An optical frequency comb (FC) is the basis of the innovation. It's a special type of laser that emits light at evenly spaced frequencies, resembling the teeth of a comb. These “combs” allow researchers to measure and compare optical frequencies with extreme accuracy, turning light into a kind of ruler.
One advanced technique, dual-comb spectroscopy (DCS), uses two combs for detection. One passes through a sample, the other acts as a reference. When the two signals are combined, they produce beat signals at much lower frequencies that can be measured more easily and in real time. This allows for rapid, accurate chemical identification without any moving parts.
Despite their promise, frequency combs face a major limitation: bandwidth. The wider the bandwidth of the comb, the more chemical species it can detect, especially in the long-wave infrared spectrum, which contains strong molecular absorption features.
But, expanding the bandwidth is difficult because of light dispersion, where different light frequencies travel at different speeds. In high-dispersion materials, this effect disrupts the uniform spacing of the comb teeth, limiting the comb’s precision. The problem is especially pronounced in long-wave infrared systems, where dispersion is much stronger than in visible or near-infrared regimes.
Researchers often add bulky compensation components to manage dispersion, which results in reduced efficiency and scalability. Some success has been seen using corrugated double-chirped mirrors (DCMs) in terahertz FCs, but adapting this to the shorter infrared wavelengths is a difficult engineering challenge.
Air–Dielectric Double-Chirped Mirrors
In this new study, published in Light: Science & Applications, researchers introduced a compact, ultrabroadband laser frequency comb with a specially designed air-dielectric double-chirped mirror (DCM). This type of mirror is made up of alternating layers with graduating thicknesses and refractive indices.
The DCM used here takes advantage of the large refractive index difference between air and semiconductor materials, allowing it to deliver exceptionally high dispersion per unit length while eliminating the metallic losses seen in earlier corrugated mirror designs. This combination makes it ideal for handling the complex dispersion dynamics in long-wave infrared combs.
Using deep dry etching and electron-beam lithography, the team fabricated a DCM structure with ultra-precise features, achieving one of the highest known aspect ratios for this kind of nanophotonic device.
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The researchers built a custom on-chip segmented dispersion measurement platform to characterize the comb’s performance in real operating conditions. This setup can measure gain-induced dispersion above the lasing threshold, using only the broadband emission spectrum from the quantum cascade laser under pulsed bias.
This method sidesteps the need for external interferometers or time-domain systems, significantly simplifying integration and reducing the system’s physical footprint, making portable sensing more viable.
The researchers found that this approach produces a stable, coherent frequency comb with bandwidths exceeding 100?cm-1 at a central wavelength of 9.6?µm, all while operating at room temperature.
Next Steps and Future Applications
The work successfully addresses the whole chain of challenges; Suppressing unwanted dispersion across the gain bandwidth, avoiding metallic losses, achieving precise fabrication of sub-wavelength mirror layers, and enabling on-chip dispersion characterisation.
Looking ahead, the team plans to extend this approach to other laser systems to achieve even higher power outputs and broader spectral coverage. That could make frequency combs even more effective for detecting a wider range of chemicals or for use in advanced communication systems.
The ability to generate stable frequency combs in the LWIR region, compactly and at room temperature, is a significant milestone. It could begin a new generation of portable, high-precision sensors capable of real-time, multi-compound chemical analysis in the field.
Journal Reference
- Zeng, T., Dikmelik, Y., Xie, F., Lascola, K., Burghoff, D., Hu, Q. (2025). Ultrabroadband air-dielectric double-chirped mirrors for laser frequency combs. Light: Science & Applications, 14(1), 1-12. DOI: 10.1038/s41377-025-01961-4, https://www.nature.com/articles/s41377-025-01961-4
- Zewe, A. (2025) New laser “comb” can enable rapid identification of chemicals with extreme precision [Online] Available at https://news.mit.edu/2025/new-laser-comb-can-enable-rapid-identification-chemicals-extreme-precision-0820 (Accessed on 04 September 2025)