Research activity

Atmospheric optics (2006-present)

Fundamental and applied research on aerosols and greenhouse gases: laboratory and field experiments in optics.

Keywords: spectroscopy, polarimetry, aerosols, molecules, backscattering, laboratory experiments, lidar remote sensing experiments.

Institute of Light and Matter (iLM), ATMOS research group

Studied objects : Molecules (gas phase), Aerosols (condensed phase)

Major advances have been made at the iLM in understanding the interaction of light with atmospheric trace gases and aerosols. The originality of my research lies in the unique synergy between laboratory experiments and observations in the atmosphere. By revealing the intrinsic and in-situ complexity of these objects, using light spectroscopy and polarization, this synergy makes it possible to carry out both fundamental physics studies and aerosol metrology, as well as technology transfer, innovation and industrial development.

Contributions to the molecular phase: greenhouse gas spectroscopy.
Optical molecular spectroscopy for trace gas detection
The question, resolved during the theses of B. Thomas and C. Anselmo, is how to determine the concentration of a gas from an optical absorption measurement based on Beer-Lambert's law, taking into account finite spectral resolution and strong absorption. By coupling lidar remote sensing with correlation optical spectroscopy, a new method for measuring the concentration of a trace gas has been developed [30], making the assumptions of monochromaticity and low absorption of the process obsolete. The mathematical resolution of this integral problem was carried out in collaboration with the Institut Camille Jordan de Mathématiques at the Université Lyon 1 (Dr. Welschinger). Numerical simulations [21, 24], spectral calibration in the laboratory [30] and experimental proof of concept in a real atmosphere [23, 32] have been published, as well as an international patent with TOTAL. Although interesting, this topic has been superseded by research activity related to the condensed phase. However, this subject has been perpetuated by the recruitment of a new permanent member of the ATMOS team in 2016. Finally, the work carried out on molecular optical scattering (M. Abou Chacra's thesis, 2006-2009) had an impact on the calibration of polarisation-resolved aerosol lidar measurements and contributed to the advent of the HSRL lidar with high spectral resolution.

Contributions to the condensed phase: spectroscopy and polarimetry of atmospheric aerosols.
With regard to the condensed phase, the question explored during the theses of G. David, T. Mehri and D. Cholleton was how to reveal the optical backscattering properties of a set of inhomogeneous particles of any shape suspended in the air. There is no analytical solution to Maxwell's equations for this type of particle, for which Mie's theory is inapplicable. The most significant experimental achievement has therefore been the development of a laboratory polarimeter, the only one of its kind in the world [26], which provides an absolute measurement of the elements of the scattering matrix of a set of inhomogeneous particles of any shape suspended in the air. Overcoming the obstacles posed by the coaxial source-detector geometry (180.0 ± 0.2°) and the very low intensity of the backscattered wave, this Pi-polarimeter has opened the way to absolute measurement of the elements of the scattering matrix of these particles over the entire phase function. It also provides a unique added value by revealing the spectral (UV, VIS, IR) and polarimetric signatures of these particles, accurately (1%) and unambiguously (bias < 10-7), laying the foundations for an optical database for these particles as well as for molecules. The polarimeter developed at UCBL and iLM has thus become a reference instrument for the scientific community [18-42]. Several teams have come to the iLM to study the optical backscattering of soot (ONERA, [39]) and complex core-shell chemical objects (IRCELYON), as published in the prestigious journal PCCP [38]. These developments have been referenced as iLM highlights (PNAS, ACP, OptLett, PCCP).

Laboratory experiments
Linked publications : [17-42]

Understanding of the interaction of light with these particles has been deepened by identifying the role of optical absorption and the size of the scattering centres on these spectral and polarimetric signatures [29,33,42]. The Pi-polarimeter has thus made it possible to extend ([35], 3) the historic approach of Ångström (1929), which sought to reveal the size of mineral particles in the atmosphere through the spectral dependence (alone) of optical scattering. Finally, the 1% accuracy obtained on the elements of the scattering matrix made it possible to discuss the validity of the numerical models (T-matrix, DDA) used in climatology by the radiative transfer scientific community (CNES, ESA, NASA) [29,31,33]. Funding has been obtained from CNES since 2020.

Lidar remote sensing field experiments
Linked publications : [16-27; 34-36]

This section concerns development of new lidar methodologies in the real atmosphere, carried out by implementing this polarimeter in the iLM lidar platform, in order to study mixtures of atmospheric particles. The precision of the Pi-polarimeter has made it possible to reveal the vertical structure and temporal evolution of these particles from urban [16], volcanic [17-19] and desert [22,25,27,31,34] sources in the atmosphere. The fundamental nucleation process, coupled with photo-catalytic processes on the surface of naturally semi-conducting nanoparticles from deserts [22], was thus observed in the atmosphere, revealing a new pathway for this process under normal temperature and pressure conditions, as published in the prestigious journal PNAS (Impact factor: 11.2, [22]), with an OSA Spotlight Award and two new publications [27, 34] extending this methodology. Soot particles, which are absorbent, were studied in an original way by observing their laser-induced incandescence in the atmosphere (weaker than the vibrational Raman of water), coupled with a lidar remote sensor enabling time-altitude mapping of their radiative emission according to Planck's law [28]. Finally, a technology transfer is underway between the iLM (INP) and the LA (INSU) in order to implement the laboratory polarimeter on a mobile LA remote sensor.

	Optical laboratory experiments on aerosols		Optical lidar remote sensing field experiments on aerosols

- Molecular spectroscopy of atmospheric greenhouse gases (2009-2016).

Polarisation and optical spectroscopy in quantum optics (PhD, post-doctorate, 2001-2006)

Atom interferometry with laser diffraction (PhD), Spectroscopy and optical trapping of ions in a Paul trap by laser cooling techniques (Post-Doctorate).
Phd : Laboratoire Collisions, Agrégats, Réactivité (LCAR), Université Toulouse III. 
Post-doctorate : Laboratoire de Physique des Interactions Ioniques et Moléculaires (PIIM),Université Aix-Marseille I.