Dual-comb spectroscopy enables broadband analysis of key molecules with unparalleled frequency resolution and exceptional signal-to-noise ratios across various spectral regions. However, fully harnessing its potential for broadband spectroscopy with high sensitivity and spectral resolution depends critically on selecting the appropriate frequency combs with optimized (comb) parameters tailored to specific applications. This study compares dual-comb spectroscopy systems operating at 80 MHz and 1 GHz repetition rates, in the near infrared and visible spectral regions. The 80 MHz system provides high spectral resolution, ideal for resolving complex spectra, showcased with measurements of NH3 vibrational bands and I2 hyperfine transitions. Utilizing phase-locked feed-forward stabilization, the system delivers excellent signal-to-noise ratios but faces limitations in temporal resolution. The free-running 1 GHz system offers superior temporal resolution and compactness, making it suitable for real-time environmental monitoring in laboratory and field settings. A self-correction algorithm enhances the high mutual coherence, enabling high-signal-to-noise measurements without additional electronics. With its 1 GHz resolution, it excels in monitoring NH3 transitions or NO2 lines at high speeds. This work highlights the complementary strengths of these systems for high-resolution spectroscopy and real-time trace gas sensing.

The light induced trans-cis isomerization of azobenzene allows studying and controlling the properties of photosensitive materials. Promising applications include optical switching, data storage, and light-driven nanomechanical devices. Understanding switching in molecular assemblies is essential to scale these photoinduced mechanisms for practical applications. In this study, we analyze the switching behavior of single azobenzene derivatives in large assemblies (n >> 103) with advanced computer vision techniques. This approach enables us to detect subtle variations in switching yields among densely packed, similarly oriented molecules. Our findings provide new insights into the switching of molecular assemblies.

Ultraviolet (UV) spectroscopy reveals electronic transitions in matter that underpin atmospheric photochemistry and molecular dynamics. While dual-comb spectroscopy (DCS) has revolutionized precision measurements in the infrared region and expanded to the THz to the visible regions, its extension into the UV has been accomplished only very recently with demanding experimental efforts. Here we introduce free-running ultraviolet dual-comb spectroscopy (UV-DCS), a straightforward, high-fidelity method for absolute absorption cross sections and rapid molecular fingerprinting in the atmospheric UV window. The approach delivers high spectral resolution (1 GHz), broad bandwidth (12 THz), and fast acquisition (500 ms) without active stabilization, resulting in a UV-DCS quality factor that exceeds previously reported values.. Applied to formaldehyde (HCHO), a key atmospheric pollutant and photochemical driver, the technique yields an unprecedented count of rovibrational transitions in this window, enriching the molecular line list used for atmospheric chemistry and remote sensing. The measurements produce refined rotational constants, enabling high-accuracy quantum simulations of molecular eigenstates and informing ab initio models. Beyond robust absolute cross sections, UV-DCS provides a universal, rapid fingerprinting tool for transmissive species, offering fast atmospheric sensing and a rigorous benchmark for quantum theory without stabilization requirements. Coupled with advanced formaldehyde synthesis, these results support improved atmospheric monitoring, more reliable retrievals of HCHO abundances, and enhanced validation of fundamental molecular physics. Overall, this work realizes a free-running UV-DCS platform combining GHz resolution, multi-terahertz bandwidth, and sub-second acquisition, and applies it to refine formaldehyde ultraviolet spectroscopic parameters relevant to atmospheric and molecular science.

Photochemistry in the earth's atmosphere is driven by the sun, continuously altering the concentration and spatial distribution of pollutants. Precisely monitoring their atmospheric abundance relies predominantly on optical sensing, which requires the knowledge of exact absorption cross sections. One key pollutant which impacts many photochemical reaction-pathways is formaldehyde. Agreement on formaldehyde absolute absorption cross section remains elusive in the photochemically-relevant ultraviolet spectral region, hampering sensitive concentration tracking. Here, we introduce free-running ultraviolet dual comb spectroscopy, combining high spectral resolution (1 GHz), broad spectral coverage (12 THz), and fast acquisition speed (500 ms), as a novel method for absolute absorption cross section determination with unprecedented fidelity. Within this bandwidth, our method uncovers almost one order of magnitude more rovibrational transitions than detected before which leads to refined rotational constants for high-level quantum simulations of molecular eigenstates. This ultra-resolution method can be generalized to provide a universal tool for fast electronic fingerprinting of atmospherically-relevant species, both for sensing applications and to benchmark improvements of ab-initio quantum theory.

We radically simplify coherently averaged dual-comb spectroscopy by introducing a real-time self-correction system: a radio frequency system-on-chip computes each incoming dual-comb interferogram's phase, frequency, and arrival time; calculates changes in the combs' carrier-envelope offset frequency and repetition rate difference; and immediately phase-corrects the incoming interferogram data stream. The algorithm combines fast measurement times with broadband optical detection. Using this system, we achieve comb-resolved spectroscopy with Fourier-limited linewidth, coherent averaging over arbitrarily long durations, and high signal-to-noise ratios. Iodine and acetylene spectroscopy yield good agreement with literature over an optical bandwidth of 10 THz in the visible and near-infrared. Common dual-comb spectroscopy self-correction requires a continuous interferogram train. We lift this requirement by introducing cross-channel correction: the algorithm measures phase fluctuations from a reference channel to predict and correct a signal channel. This enables correcting unstable or intermittent signals (typical, e.g., in field measurements), or low-amplitude signals with amplified phase fluctuations (relevant for nonlinearly upconverted combs). The approach makes instantaneous dual-comb spectroscopy available to everyday applications.

