Abstract

Mirrors for storage ring free-electron lasers in the vacuum ultraviolet must provide adequate reflectivity and resistance against synchrotron radiation. The free-electron laser system at ELETTRA (Trieste, Italy) is targeted to lase in the spectral range between 155 and $200\text{\hspace{0.17em} nm}$. It was demonstrated that dense oxide multilayer coatings allow lasing down to $189.9\text{\hspace{0.17em} nm}$. However, pure oxide systems show significant absorption at lower wavelengths and cannot be employed below 189.9 nm. Fluoride stacks can be deposited down to $130\text{\hspace{0.17em} nm}$ with high reflection values above $95\%$, but their resistance against the harsh synchrotron environment is poor. They rapidly degrade; lasing cannot be realized with this mirror approach. For the range between 170 and $190\text{\hspace{0.17em} nm}$, hybrid systems—combining fluoride and oxide materials—have been manufactured. With appropriate deposition procedures, mirrors achieve reflectance values up to $99\%$ and an adequate radiation resistance simultaneously. A mirror based on a conventional fluoride stack protected by a dense silicon dioxide protection layer was deposited and successfully employed for free-electron lasing at $176.4\text{\hspace{0.17em} nm}$.

© 2006 Optical Society of America

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