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Accueil > Séminaires & Soutenances > Séminaires, soutenances de thèses et HDR précédents > 2019 > Séminaires 2019 > Le lundi 21 janvier 2019 à 10h
Le lundi 21 janvier 2019 à 10h
Lieu : Ecole Polytechnique, amphi Monge
Orateur : Elmar Slikboer
Titre : Investigation of Plasma Surface Interactions using Mueller Polarimetry (PhD work)
Résumé :
This thesis examines a new diagnostic method called Mueller Polarimetry for the investigation of plasma-surface interactions. This imaging technique allows for the time-resolved optical characterization of targets under plasma exposure. The measured Mueller matrices are analyzed using the logarithmic deposition provid- ing polarimetric data on diattenuation, depolarization and birefringence. The lat- ter is used by examining materials that possess optically active behavior to identify specific aspects of the plasma interaction, e.g. electric fields or temperature.Retour ligne automatique
This work focusses on electro-optic targets, which primarily allows for the de- tection of electric fields induced by surface charge deposited during the interac- tion. The birefringence is coupled analytically to the externally induced electric field, by relating the phase retardance for the probing polarized light beam to the perturbed index ellipsoid, according to the Pockels effect. Through this analytical approach, materials with specific electro-optic properties can be chosen in such a way – together with the orientation of the Mueller polarimeter itself – that all the individual electric field components (axial and radial) induced inside the sam- ple are imaged separately. This has never been done before and allows to better understand the plasma dynamics in the vicinity of a dielectric surface.Retour ligne automatique
It is used to investigate the surface impact by guided ionization waves gener- ated by a kHz-driven atmospheric pressure plasma jet. These non-thermal fila- mentary discharges are generally applied to various samples for e.g. surface func- tionalization of polymers or biomedical treatment of organic tissues, but available diagnostic tools are limited. Imaging Mueller polarimetry applied to electro-optic targets examines the axial and radial field patterns in terms of amplitude (3-6 kV/cm), spatial scales (≤ 1mm axial and ≤1cm radial) and timescales (≤ 1μs pulsed and ≤ 10μs AC) for various operating parameters of the jet, e.g. voltage amplitude and surrounding gas.Retour ligne automatique
Simultaneous with the transient birefringence induced by the electric field also a constant background pattern is observed. This is induced by temperature (gra- dient) induced strain inside the targeted material due to an inhomogeneous tem- perature distribution. An analytical relation is obtained following the photo-elastic effect, which allowed for a fitting procedure to be designed to retrieve the temper- ature pattern. This procedure is used, after calibration, to show that the tempera- ture of the sample can vary up to 25 degrees relative to room conditions – while changes in electric field are seen as well – depending on the operating frequency of the AC driven plasma jet. The accurate determination of the temperature is important since most applications involve temperature sensitive samples.Retour ligne automatique
Lastly, this work shows how complex samples can be examined during a plasma- surface interaction, by combining them with the electro-optic targets. Due to the addition of a (thin) complex sample, depolarization is added to the system, through scattering of the polarized light beam. In-vitro changes of depolarization relate to the evolution of the complex sample during the plasma treatment. This, coupled to the simultaneously monitored electric field patterns, provides a unique diagnostic tool to examine plasma-surface interactions. This has been applied for a test case where a single layer of onion cells is exposed to the ionization waves generated by the non-thermal plasma jet.
A pdf copy of Elmar’s thesis can be downloaded with the following link : https://bit.ly/2rq8rZN
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