Transmission of fused silica, CaF 2 , LiF, and MgF 2 is measured using 450-fsec, 248-nm pulses in the range 10-120 GW/cm 2 . Different loss mechanisms such as scattering of transmitted radiation, color-center formation, and multiphoton absorption were studied separately. For fused silica a two-photon absorption mechanism is found, while for CaF 2 , LiF, and MgF 2 three-photon absorption and absorption due to color-center formation are found as dominant absorption mechanisms.With existing high-brightness KrF laser systems, optical powers of the order of typically 10-100 GW are generated. [1][2][3][4][5][6][7] The power densities of the output beams of these systems typically range from 10 to 100 GW/cm 2 . At those high power densities nonlinearoptical properties of window materials become important.Previous studies of two-photon absorption were performed at 355 and 266 nm. 8 Measurements of nonlinear absorption of window materials at 248 nm were recently reported in Refs. 6,9, and 10. However, the relative contributions of the different loss mechanisms (multiphoton absorption, color-center formation, and scattering) were not considered. In Ref. 8 the two-photon absorption mechanism found for fused silica was simply adapted for CaF 2 . Surprisingly, the two-photon coefficient of CaF 2 was dependent on the sample studied. Even in the case of LiF andMgF 2 , where two-photon absorption is improbable, an upper bound for a two-photon absorption coefficient was defined. In addition, the results reported in the publications cited above are not in total agreement. The above problems and the importance and necessity of having exact data led us to carry out comparative measurements on the intensity-dependent transmission of UV windows of different materials and from different suppliers. In this Letter we report on the nonlinear loss mechanisms observed in fused silica, CaF 2 , LiF, and MgF 2 using 450-fsec pulses at 248 nm. While for fused silica our results show good agreement with the two-photon absorption mechanism found in Refs. 8 and 9, we found that the data for CaF 2 , LiF, and MgF 2 cannot simply be approximated by two-photon absorption but can be described by the combined effect of three-photon absorption, scattering, and color-center formation.For the transmission measurements, 8-mJ, 450-fsec pulses at 248 nm from a high-power KrF laser system 6 were used. The pulse energy is measured with a Gentec ED-500 detector. The pulse duration is determined by an autocorrelation measurement using twophoton ionization in NO. 2 , 6 " 11 ,12 From the autocorrelation trace a +4% fluctuation is obtained for the energy and the pulse width. The amplified spontaneous emission is -5% of the total energy.The experimental setup is as follows: The relatively homogeneous middle part of the beam is selected by an approximately 3.5-mm-diameter circular aperture. The plane of this aperture is imaged onto the sample by a 1-m focal-length fused-silica lens. The spot size and the intensity distribution on the sample were checked by a linear d...