Variable reflectivity dielectric mirrors for THz-FEL applications

We have constructed and tested multilayer dielectric mirrors with tunable reflectivity to be used as efficient, broad band outcouplers for THz FELs. The test outcoupler mirrors are assembled by stacking up to five cyristalline (z-cut) Quartz wafers separated by vacuum. Reflectivities of 99.5% and a tunability ranging from 0 up to ~30% have been measured. In order to attain reflection values ges 99.9% (as a high reflectivity cavity mirror) we employed a hybrid structure that is composed of a metal layer (gold coated) and multiple dielectric layers.