Bubble creation and collapse during excimer laser ablation of weak absorbing polymers

We present evidence suggesting that XeCl laser ablation of a weakly absorbing poly-methyl-methacrylate (PMMA) polymer, done by chemical, thermal bond breaking of the polymer chain or optical breakdown of the material, which involves plasma generation, creates a cloud of small asymmetric near the surface bubbles, which subsequently expand and aggregate during the same laser pulse duration or in subsequent pulses depending on the laser pulse energy. When a critical volume is reached each bubble collapses in a high pressure and temperature central point and rebounds ejecting a hot jet of material on the non-irradiated area of the polymer and creating craters on the surface. A characteristic bipolar pressure wave corresponding to the bubble collapse, explosion and rebound is observed. The number density of the craters on the surface is a function of the laser pulse sequence number and the laser pulse energy density.