We have experimentally observed anisotropic ion emission from Xe and Ar clusters under intense ultrashort (∼30  fs) laser irradiation, with up to 1.8 times more energetic ions emitted in the direction perpendicular to the laser polarization than in the parallel direction. As the pulse length was varied in the range of 8-160 fs, we found this anisotropy to first grow and then diminish. Treating electrons inside the unexpanded cluster as a harmonic oscillator qualitatively demonstrates how intracluster electric field can result in an ion emission anisotropy of this kind. Our observations give direct access to the initial charging dynamics present in the first few cycles of an intense laser field interacting with any nanoscale dielectric.

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We have experimentally observed anisotropic ion emission from Xe and Ar clusters subject to intense ultra-short (∼ 30 fs) near-infrared laser pulses, with up to 80% more energetic ions emitted in the perpendicular direction to the laser polarisation than in the parallel direction. Our pulse duration investigation in the range of 8 − 160 fs revealed that this anisotropy is present in the interaction for a specific range of pulse durations. Treating electrons inside the unexpanded cluster as a harmonic oscillator qualitatively demonstrates how the intracluster electric field can result in an ion emission anisotropy consistent with our observations. Our observations and modelling therefore give direct access to the dynamics that will be present in the first few cycles of an intense laser field interacting with any nanoscale dielectric.