Ultrafast imaging of 0.532-μm laser ablation of polymers: Time evolution of surface damage and blast wave generation

An ultrafast two-color laser spectrometer with image acquisition capability is used to study surface ablation of a transparent polymer, PMMA (polymethyl methacrylate). Surface ablation was produced by 100-ps, 0.532-μm pulses and probed by 2-ps, 0.570-μm pulses. Computer-digitized images were obtained over the time range 10^-12 -10⁰ s. The images were analyzed to obtain the time-dependent behavior of the damaged solid, and the blast wave generated at the solid-gas interface. Near the peak of the ablation pulse, self-focusing begins and produces a small-diameter filament lasting for 20 ps. The polymer irradiated by the filament then undergoes explosive thermal decomposition, ejecting particles from a conical volume into the atmosphere above the surface. This ablated matter produces a hemispherical, supersonic blast wave whose kinetic energy is one-fourth of the ablation pulse energy. The evacuated pit produced in the polymer is very hot, and the surrounding solid softens and flows, resolidifying in about 1 s. A mechanism for the ablation process involving nonlinear absorption is proposed. The steeply rising envelope of the ablation pulse simultaneously increases the absorption coefficient and decreases the absorption length, resulting in a runaway heating process with a rate of ≊10¹³ K/s. The polymer is overheated far beyond the normal decomposition temperature. Thermal decomposition then proceeds with a large, negative free energy.