Ion beam sputtering of large scale dichroic mirror for fifth-harmonic separation

This contribution describes an approach to realize dichroic mirrors for fifth-harmonic separation with a diameter of 12”. The optics will be used at the National Ignition Facility to build a diagnostic based on Optical Thomson Scattering, which requires a high energy, pulsed laser operating at 211nm wavelength. Since the ultra-violet absorption edge of the most commonly used high refractive coating materials is above 211nm, only a few oxide materials as for example alumina are suitable for this wavelength. The applied material combination Al2O3/SiO2 provides a small refractive index contrast of about 0.2, which requires a coating process with a very high precision and uniformity to realize complex thin film designs. To achieve a physical layer thickness uniformity better than 0.5%, the linear motion concept of the MAXIMA ion beam sputtering machine [1] is combined with an additional substrate rotation. The layer thickness is controlled precisely by optical broad band monitoring in the wavelength range from 220nm to 1050nm. To realize a surface figure of λ/10 at 633nm for a clear aperture of 250mm diameter, the multi-antireflection coating on the backside is utilized for stress compensation. Experimental results regarding the spectral performance, the mechanical stress, the surface roughness and the laser damage resistance will be presented and discussed.