Ablation of optical materials due to an intense picosecond free-electron laser

An intense pico second FEL induces a surface ablation on optical materials, which are used for focusing and sharing of the FEL power in application researches. The FELs at the FELI are available in 40 to approximately 0.2 micrometer wavelength region, and therefore various kinds of optical materials are used for output windows and coupling mirrors such as ZnSe, SiO2 coated with Ta2O5 or HfO2 in the facility. In addition, the FEL macropulse is continued for approximately 20 microseconds of several MW micropulse train. In high power applications the focused FEL power density is over several GW/cm2. When the power density of the FEL is exceeded over the threshold of the optical materials, the luminescence emits from the material surface. We found the ablation threshold of ZnSe and SiO2 due to both 9.2 micrometer FEL and 1.054 micrometer Nd:YLF laser irradiation in pico-second duration. In case of ZnSe material we have observed that the luminescence is synchronized with 22.3 MHz FEL micropulse, and that the ablation thresholds vary of the square root of the laser duration scaling law in 3 - 12 ps duration.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.