Laser conditioning and nonlinear absorption of LaF3/MgF2 dielectric multilayers at 193 nm

A sensitive and simple pulsed surface thermal lens (TL) technique is used i n situ to investigate the laser conditioning and to measure the nonlinear a bsorption of LaF3/MgF2 dielectric multilayers deposited on CaF2 substrates at 193 rim. Due to the high single-shot sensitivity of the surface TL techn ique, the laser conditioning can be monitored from the first shot of irradi ation on a shot-by-shot basis. The LaF3/MgF2 multilayers show a very strong conditioning effect. The ratio of the absorption before and after the lase r irradiation is in the range 4-8 for a highly reflective (LH)(20) LaF3/MgF 2 multilayer, and 3-4 for (1L3H)(7) and (3L1H)(7) multilayers. In compariso n, a (LH)(20) LaF3/AlF3 multilayer shows only a weak conditioning effect, w ith an absorption ratio of approximately 1.4. Our experimental results sugg est that the strong conditioning effect of the LaF3/MgF2 multilayer is due to the absorption conditioning of the LaF3 layers. However, the MgF2 layers are shown to be responsible for the considerable increase in LaF3 absorpti on as well as the ability to condition the absorption. The fluoride multila yers present non-negligible nonlinear absorption and the two-photon absorpt ion coefficient of the multilayers is estimated to be approximately 5 x 10( -7) cm/W.