Reflectivity transients on solid surfaces induced by 0.5 ps high power excimer laser irradiation

Changes in reflectivity of various solid materials upon irradiation with hi gh power (10(12) W/cm(2)) ultrashort (500 fs) KrF excimer laser pulses are reported. The excimer pulse is focused by a cylindrical lens onto a line at the sample surface while a tilted wavefront (45 degrees) probe pulse (lamb da = 496 nm, 500 fs pulse length) is scanning over the ablated area. Thus t he spatial distribution of the reflected probe pulse corresponding to the t emporal reflectivity transient, was recorded. For polymeric and semiconduct or samples the reflectivity increases with a factor of 1.5-5 compared to th e original value when the excimer laser pulse impinges onto the surface. It is shown that increasing the absorption of PMMA samples with naphthalene d oping leads to an increase in maximum reflectivity. In case of glass reflec tivity decrease is observed. The reflectivity increase or decrease is inter preted as a competing process of formation of high density free electron ga s due to fast ionization processes, which leads to higher reflectance, and expansion of the ablation plume which leads to reflectivity decrease due to light scattering processes.