High Resolution Molecular Spectroscopy with Quantum Cascade Laser Frequency Combs

Project Information

HiResCombs

Grant agreement ID: 101032761

DOI

10.3030/101032761

Project closed

EC signature date 14 April 2021

Start date 1 September 2021

End date 31 August 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 191 149,44

EU contribution

€ 191 149,44

Coordinated by

SENSIRION AG
Switzerland

Periodic Reporting for period 1 - HiResCombs (High Resolution Molecular Spectroscopy with Quantum Cascade Laser Frequency Combs)

Reporting period: 2021-09-01 to 2023-08-31

Summary of the context and overall objectives of the project

Infrared spectroscopy provides a detailed view into the structure and bonds of molecules. It allows unambiguous identification and quantification of molecules, and can provide detailed insights into the strength of chemical bonds. Scientists can use this information to study chemical reactions in the atmosphere, the composition of the atmosphere of exoplanets, and much more. To study the energy landscape of a molecule at the level of rotational and vibrational quantum states requires instrumentation with high spectral resolution, typically based on lasers. Lasers typically only cover a very small range of the infrared spectrum, resulting in a trade off between spectral coverage and spectral resolution. Optical frequency combs are an emerging new class of lasers that have been recognized with a Nobel prize in physics. Theses laser sources emit light over a broad spectral range at exactly defined intervals, resulting in a spectrum resembling a comb.
IRsweep, the beneficiary of the HiResCombs project, has developed the first commercial spectrometer based on quantum cascade laser frequency combs in the mid-infrared spectral region. The spectral resolution of the spectrometer of 0.3 cm-1 is well suited for measurements of liquid and solid samples, but insufficient for advanced studies of gases that require a high spectral resolution better than 0.001 cm-1.
During the HiResCombs project, this spectral resolution will be achieved by tuning of the laser center wavelength. With this development, we want to make a turn-key spectrometer for high-resolution mid-infrared spectroscopy available to researchers of divers fields, allowing them to perform measurements over a broad spectral range with high resolution in minutes.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The frequency combs used in IRsweep's spectrometers sample the infrared spectrum at well defined frequencies at an interval of ~ 0.3 cm-1. To fill the gaps between data-points, the frequency comb can be spectrally shifted in small increments, and the recorded spectra can be combined to obtain a quasi continuous spectrum with increased resolution. To this end, the exact frequency increments between all measurements must be known or measured with high accuracy.
During the project, methods to tune the center frequency of a frequency comb in a dual-comb spectrometer were investigated and implemented, yielding scripts that automatically step the wavelength across the 0.3 cm-1 gap between consecutive spectral data points. Further, a method to measure the exact frequency increments of all steps was implemented that does not rely on any additional hardware compared to the standard spectrometer.
Procedures to identify ranges of laser temperature and current for which the quantum cascade frequency combs can be tuned across ~0.3 cm-1 were established. This allows delivering lasers with pre defined and tested tuning currents.
The technique was tested successfully in collaborative measurement campaigns with international academic partners. The measured spectra allowed referencing results, like absorption line positions, against measurements performed by others with unique and elaborate laboratory scale setups. The found excellent agreement confirms the high spectral resolution and accuracy achieved, and lays the foundation for extensive future measurements by customers of IRsweep that will produce new spectroscopic data of improved quality and coverage.
Besides minute-scale measurements of spectra covering ~60 cm-1 with 0.0005 cm-1 resolution, we further demonstrated time-resolved high-resolution measurements. Spectra with 1 µs time resolution and 0.0005 cm-1 spectral resolution can be recorded, enabling measurements in advanced experimental configurations like pulsed supersonic jet expansions and photolysis initiated gas phase reaction kinetics.
The results were presented at five international conferences, one peer-reviewed article has been published and further manuscripts have been submitted for publication.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

With the developments implemented in the HiResCombs project, the QCL dual-comb spectrometer has become a unique tool for high resolution molecular spectroscopy in the 2000 cm-1 - 1000 cm-1 spectral region for which the availability of commercial laser spectrometers has been very limited.
With these instrumental developments, researchers are, for the first time, able to measure spectra covering 10s of cm-1 with 0.0005 cm-1 resolution in a few minutes, and even with µs time resolution, if necessary. We are confident that this will result in a large amount of new spectroscopic data that will be used to improve our understanding of the earth's atmosphere, study the atmosphere of remote planets and answer fundamental scientific questions.

measured spectrum of methane. Left: full range. Right: Zoom on one absorption line.