High contrast plasma mirror: Spatial filtering and second harmonic generation at 1019 W cm-2

Abstract

Recently, the use of plasma optics to improve temporal pulse contrast has had a remarkable impact on the field of high-power laser-solid density interaction physics. Opening an avenue to previously unachievable plasma density gradients in the high intensity focus, this advance has enabled researchers to investigate new regimes of harmonic generation and ion acceleration. Until now, however, plasma optics for fundamental laser reflection have been used in the sub-relativistic intensity regime (10¹⁵-10¹⁶W cm ^-2) showing high reflectivity (∼70%) and good focusability. Therefore, the question remains as to whether plasma optics can be used for such applications in the relativistic intensity regime (>10¹⁸ Wcm ^-2). Previous studies of plasma mirrors (PMs) indicate that, for 40 fs laser pulses, the reflectivity fluctuates by an order of magnitude and that focusability of the beam is lost as the intensity is increased above 5 × 10¹⁶W cm^-2. However, these experiments were performed using laser pulses with a contrast ratio of ∼10⁷ to generate the reflecting surface. Here, we present results for PM operation using high contrast laser pulses resulting in a new regime of operation-the high contrast plasma mirror (HCPM). In this regime, pulses with contrast ratio >10 ¹⁰ are used to form the PM surface at > 10¹⁹ W cm ^-2, displaying excellent spatial filtering, reflected near-field beam profile of the fundamental beam and reflectivities of 60 ± 5%. Efficient second harmonic generation is also observed with exceptional beam quality suggesting that this may be a route to achieving the highest focusable harmonic intensities. Plasma optics therefore offer the opportunity to manipulate ultra-intense laser beams both spatially and temporally. They also allow for ultrafast frequency up-shifting without detrimental effects due to group velocity dispersion (GVD) or reduced focusability which frequently occur when nonlinear crystals are used for frequency conversion. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.