers had pulses that were too short and contained too many frequencies. They tended to introduce a bunch of unwanted light—both at the edge of the chip and through its middle—instead of the edge-limited light that the chip was designed to diffuse into a frequency comb. Because of the long time and expense involved in getting new chips, the team had to make sure they found a laser that balanced peak power delivery with longer durations and tunable pulses.

“I emailed basically every laser company,” says Flower. “I asked to find someone to make me a custom, long pulse laser. Most said [that] they [didn’t] do it, and they are too busy to make custom lasers. But a company in France came back to me and said, ‘We can do this. Let’s talk.'”

His persistence paid off, and after several shipments from France to install a more robust cooling system for the new laser, the team finally sent the right kind of light to their chip and saw an overlapping frequency comb.

The team says that while their experiment is specific to a chip made of silicon nitride, the design can easily be translated to other photonic materials that can create combs in different frequency bands. They also consider their chip to introduce a new platform for the study of topological photonics, particularly in applications where there is a threshold between relatively predictable behavior and more complex effects – such as generating a frequency comb.

More information:
Christopher J. Flower et al, Observation of topological frequency combs, science (2024). DOI: 10.1126/science.ado0053

Provided by the Joint Quantum Institute

citation: New photonic chip spawns comb with overlapping topological frequency (2024, June 20) retrieved June 21, 2024 from

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