Ultrafast Breakdown of dielectrics: Energy absorption mechanisms investigated by double pulse experiments

We investigate the mechanisms involved in the modification of dielectric materials by ultrashort laser pulses. We show that the use of a double pulse (fundamental and second harmonic of a Ti-Sa laser) excitation allows getting new insight in the fundamental processes that occur during the interaction. We first measure the optical breakdown (OB) threshold map (intensity of first pulse versus intensity of second pulse) in various materials (Al2O3, MgO, α-SiO2). Using a simple model that includes multiphoton excitation followed by carrier heating in the conduction band, and assuming that OB occurs when a critical amount of energy is deposited in the material, we can satisfactorily reproduce this evolution of optical breakdown thresholds. The results demonstrate the dominant role of carrier heating in the energy transfer from the laser pulse to the solid. This important phenomenon is also highlighted by the kinetic energy distribution of photoelectrons observed in a photoemission experiment performed under similar conditions of double pulse excitation. Finally we show, in the case of α-SiO2, that the initial electronic excitation plays a key role in the formation of surface ripples and that their characteristics are determined by the first pulse, even at intensities well below OB threshold.