Researchers at CU Boulder have developed highly efficient optical microresonators that could support a new generation of powerful sensor technologies.
A microresonator is a microscopic structure designed to confine light in a small space. As light circulates inside, its intensity increases. When that intensity reaches a sufficient level, scientists can carry out specialized optical processes that enable sensing and other advanced functions.
“Our work is about using less optical power with these resonators for future uses,” said Bright Lu, a fourth year doctoral student in electrical and computer engineering and a lead author on the study. “One day these microresonators can be adapted for a wide range of sensors from navigation to identifying chemicals.”
The research was published in Applied Physics Letters.
Racetrack Resonator Design Reduces Light Loss
To achieve stronger performance, the team focused on “racetrack” resonators, which are named for their elongated loop shape that resembles a running track.
They incorporated “Euler curves” — a type of smooth curve also found in road and railway design. Just as vehicles cannot navigate sudden right angle turns at speed, light does not travel efficiently through sharp bends.
“These racetrack curves minimize bending loss,” said Won Park, Sheppard Professor of Electrical Engineering and a co advisor on the project. “Our design choice was a key innovation of this project.”
By steering light through gradual, carefully engineered curves, the researchers significantly limited the amount of light that escaped. This allowed photons to circulate longer within the resonator and interact more intensely.
Lu explained that excessive light loss prevents the device from reaching the high intensities required for optimal operation.
Precision Nanofabrication at COSINC
The microresonators were fabricated at the Colorado Shared Instrumentation in Nanofabrication and Characterization (COSINC) clean room using a new electron beam lithography system.
Such facilities maintain tightly controlled conditions that are essential for producing reliable devices at extremely small scales. Many optical and photonic components are smaller than the width of a sheet of paper, so even tiny dust particles or minor surface imperfections can interfere with how light travels through them.
“Traditional lithography uses photons and is fundamentally limited by the wavelength of light,” Lu said. “However, electron beam lithography has no such constraint. With electrons, we can realize our structures with sub-nanometer resolution, which is critical for our microresonators.”
Lu described the fabrication process as one of the most rewarding parts of the project.
“Clean rooms are just cool. You’re working with these massive, precise machines, and then you get to see images of structures you made only microns wide. Turning a thin film of glass into a working optical circuit is really satisfying.”
Chalcogenide Glass Enables Ultra Low Loss Performance
A major milestone for the team was successfully building the devices using chalcogenides, a family of specialized semiconductor glasses.
“These chalcogenides are excellent materials for photonics because of their high transparency and nonlinearity,” Park said. “Our work represents one of the best performing devices using chalcogenides, if not the best.”
Chalcogenides allow intense light to pass through with minimal loss, which is essential for high performance microresonators. At the same time, they are challenging materials to process, requiring careful balance during fabrication.
“Chalcogenides are difficult, but rewarding materials to operate for photonic nonlinear devices,” said Professor Juilet Gopinath, who has collaborated with Park on this project for more than 10 years. “Our results showed that minimizing the bend loss enables ultra-low loss devices comparable to state-of-the-art in other materials platforms.”
Laser Testing and Resonance Measurements
After fabrication, the devices were evaluated under the leadership of James Erikson, a physics PhD student who specializes in laser based measurements. He precisely aligned lasers with microscopic waveguides to send light into and out of the resonators while monitoring its behavior inside.
The team searched for “dips” in the transmitted light signal that indicate resonance, which occurs when photons become trapped and circulate within the structure. By studying the shape of those dips, they were able to determine properties such as absorption and thermal effects.
“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,” Erikson said. “We’ve been chasing this kind of resonator for a long time, and when we saw the sharp resonances on this new device we knew right away that we’d finally cracked the code.”
Erikson noted that understanding how much light is absorbed compared to how much is transmitted is critical for device performance. Increasing laser power can introduce heating, which in turn can alter material properties or even damage the device.
“The way most materials interact with light also changes depending on the temperature of the material,” said Erikson. “So as a device heats up its properties can change and cause it to work differently.”
Toward Microlasers and Quantum Photonics
Looking ahead, these microresonators could be used to create compact microlasers, highly sensitive chemical and biological sensors, and tools for quantum metrology and networking.
“Many photonic components from lasers, modulators and detectors are being developed and microresonators like ours will help tie all of those pieces together,” Lu said. “Eventually, the goal is to build something you could hand to a manufacturer and create hundreds of thousands of them.”