Abstract

The future operation of broadly tunable free-electron laser (FEL) oscillators below 100 nm is very dependent on the development of resonator end mirrors with reflectance greater than 50 per cent. We have determined that a multifaceted mirror configuration of a few metal films, e.g., Al, Si, Rh, and Ag, offers the potential of achieving large values (≥50 per cent) of XUV retroreflectance. Even larger net reflectance values are possible for 90° beam-turning mirrors. These make use of the principle of quasi-total external reflectance for incidence angles larger than the critical angle (≥60°) and with S-plane polarization. Only when the optical constants n and k both are sufficiently less than unity is there a large increase over a single, normal-incidence reflection.

© 1986 Optical Society of America