Mitchell Physics Building
College Station, Texas 77843-4242
High-order harmonic generation (HHG) in plasmas induced by ultrashort, relativistic-intensity laser pulses on solid surfaces can provide an efficient source of attosecond pulses and opens routes toward new regimes of laser-matter interactions, x-ray generation, laser particle acceleration, and relativistic nonlinear optics. However, field intensities in the range of I relâˆ¼1019 Wâˆ•cm2 are typically needed to achieve the relativistic regime of HHG in experiments with near-infrared laser pulses. Here, we show that, in the midinfrared range, due to the Î»âˆ’2 scaling of I rel with the driver wavelength Î», relativistic HHG can be observed at much lower levels of laser field intensities. High-peak-power 80-fs, 3.9-Î¼m pulses are focused in our experiments on a solid surface to provide field intensities in the range of 1017 Wâˆ•cm2. Remarkably, this level of field intensities, considered as low by the standards of relativistic optics in the near infrared, is shown to be sufficient for generation of high-order harmonics with signature properties of relativistic HHG-beam directionality, spectra with extended plateaus, and a high HHG yield sustained for both p- and s-polarized driver fields.
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