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

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Abstract
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.