The CHROME laboratory harnesses the Nobel Prize-winning innovation of optical frequency combs to conduct two main research programs. One focuses on trace gas detection for environmental, medical, and combustion applications, while the other targets precision spectroscopy and optical metrology for fundamental physics. A summary of these two research areas is presented below.
Trace Gas Sensing
The approach pursued for trace gas sensing exploits dual-comb spectroscopy (DCS)—a powerful spectroscopic technique that enables the simultaneous detection of numerous molecular species in a gas sample with exceptional sensitivity and high temporal and spectral resolution. This method is particularly advantageous for applications such as: i) identifying biomarkers in human breath, ii) monitoring industrial processes, iii) investigating combustion chemistry, and iv) measuring air pollutants. The core advantage of DCS lies in the use of two optical frequency combs, one serving as a probe for the sample’s absorption features, the other providing a reference grid of equally spaced frequencies. This allows parallel acquisition across thousands of spectral lines, significantly enhancing speed and chemical detail.
At CHROME, we are developing a compact and robust DCS platform that employs electro-optic modulators in the near-infrared to translate microwave signals into optical frequency combs. The generated combs are then shifted into the mid-infrared—a region rich in fundamental molecular transitions—using nonlinear optical processes such as optical parametric oscillation (OPO) and difference frequency generation (DFG). The goal is to cover an unprecedented range from 3 to 12 µm with a setup designed for portability and adaptability across scientific, industrial, and environmental applications.
Optical Metrology
Our second research focus investigates molecular hydrogen (H₂), the simplest and most abundant molecule in the universe. Owing to its simple structure, H₂ represents a benchmark for quantum molecular physics, allowing ab initio calculations of its rovibrational energy levels, including relativistic and quantum electrodynamics (QED) corrections. The goal of our research is to achieve ultra-high precision measurements of these transitions to rigorously test QED predictions and possibly uncover discrepancies that hint at new physics.
To this end, we move beyond traditional absorption techniques and apply Coherent Raman Spectroscopy (CRS)—a nonlinear method that circumvents the weak quadrupole transitions inherent to homonuclear molecules like H₂. In a recent work on the Q(1) line at approximately 4155 cm⁻¹, we achieved a combined uncertainty of 1.0 × 10⁻⁵ cm⁻¹ (310 kHz), exceeding the current theoretical benchmark by over an order of magnitude. Future plans include: i) expanding measurements to other Q and S lines in the 1–0 vibrational band at varying temperatures to support robust spectral fitting and extrapolation to zero pressure, ii) extending the spectral coverage to pure rotational bands, and iii) applying this methodology to other infrared-inactive molecules, such as CO₂, N₂, and benzene (C₆H₆). To further boost the sensitivity, we are transitioning from traditional multi-pass gas cells to hollow-core photonic crystal fibers, which offer tighter beam confinement and enhanced interaction lengths.
