Comparison of high power sputtered dielectric mirrors for PW laser

Abstract

Latest advances of high intensity laser facilities enable the beam transport of petawatt laser pulses and can provide novel fundamental insights in high energy plasma physics or laser fusion. The very high peak intensities put enormous demands on the required large sized optics. Beam transport mirrors reflect pulses with only several tens of fs and maintain their phase while providing best possible laser induced damage threshold. State of the art, such mirrors are mostly manufactured by thermal evaporation techniques as they provide a large and uniform deposition area. Their porous layer structure causes changing spectral characteristics and wavefront when vacuum-air cycled. Especially large sized mirrors can show crazing and thereby decrease up-time of PW beamlines. In contrast, sputtered layers are very compact and provide non changing characteristics. Stable and reproducible sputter processes enable the deposition of more complex design structures necessary for further optimization of the laser induced damage threshold. However, deposition rate is slow and an uniform large sized area difficult to achieve for sputtered coatings. In our study, we show a self-constructed and built-up ion beam sputtering (IBS) machine capable to deposit large sized substrates up to a diameter of 550 mm. A design study is presented to evaluate best HR810nm mirror to meet demanding spectral requirements and providing maximized laser damage threshold for HAPLS at ELI beamlines. In the end, a field optimized design is applied with a measured LIDT of 0.9 J/cm2 at 42 fs and 1 kHz. This design is used to manufacture beam transport mirrors for HAPLS applying IBS.