Interference Coatings for High Energy and Average Power Femtosecond-class Lasers in the 0.8-2um Wavelength Range

The problem/situation that is being addressed concerns the development of ultrafast coatings that will meet the specifications of the DoE solicitation: “broad-bandwidth, laser damage threshold of 0.5 J/cm2 (1 ps), engineered using a process that is scalable to large areas.” Development of interference coatings (ICs) with superior laser damage is strongly motivated by the needs to scale near infrared femtosecond laser systems to kilo-Watts average power for laser-driven accelerator applications. Output power of these lasers is limited by the lowest damage threshold of their optical components that is usually attributed to the ICs. Therefore, ultra-high intensity high average power femtosecond laser sources critically rely on superior performance of ICs for maximum power extraction. However, when exposed to intense femtosecond pulses, the coatings damage at a fluence below 1 J/cm2. XUV Lasers Inc. in collaboration with the Ohio State University (OSU), University of New Mexico (UNM) and Colorado State University (CSU), proposes a combined modeling and experimental effort to develop high damage threshold ultrafast ICs beyond state of the art. In Phase I, we extended the Gruzdev model of laser damage of wide bandgap materials to describe the interaction with ultrabroad bandwidth ultrashort pulses. We also included field ionization, plasma and propagation effects to create a dynamic finite difference time domain (FDTD) model for 1D for multilayer stacks. The simulation results of bi- and quad-layers IC convincingly show laser damage depends on pulse duration, pointing towards importance of the extensive modeling framework that is needed to capture and enhance the understanding of the whole process, so that we can design and construct higher damage threshold optics. The Phase II effort will expand the model framework to multi-time-scale multi-physics 2D, capturing field-ionization, temperature evolution and hydrodynamics while incorporating critical microscopic material parameters that are unknown for the disordered amorphous oxides more accurately through theory and experimentation. These modifications will significantly enhance the predictive capabilities of the model to capture the failure of IC systems under ultra-broadband ultrashort pulse laser irradiation more accurately. A close interaction with the coating and fabrication effort will enable us to build a robust, predictive framework for IC damage at femtosecond time scales. This iterative process will be key to optimize the performance of IC beyond state-of-the-art. The focus of the Phase II effort on growth, characterization and modeling of laser/matter interaction will be on ICs for high average power ultrafast lasers operating in the 0.8-2 m range. The outcome of this Phase II will be ICs with beyond state-of-the- art performance that XUV Lasers will commercialize. Interference coatings demonstrate a solid performance for low-fluence narrow-bandwidth lasers but fail dramatically when tested with ultrashort pulses because of their large bandwidth. This challenge coupled with a scarce supplier market open up enticing commercial opportunities. Ultrafast coatings with superior laser damage performance will advance the engineering of high average power femtosecond lasers for next-gen accelerator-type applications. If realized, they can create a $50 million/year high power-million to billion shot optics market for laser-based accelerators in scientific and medical markets. Considering the growing investments in high average power femtosecond lasers, the proposed efforts will enable the United States to remain competitive in this critical technology.