Nonintrusive monitoring and quantitative analysis of strong laser-field-induced impulsive alignment

Abstract

We report the observation of impulsive alignment of $\mathrm{C}{\mathrm{O}}_{2}$ molecules produced through their interaction with a nonresonant, strong laser pulse. The periodic alignment is monitored using a polarization technique generally employed in optical Kerr effect experiments; the birefringence produced by alignment of the molecular sample is measured with a weak pulse, time-delayed with respect to the alignment pulse. The technique provides a signal proportional to $⟨{\mathrm{cos}}^{2}\phantom{\rule{0.2em}{0ex}}\ensuremath{\theta}⟩\ensuremath{-}\frac{1}{3}$, where $\ensuremath{\theta}$ is the polar angle between the molecular axis and the strong-field polarization axis. Experiments are conducted at room and at low temperatures. Two methods of analysis are presented. The first one consists in comparing the signal with the prediction of the time-dependent Schr\"odinger equation. From a fine analysis of the temporal signal shape one can then deduce the value of $⟨{\mathrm{cos}}^{2}\phantom{\rule{0.2em}{0ex}}\ensuremath{\theta}⟩$. The second one allows us to extract $⟨{\mathrm{cos}}^{2}\phantom{\rule{0.2em}{0ex}}\ensuremath{\theta}⟩$ through a calibration of the birefringence signal obtained by performing the experiment in a reference atomic gas sample. Both analyses are compared and found in good agreement for the different laser intensities investigated. Saturation of the alignment process is observed at a laser intensity that agrees with the ionization saturation intensities of $\mathrm{C}{\mathrm{O}}_{2}$.