TIME-RESOLVED DETECTION OF PARTICLE REMOVAL FROM DIELECTRICS ON FEMTOSECOND LASER-ABLATION

The dynamics of the ablation process in different oxides (amorphous an d crystalline quartz. sapphire and MgO) after intensive lasts excitati on was studied by a combination of the femtosecond pump-probe techniqu e with optical surface scattering. A 120-fs laser pulse (lambda = 800 nm) was focused onto the sample with a fluence at least twice the sing le-shot surface damage threshold. A much weal;er probe laser pulse opt ically aligned collinear to the pump beam illuminated the excited area at variable delay times up to 100 ps. We observe a material dependent increase in the scattering signal after a certain delay time which we interpret as the onset of the ablation process. This occurs much earl ier for amorphous and crystalline silica than for sapphire or MgO, as a consequence of differences in the electron-phonon coupling strength. The morphology of the irradiated spot on sapphire, amorphous and crys talline quartz is indicative of melting and vaporisation in contrast t o that on MgO which clearly shows fracturing. (C) 1998 Elsevier Scienc e B.V.