Contrast Improvement of Relativistic Few-Cycle Light Pulses

Laser-plasma interaction has been a widespread and popular field of research since the birth of the laser. It involves numerous interesting phenomena such as inertial confinement fusion; generation and acceleration of electron, positron, neutron, proton and ion particle beams; source of electro-magnetic radiation in the IR, visible, UV and x-ray range, just to mention some important applications. The continuously increasing power and the development of chirped pulse amplification lead to electric fields in which oscillating free electrons reach almost the speed of light. This so called relativistic optics regime starts at intensities about 1018 W/cm2 at 1 μm wavelength. The interaction of relativistically intense laser pulses with solid state -possibly liquidtarget material is always accompanied by the creation of inhomogeneous plasmas, as a given amount of extended preplasma is formed in front of the target before the main laser pulse arrives. This preplasma is produced by undesired laser light impinging onto the target before the short main pulse. Its density decreases rapidly farther from the target due to the hydrodynamic expansion of the hot plasma during the time till the interaction pulse comes. The extension of the preplasma significantly influences the interaction and leads to completely different types of processes depending on its value. Typical lasers produce an extended preplasma before the main pulse, because they amplify preand postpulses and pedestals from various origins. Therefore a certain types of processes involving interaction of intense laser pulse with extended low density plasmas dominate these investigations. On the other hand, practically preplasma-free environment is required by various relativistic laser-plasma experiments involving ultra-intense laser and high-density plasma interaction like surface high harmonic generation Monot et al. (2004); Thaury et al. (2007), laser-driven proton and ion acceleration Hegelich et al. (2006) or laser interaction with few nanometer thick diamond-like-carbon foils, so called nanofoils. This preplasma formation is influenced by the contrast, which characterizes the laser intensity before and after the main pulse relative to this pulse. Therefore precise characterization and improvement of the contrast are essential to successfully conduct these types of experiments. Laser pulses with a duration of only a few optical cycles at moderate intensities opened up the new era of attosecond physics Brabec & Krausz (2000); Krausz & Ivanov (2009) via the controlled reproducible generation of weak XUV pulses with a duration of hundred(s) of attoseconds precisely synchronized to the laser pulse. These pulses allow the investigation of electron motion in atoms, molecules and solid states in an XUV pump and visible / near infrared probe configuration. Relativistic intensity few-cycle sources hold the promise to generate XUV pulses with unprecedented energy and thus form the basis of the novel research 14