TeraHertz time domain spectrometer for the determination of optical properties of suspended micro and nanoparticles in the air in a controlled environment – STEPSON

To achieve the above objectives, two main approaches are used:
(i) the morphological and chemical characterization of the samples studied
(ii) the development of a broadband TeraHertz spectroscopy bench dedicated to the study of resuspended particles (aerosols)

(i) Knowing the particle size but also the shape and the chemical composition of the particles studied is essential in order to link these properties to the optical properties deduced from the spectroscopic study. As such, scanning electron microscopy coupled with EDX analysis was used in the project (Microscope located on the IRENE platform, University of Littoral Côte d'Opal with the scientific expertise of the LPCA). On the one hand, this makes it possible to check on samples of 'unitary' composition (amorphous silica, quartz, calcium carbonate type) to check the particle size provided by the particle counter and to know / check their morphology (sphericity, rhombohedral , form factor...)
On the other hand, and in particular for «complex« samples such as desert dust, this method also makes it possible to know the chemical elemental analysis on a large number of particles (statistical analysis on more than 1000 particles) and even, to know the distribution of these chemical elements within the same particle (mapping).
Coupled with a crystallographic analysis by X-ray diffraction, it is possible to go back to the geological classes and to quantify them within this complex sample.


(ii) Time domain spectroscopy provides access to the value of the electric field and therefore to both its amplitude and its phase. Access to these two pieces of information makes it possible to determine the real and imaginary parts of the complex index of refraction of the material studied.
The developed bench is based on a compact and commercial time domain spectroscopy technology. However, the configuration of this compact THz-TDS has been specially designed to be able to respond to the problems of studies at different path lengths and on longer optical paths (1m50) than usual in its original configuration.
The resuspension set-up and the measurement cell were designed, then assembled and adjusted on the bench. Certain mechanical parts such as the cell's French windows were manufactured in the laboratory. An optimization part of the optical block part for optimized THz beam propagation is also developed and even manufactured in the laboratory.
An optical particle size analyzer makes it possible to know the size distribution of the particles having passed through the spectroscopy cell

The STEPSON bench was designed, assembled, optimized and characterized (resuspension parts, flows, flow rates, losses, characterization of the beam and the temporal signal)

A study on «transparent« materials in the TeraHertz range was carried out in order to determine the best windows to use for the cell (taking into account the mechanical and economic properties) (communications with proceeding)

A sample of desert dust was studied by SEM-EDX and DRX (publication in preparation)

Scientific popularization: a Stepson poster and animations related to satellite observations and the optical properties of aerosols were carried out as part of the science festival on the Villeneuve d'Ascq site

The next medium-term prospects are to carry out measurement campaigns with aerosols in suspension in the cell, first of all in «unitary« compounds of the calcium carbonate type, then to pass «complex« samples such as desert dust.
It is necessary to optimize and adapt the existing ICR extraction algorithm developed in the laboratory for the study of suspended particles.
In collaboration with the PC2A and LOA laboratories, the goal is to obtain complete ICR data sets of aerosols from UV to THz under similar operating conditions. In the longer term, having the different spectrometers required «on-line« (UV-Vis, Infrared and THz spectrometers in the same room and under the exact same operating conditions) and thus obtaining the complete set of data in one experiment.

1. J. Bichon, A. Pillet, A. Sklia, D. Petitprez, Romain Peretti et al. Complex refractive index determination of PTFE, TPX and polypropylene windows for TeraHertz broadband spectroscopy
2022 47th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Aug 2022, Delft, Netherlands. pp.1-2, ?10.1109/IRMMW-THz50927.2022.9895520? (poster)
2. Jeyan Bichon, Melanie Lavancier, Denis Petitprez, Alexandre Deguine, D. Hourlier-Bahloul et al. Complex refractive indices in the TeraHertz domain of samples from atmospheric aerosol sources
2021 46th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Aug 2021, Chengdu, China. pp.1-2, ?10.1109/irmmw-thz50926.2021.9566988? (Talk)

3. J. Bichon, K. Deboudt, L. Deschutter, H. Herbin, D. Petitprez and S. Eliet, “Chemical and morphological single particles characterization of Gobi Desert dust for the complex refractive index determination from ultraviolet to TeraHertz” , IAC 2022, International Aerosols Conference, Athenes, Grèce 4-9 Sept 2022 (poster)

The study of climate change, in order to react best, requires a detailed knowledge of the physicochemical properties of the atmosphere. In particular, the calculation of the radiative balance which requires the knowledge of the transmission and the reflection of the atmosphere in the solar spectral band (direct forcing) and the spectral range IR of thermal emission of the earth (indirect forcing). Current methods require precise knowledge of the real and imaginary parts of aerosol refractive indices. However, these complex refractive index (CRI) are poorly known in the lower frequency bands and in particular in the TeraHertz (100 GHz-30THz). It is therefore essential to propose experiments to measure them. STEPSON will measure CRI of suspended particles based on their size in a controlled environment. Transportable, it will be able to participate in measurement campaigns coupled with other spectral techniques.