Time-dependent self-focusing and a 20 ps delay in laser ablation of polymers

Laser surface ablation of polymethylmethacrylate by ultrafast 0.532 μm pulses is studied using an imaging apparatus with 2 ps resolution. Coherent two-photon absorption rapidly heats the sample, inducing explosive thermal decomposition. Electron microscopy is used to characterize the damaged surface. Ultrafast imaging shows that surface damage is accompanied by the production of a transient optical filament. The intensity dependence shows that self-focusing results from an accumulative, rather than instantaneous, relaxation of the transient refractive index. At all intensities, there is a 20 ps delay before ablation commences.