Efficient Generation of Isolated Attosecond Pulses E. Power1, N. Naumova1, J. Nees1, V. Yanovsky1, A. Maksimchuk1, I. Sokolov2, B. Hou1, and Gerard Mourou1,3 1. Center for Ultrafast Optical Science, University of Michigan, Ann Arbor, Michigan 48109 2. Space Physics Research Laboratory, University of Michigan, Ann Arbor, Michigan 48109 3. Laboratoire d’Optique Appliquée, ENSTA – Ecole Polytechnique, Palaiseau Cedex, France Particle-in-cell (PIC) simulations of the interaction between a relativistic intensity fewcycle laser pulse focused to a volume of a few λ3 and critically dense plasma indicate that isolated attosecond pulses can be formed efficiently through relativistic reflection, deflection, and compression. The Doppler compression leading to attosecond pulse formation is expected to be efficient, with as much as 10% of the input optical energy converted into a single attosecond spike. Additionally, 1-D boosted frame models indicate that the duration of the generated attosecond pulses scales inversely with driving field amplitude, potentially providing access to intensities orders of magnitude higher than the focused laser intensity. 3- D PIC simulations validate the 2-D predictions for attosecond pulse formation, and also indicate that the relativistic deflection process only occurs in the plane containing the electric field vector. In the plane perpendicular to the plane of incidence, no deflection is predicted. However, the preservation of symmetry in the critical surface in this perpendicular plane leads to a higher concavity of the surface; consequently, the divergence angle in the plane of incidence is smaller than the divergence angle perpendicular to the plane of incidence. For several-cycle pulses, we expect relativistic deflection to occur, however the presence of multiple cycles leads to the formation of pulse trains in non-specular directions. Shack-Hartmann, direct imaging, and spatio-spectral imaging measurements all point to the presence of relativistic deflection. For the experimental conditions currently available, we observed the expected result of multiple spots within the focal zone of each microlens element of the Shack-Hartmann detector. Direct imaging shows that at low intensity, the reflected radiation lies within the specular cone: only 1% of the reflected radiation lies outside the specular cone at I = 6×1016 W/cm2. At I = 2×1018 W/cm2, 65% of the reflected radiation lies outside the specular cone, predominantly spread along the plane containing the electric field. Spatio-spectral measurements at I = 1.5×1018 W/cm2 show that for a large plasma density gradient, no relativistic deflection is observed. After inserting a pre-pulse to generate an estimated 0.5λ scale-length plasma, shots at I = 1.5×1018 W/cm2 were observed to be split into three spatial lobes. Two bright lobes, containing ~45% of the integrated energy each were observed near the specular axis, and showed a modest 8% increase in spectral width. The third lobe, deflected at a larger angle towards the target surface, contained ~10% of the total integrated energy and showed a bandwidth increase of 18%. Modifying the diagnostics to spatially resolve the dimension perpendicular to the plane of incidence showed no evidence of deflection, however the divergence angle was broader than the specular cone width. These spatio-spectral experimental results are in strong qualitative agreement with the 3-D PIC predictions. To time resolve relativistically generated attosecond pulses, we propose a technique using a dispersion-free split mirror interferometric autocorrelator. Phase retrieval over a multioctave spectrum remains an active area of work, as does the nature of the required nonlinear detector. Current work necessary to achieve optimum conditions for performing time resolved measurements is discussed, and solutions to outstanding technical issues are proposed.
University of Michigan, Ann Arbor, Michigan, USA