Observation of ultrafast electron behavior in femtosecond optical breakdown of dielectrics

Summary form only given. Many studies have been conducted on the ultrafast breakdown of dielectrics in order to understand the different processes involved. Such investigations are important to diverse fields such as micromachining, medical physics and solid state physics. Generally, laser induced breakdown in dielectrics is described in terms of three major processes: (i) multiphoton ionization (MPI) and/or tunneling causing the excitation of electrons to the conduction band, (ii) electron-electron collisional ionization (avalanche process) due to Joule heating, (iii) plasma energy transfer to the lattice. While the first two processes deposit energy in the plasma, the third process releases the deposited energy to the lattice thereby inducing the actual damage. This transfer of energy to the lattice is expected to occur after the laser pulse. Until recently, the above processes have been studied by measuring the pulse duration dependence of OBT. Although single pulse OBT measurements have been extended to the 5 fs-range, it is very difficult to extract information regarding electron dynamics from such measurements, especially the time scale for the plasma energy decay. We have developed a new pump-probe double pulse experiment, which is more sensitive to dynamic behavior.