Attosecond extreme-ultraviolet (XUV) pulses from laser-matter interactions have provided a unique tool for controlling and measuring electronic dynamics on the atomic scale . Currently these pulses are typically generated via high harmonic generation (HHG) in gases where the highest pulse energies are limited to the microjoule range due to phase-matching effects and ground-state depletion. HHG from relativistic interactions with plasma surfaces has the potential to surpass these limitations by employing significantly higher intensities. A key challenge for the application of these pulses is the need to isolate individual attosecond pulses from the pulse train that is responsible for the emitted radiation appearing as harmonics rather than as a continuous spectrum. For normal incidence interactions it has previously been shown that manipulating the polarization of the laser pulse so that it switches from circular to linear to circular again can restrict the attosecond emission to only the linear period, thus gating the mechanism . For this process to be efficient, however, the initial laser pulse must already only consist of a few cycles which is a major challenge for lasers with enough power to drive such a relativistic process. Here we present simulations that show how this restriction can be softened by combining this polarization effect with two-color fields where the addition of the 2nd harmonic of the laser switches the attosecond pulse emission rate from twice per cycle to once per cycle as well as increasing the intensity of the resulting isolated attosecond pulse. This Double Optical Gating technique has previously been employed for HHG in gases  but our simulations demonstrate its applicability to relativistic harmonics which will permit much higher attosecond pulse energies suitable for intense attosecond pump-probe studies.
 F. Krausz and M. Ivanov, Rev. Mod. Phys., 81, 163 (2009)
 S. Rykovanov et al., New J. Phys., 10, 025025 (2008)
 H. Mashiko et al., Phys. Rev. Lett., 100, 103906 (2008)
|Working group||Secondary radiation generation & applications|