Abstract

The temporal confinement of highly nonlinear strong-field ionization enables the petahertz-bandwidth of non-linear photoconductive sampling (NPS). In this measurement technique, an intense few-cycle laser pulse creates freely moving electrons in a gas[4] or in a solid[3], while a much weaker, orthogonally polarized probe pulse accelerates the charge carriers. A pump-probe setup measures the component of the emerging drift current that flows in the direction of the probe pulse. Simple classical picture suggests that the measured delay-dependent signal should closely match the vector potential of the probe pulse, and the experiments indeed found a good, albeit not perfect agreement. The discrepancies are of practical, as well as fundamental significance as they represent the attosecond-scale electron dynamics that accompany strong-field ionization, often in the intermediate regime between quantum tunneling and multi-photon absorption. We did a systematic theoretical investigation of NPS for the case where the medium is an atomic gas. To investigate the role of the Coulomb interaction between a free electron and its parent ion, we compare the outcomes of the numerical solution of the time-dependent Schrödinger equation (TDSE) to those of an analytical model derived in the strong-field approximation [1] (SFA). When the electric field of the probe pulse is so weak that its interaction with matter can be considered linear, the drift electric current per atom is $〈S_{\text{drift }}(\tau )〉=-e〈{\widehat{p}}_{\text{probe }}(\tau )〉$. We found an analytical solution for the drift current for any given combination of pump and probe pulses using SFA. We then compared the results with numerical solution of the full TDSE using a software package called tRecX[2].

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