Evidence of noncollisional femtosecond laser energy deposition in dielectric materials

Electron dynamics in the bulk of large band gap dielectric crystals induced by intense femtosecond laser pulses at 800 nm is studied. With laser intensities (a few 10 TW/cm 2) under the ablation threshold, electrons with unexpected energies in excess of 40-50 eV are observed by using the photoemission spectroscopy. A theoretical approach based on the Boltzmann kinetic equation including state-of-the-art modeling for various particles interactions is developed to interpret these experimental observations. A direct comparison shows that both electron heating in the bulk and a further laser field acceleration after ejection from the material contribute equivalently to the final electron energy gain. The laser energy deposition in the material is shown to be significantly driven by a noncollisional process, i.e., direct multiphoton transitions between subbands of the conduction band. This work also sheds light on the contribution of the standard electron excitation/relaxation collisional processes, providing a new baseline to study the electron dynamics in dielectric materials and associated applications as laser material micromachining. To support such applications, a simple expression to evaluate the energy deposition by noncollisional absorption is provided.