CU Boulder racetrack microresonator keeps light circulating longer on a glass chip

Engineers have shown that a minute loop patterned into a glass chip can keep light circulating for much longer than comparable devices.

This longer circulation time concentrates optical energy into a region narrower than a human hair, suggesting a route to smaller, more efficient sensors and other light-based technologies.

Light stayed in the loop

Within a slim, oval loop etched into the chip, laser light completed lap after lap rather than escaping at every bend.

By studying the pronounced dips in the measured signal created by that repeated circulation, Bright Lu at the University of Colorado Boulder (CU Boulder) linked the unusually persistent light to lower bending loss inside the loop.

Because the turns were gentle, photons were guided around each circuit without the abrupt “kicks” that typically drive light outwards, allowing more of the initial light to survive each round trip.

Trapping light this well boosted intensity without requiring excessive input power, paving the way for the design decisions that enabled those smooth bends.

Racetrack light loops

At the centre of the chip sat a microresonator - a loop that sends light round and round many times.

On every pass, the incoming wave adds to the wave already trapped in the loop, so the intensity builds without the need for a very powerful external laser.

Once intensity is high enough, the light can produce nonlinearity: the material’s response varies with optical power, which supports frequency conversion and sensing.

“ ჩვენი work is about using less optical power with these resonators for future uses,” said Lu.

Smoother turns matter

When a straight section meets a curve, sharp geometry tends to push light outwards, so the biggest losses often occur right at those transitions.

To reduce that effect, the researchers used Euler curves - bends whose curvature changes progressively - to shape each corner.

“These racetrack curves minimize bending loss,” said Professor Wounjhang Park, the Sheppard Professor of Electrical Engineering at CU Boulder.

Even with improved bends that limited leakage, the racetrack still needed to fit within the very small footprint of an on-chip device.

Choosing the right glass

The material was as important as the layout, because the loop needed to remain optically clear even as intensity rose.

For the core of the device, the team selected a specialist glass called chalcogenide, which transmits light efficiently while still changing its behaviour under high power.

“These chalcogenides are excellent materials for photonics because of their high transparency and nonlinearity,” said Park.

Working with the glass was challenging, as any surface roughness can introduce scattering that steals light before the loop achieves its purpose.

Building nanoscale circuits

Because dust and microscopic surface defects can compromise a photonic circuit, the loops were made in a cleanroom.

To pattern features with crisp boundaries, the team used electron-beam lithography, which writes structures using a focused beam of electrons.

“Traditional lithography uses photons and is fundamentally limited by the wavelength of light,” said Lu.

That precision helped produce smoother edges - important because rough sidewalls can scatter light out of the loop.

Reading the resonances

Following fabrication, the researchers searched each measurement scan for resonance - the condition in which light in the loop reinforces itself.

They carefully aligned the set-up to inject laser light into tiny on-chip channels and then collect the light coming back out.

“The most obvious indicator of device quality is the shape of the resonances and we want them to be deep and narrow, like a needle piercing through the signal background,” said James W. Erikson, a physics Ph.D. student at CU Boulder.

Those resonance profiles allowed the team to distinguish optical loss from heating effects, helping them assess how strongly the chip could be driven.

Measuring optical efficiency

A handful of figures reveals what the loop can deliver when engineers test its limits.

In the preprint, the quality factor - a measure of how long light remains in the resonator before leaking away - reached about 4.5 million.

As light travelled around the loop, only a very small amount diminished, and the glass itself absorbed just a tiny fraction.

Despite a cross-section below 10 billionths of a square centimetre (under 1.6 billionths of a square inch), the device achieved performance comparable with other leading on-chip platforms.

Heat and stability

Heat posed a subtle problem, because even modest losses can warm the glass and shift the resonance condition.

As temperature increases, the speed of light through the glass changes, so the preferred wavelength moves and the signal shape alters.

“The way most materials interact with light also changes depending on the temperature of the material,” explained Erikson.

By reducing loss, the device remained more stable, since less of the circulating light was converted into heat inside the glass.

From lab to factory

Turning a laboratory demonstration into a product requires producing the same tiny loop repeatedly, with minimal variation.

Because uniformity relies on repeatable manufacturing steps, the team pursued designs that a foundry could reproduce across many chips.

“Eventually, the goal is to build something you could hand to a manufacturer and create hundreds of thousands of them,” said Lu.

If such loops could be made at scale, more light-based circuits could reach the market, although practical products will also require packaging that keeps both contamination and heat under control.

Next steps for photonics

By pairing smoother racetrack bends with a chalcogenide glass core and careful fabrication, the group extended how long light could circulate on chip.

Future work will focus on reducing sidewall roughness further and integrating additional on-chip components, so these resonators can progress towards deployable, field-ready sensors.

Image caption: Optical waveguide microresonators on a chip, which are ten times thinner than a human hair. Credit: University of Colorado Boulder