In a groundbreaking advancement in photonics technology, researchers at the University of Colorado Boulder have engineered highly efficient optical microresonators with the potential to revolutionize sensor technologies across multiple industries. These microresonators, minuscule devices capable of confining light and amplifying its intensity, provide a new platform for intricate light manipulation at scales far smaller than previously possible, opening a plethora of avenues for future applications ranging from advanced navigation systems to chemical detection.
At the heart of this innovation lies the microresonator’s ability to trap light within an ultra-small footprint, allowing photons to circulate and intensify. Increasing the intensity within these microscopic cavities is pivotal because it enables a range of nonlinear optical processes that are essential for developing sensitive and compact photonic components. The team’s focus on reducing the optical power required to achieve these high intensities marks a significant stride toward practical, scalable photonic devices that can be integrated into everyday sensors and communication systems.
The researchers adopted a “racetrack” geometry for their resonators—a design inspired by running tracks with elongated loops—which plays a critical role in optimizing light confinement. Unlike conventional shapes, these racetrack resonators incorporate smooth Euler curves, a concept borrowed from road and railway engineering, which allows light to navigate bends without abrupt changes in direction. This minimizes bending losses, a common source of inefficiency where photons escape or are absorbed due to sudden curvatures, thereby enhancing the resonator’s quality and performance.
The implementation of Euler curves is a deliberate design innovation that ensures photons maintain coherence and energy as they circulate within the device. By mitigating the detrimental effects of sharp bends on light propagation, the team succeeded in increasing the residence time of photons inside the resonator. This extended interaction time boosts the efficacy of nonlinear processes, crucial for applications demanding precision and sensitivity such as quantum computing components and high-fidelity sensors.
Fabrication of these ultra-thin microresonators—astonishingly ten times thinner than a human hair—was achieved using advanced electron beam lithography at the Colorado Shared Instrumentation in Nanofabrication and Characterization (COSINC) facility. Unlike traditional photolithography, electron beam lithography achieves resolutions at sub-nanometer scales by directly writing patterns with electrons instead of photons, overcoming fundamental wavelength limitations. This precision manufacturing is vital to realize the intricate geometries and smooth curves demanded by the racetrack design to ensure minimal optical losses.
Working at the nanoscale, researchers had to maintain extreme environmental control to prevent surface imperfections and microscopic dust particles from disrupting optical pathways. The COSINC cleanroom environment provides the stringent conditions necessary to achieve this, resulting in devices that exhibit exceptional optical quality and reproducibility—key attributes for translating laboratory prototypes into commercial products.
One of the most noteworthy materials integrated into these microresonators are chalcogenides, a group of specialized semiconductor glasses known for their extraordinary transparency and optical nonlinearity. These materials allow light to pass through with minimal attenuation even at high intensities, which is essential for the functionality of microresonators designed to amplify light through repeated circulation. However, fabricating devices with chalcogenide glasses is notoriously challenging because their delicate material properties demand precise handling and processing techniques to avoid defects that would degrade performance.
The work at CU Boulder represents some of the best performing chalcogenide-based microresonators to date, demonstrating ultra-low optical losses and a balance between material robustness and optical functionality that few previous devices have achieved. Minimizing bend losses through thoughtful geometric design combined with the advantageous optical properties of chalcogenides has culminated in devices that rival the performance of those constructed from more conventional, yet less versatile, photonic materials.
Characterizing the microresonators’ performance involved sophisticated laser-based measurements conducted by a dedicated experimental team. By carefully coupling lasers into the waveguides and analyzing the light that emerged, the researchers identified resonance “dips” where photons were tightly confined within the resonator. These features, sharp and well-defined, signal the device’s quality and are indicative of the low loss and high photon lifetime inside the cavity.
Detailed analysis of resonance shape allowed the team to extract critical parameters such as intrinsic absorption and thermal behavior, which influence device stability and efficiency. Managing thermal effects is particularly crucial because as the resonator absorbs laser power, its temperature changes, which in turn alters the optical properties and can lead to degraded or unstable operation. Understanding and mitigating these thermal influences thus ensures reliable performance under diverse operating conditions.
The implications of these advancements extend far beyond initial demonstrations. With their compact size and superior performance, these microresonators can serve as foundational elements in integrated photonic circuits, enabling the development of compact microlasers, highly sensitive chemical and biological sensors, and hardware vital to quantum communication networks. Their adaptability promises profound impacts on precision measurement and metrology, where controlling and manipulating light at the microscale is paramount.
Dr. Bright Lu, the lead doctoral researcher on the project, envisions a future where such microresonators become ubiquitous components embedded in a wide range of everyday devices. The ultimate goal is to refine fabrication techniques to the point where microresonators can be produced en masse by industrial manufacturers, facilitating advances in sensing technology that are both scalable and affordable.
This work not only highlights critical material science and engineering innovations but also underscores the interdisciplinary nature of modern photonics research, bridging conceptual design, precise fabrication, and rigorous experimental validation. The achievement of ultra-low-loss chalcogenide microresonators with novel racetrack geometry marks a significant milestone in photonic device research, pushing closer to the realization of next-generation optical technologies that harness light with unprecedented control and efficiency.
Subject of Research: Optical Microresonators for Advanced Photonics and Sensor Technologies
Article Title: High-Performance Chalcogenide Racetrack Microresonators with Ultra-Low Losses
News Publication Date: 23-Feb-2026
Web References: Applied Physics Letters, DOI: 10.1063/5.0305459
Image Credits: CU Boulder College of Engineering and Applied Science
Keywords
Optical microresonators, photonics, chalcogenides, electron beam lithography, racetrack resonators, nonlinear optics, nanoscale fabrication, light confinement, sensor technology, thermal effects, integrated photonics, quantum metrology
