Three omega mirrors for the megajoule laser: the narrow way

The possibilities of very high damage threshold optics at the Nd:YAG third harmonic are discussed for Megajoule applications. Only two possibilities which are either electron beam oxides materials like Al2O3/SiO2 or IBS fluorides like YF3/LiF seem to be able to meet the specifications. Current state of the art of the two solutions is given as well as their possible improvements.