CC BY
10LMI-I-27
Intrapulse dynamics of plasma formation in fs-laser irradiated dielectrics
A. Mermillod-Blondin1, P. Jürgens1, B. Liewehr2, C. Peltz2, B. Kruse2, T. Fennel2, A. Husakou1, M.J.J. Vrakking1
1Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, A: Attosecond Physics, Berlin, Germany
2University of Rostock, Institute of Physics, Rostock, Germany
Focusing an intense (in the TW/cm2 range) ultrashort (sub-ps) laser pulse in the bulk of a transparent material leads to permanent structural modifications of the lattice in the irradiation region. Such structural modifications alter the samples properties including its refractive index. Based on laser-induced refractive index changes, fs laser-direct writing of optical microstructures in solid dielectrics has rapidly become a standard process.
Because structural re-arrangements of the lattice require energy, understanding the mechanisms of laser energy deposition into the sample is of prime importance to describe, control and optimize the laser matter interaction. The energy transfer between the laser beam and the lattice proceeds through the promotion of electrons from the valence to the conduction band resulting in the formation of an underdense (density of ca.1019-1020 cm-3) electron-hole plasma. Strong-field ionization and electron-electron impact ionization have been identified as the two fundamental processes governing the free carrier generation. However, the relative contribution of these two channels remains an open question.
In this talk, we report on the emission of low-order harmonics during laser irradiation in amorphous fused silica and show that when the laser-intensity approaches the permanent modification threshold, the low-order harmonic formation is due to the transport of the free carriers across the bandgap. The nonlinearity of strong field ionization dominates by far all other possible sources of nonlinearities, including Kerr-type, intraband, interband as well as the so-called Brunel contributions. These results enable to detect optically the onset of strong-field ionization and provide unprecedented insights into the laser-induced plasma formation in microprocessing conditions.
