Micro-void and spherical dome formation in polycarbonate with nanojoule energy femtosecond laser pulses

We report on the formation of micro-voids inside polycarbonate samples upon irradiation with a high repetition rate and low energy femtosecond laser. The laser focal point was varied through the sample thickness and the resulting features were analyzed in a scanning electron microscope. It was found that spherical dome-like features are formed on the sample surface and that their size depends on the depth of the laser focal point. Resolidified regions and microvoids were also found below each dome and their shape was justified in terms of a change in the laser focal length due to the formation of the spherical dome that acts as a microlens.