We introduce an agile light source bridging from the near ultraviolet to the visible spectral region by covering more than 240 THz through resonant dispersive wave (RDW) emission in a gas-filled hollow-core fiber waveguide. The light source allows tuning of a 20 nm (FWHM) spectrum from ∼340 nm to 465 nm (645 to ∼885 THz) with conversion efficiencies of (1.5 ± 0.4) %, providing spectral powers up to (2.6 ± 1) mW/nm. This technique is showcased for spectroscopy with broadband absorption measurements of nitrogen dioxide, a molecular species of major atmospheric relevance. To our knowledge, this is the first demonstration of absorption spectroscopy with an RDW-based light source. The presented measurements indicate conservation of the coherence of the frequency comb seeding the frequency up-conversion process, paving the way towards ultra-broadband (dual) comb molecular spectroscopy across the highly relevant ultraviolet and visible range.

Sustained mutual coherence between 2 combs over extended periods is a prerequisite for dual-comb spectroscopy (DCS), particularly in achieving high-resolution molecular spectroscopy and precise spectral measurements. However, achieving long coherence times remains a challenge for Yb-doped frequency combs. This work introduces an experimental approach for phase-stable DCS using Yb-doped frequency combs at 1.03 μm with a novel feed-forward method, combatting the limitations of mutual coherence. Without relying on postprocessing or self-correction algorithms, we achieve a coherence time of 1,000 s—3 orders of magnitude longer than the current state of the art for DCS. This extended coherence enables time-domain averaging, resulting in a signal-to-noise ratio (SNR) of 2,045. We demonstrate high-resolution monitoring of weak overtone transitions in the P and R branches of C2H2, achieving good agreement with simulated spectra based on HITRAN parameters. The phase-locked multiheterodyne system also enables phase spectrum measurements with a scatter down to 7 mrad. Furthermore, we successfully extend our technique to the visible spectral region using second harmonic generation, achieving high-resolution spectra of NO2 with excellent SNR. The method offers high-frequency accuracy and demonstrates the potential of Yb-doped systems for multiplexed metrology, effectively extending the capabilities of DCS as a powerful tool for multi-disciplinary applications.

The first broadband ultraviolet dual-comb spectroscopy system is used to analyze the absolute absorption cross section of formaldehyde with high spectral resolution. This enables the determination of the transition strength and fundamental rotational constants.

The highly energetic photons of ultraviolet light drive electronic and rovibronic transitions in all molecular species. This radiation is thus a prime tool for strongly selective spectroscopic fingerprinting and real-time environmental monitoring if broad spectral coverage, short acquisition times, and high spectral resolution are achieved-requirements that are in mutual competition in traditional applications. As an approach with intrinsic potency in all three aspects, here we introduce ultraviolet dual comb spectroscopy using two broadband ultraviolet frequency combs centered at 871 THz and covering a spectral bandwidth of 35.7 THz. Within a 100 mu s acquisition time window, we obtain rotational state -resolved absorption spectra of formaldehyde, a prototype molecule with high relevance for laser spectroscopy and environmental sciences. To our knowledge, this is the first realization of broadband dual comb spectroscopy in the ultraviolet spectral region and a pioneering tool to allow for real-time monitoring of rovibronic transitions.

Nitrogen dioxide is of major importance to our climate. We achieve a 5 ppb sensitivity with one-minute averaging times and a spatial resolution of 90 m with a mobile dual frequency comb and reflector setup.

We introduce a portable dual-comb spectrometer operating in the visible spectral region for atmospheric monitoring of NO2, a pollution gas of major importance. Dual-comb spectroscopy, combining key advantages of fast, broadband and accurate measurements, has been established in the infrared as a method for the investigation of atmospheric gases with kilometer-scale absorption path lengths. With the presented dual-comb spectrometer centered at 517 nm, we make use of the strong absorption cross section of NO2 in this spectral region. In combination with a multi-pass approach through the atmosphere, we achieve an interaction path length of almost a kilometer while achieving both advanced spatial resolution (90 m) and a detection sensitivity of 5 ppb. The demonstrated temporal resolution of one minute outperforms the standard chemiluminescence-based NO2 detector that is commercially available and used in this experiment, by a factor of three.

The mission of the « Isolated and interacting molecular assemblies » research network (EMIE) is to bring together the French community of physicists and chemists working on molecular systems, and covering a wide range of size and complexity. The objects under scrutiny are either isolated in the gas phase or surrounded by a controlled environment. Building upon fundamental aspects of experimental and theoretical molecular physics, our community is naturally inclined to benefit from interactions with other disciplines (chemistry, biology) and to extend its fields of applications to other scientific domains with timely societal impacts (biology, atmosphere).

In this work, we present normal spectral emissivity data of solid and liquid molybdenum at a wavelength of 684.5 nm. The presented results are novel measurements on molybdenum, a material, which was already measured 15 years ago by our group. The present results indicate a lower emissivity in the liquid phase. The novel measurements were done within the European Metrology Programme for Innovation and Research (EMPIR) project 17IND11 Hi-TRACE. The optimized measuring system is an ohmic pulse-heating apparatus combined with microsecond Division of Amplitude polarimetry.