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RESEARCH PRODUCT

Alignement moléculaire par impulsions laser ultrabrèves : Mesures & Applications

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The interaction of an intense ultrashort laser pulse with a molecular sample leads, after the end of the pulse, to molecular alignment at specific time under field-free conditions. This thesis is dedicated to the measurements and applications of this effect. The degree of alignment is determined through the variation of the refractive index induced by the rearrangement of the molecular dipoles. A two-dimensional technique allowing to measure molecular alignment on a single-shot basis is reported in this document. The alignment is then exploited in order to determine physical quantities which are usually difficult to estimate through others methods. The "cross-defocusing technique" has been first implemented to provide ionization probabilities of atomic and molecular gas in the femtosecond regime. The "1D polarization technique" has been then applied to the measurement of the electronic Kerr index calibrated through molecular alignment. It has been found that the optical Kerr effect exhibits a sign reversal around about 30 TW.cm-2 for the main atmospheric gases. This effect has been described phenomenologically by high order Kerr terms (from n4 to n10). The important impact of theses results on the understanding of the filamentation mechanism is discussed