We measured the optical breakdown threshold (OBT) in dielectrics with different band gaps for single and double 25-fs 800-nm transform-limited laser pulses. Our pump-probe double pulse measurements indicate that the plasma energy in dielectrics experiences ultrafast decay which lasts only ϳ100 fs and does not follow an exponential decay curve. Therefore, a decay term must be included in the electron density rate equation. Our double pulse measurements also demonstrate that the OBT is temperature dependent. The OBT in dielectrics was determined using a novel technique, which eliminates the ambiguity in its definition and also allows real-time data acquisition. [S0031-9007(99)08660-3] PACS numbers: 78.47. + p, 42.50.Ct, 42.62.Cf Many studies have been conducted on the ultrafast breakdown of dielectrics in order to understand the different processes involved [1][2][3][4][5]. Such investigations are important to such diverse fields 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, and (iii) plasma energy transfer to the lattice [3,4,6]. 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 [3]. Until recently, the above processes were studied by measuring the pulse duration dependence of optical breakdown threshold (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. Although another pump-probe technique by Mazur et al.[7], with a resolution of ϳ100 fs, was proposed for the study of semiconductors, this is the first time that the time scale for the plasma energy decay in dielectrics has been directly measured with ultrahigh temporal resolution (25 fs). Our results from the double pulse OBT measurements indicate that this energy decay is an ultrafast process ͑ϳ100 fs͒. Therefore, for a pulse longer than 100 fs, the plasma energy decays within the pulse duration, which indicates a need to redefine the breakdown process. Although we have modified the existing electron density model to include the decay behavior, our simplified model does not fit the data very well. In order to determine a definite time scale and to investigate the impact of including such decay behavior on interpreting other single pulse experiments, a more sophisticated model is needed. Every experiment on OBT requires an experimental criterion for damage. Typically, OBT is determine...

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The aim of this paper is to examine the role of excited states and multi-electron interactions in molecular ionization by strong laser fields. We present new data on the ionization and dissociation of iodine molecules that reveal important aspects of the strong field-molecule interaction in the short pulse regime. Our data, along with previous studies, is inconsistent with the simplest and commonly accepted model of molecular ionization in a strong laser field and this has led us to examine closely the individual ionization steps. We have found that a molecule can ionize into several distinct configurations predominantly through multi-electron interactions and the abundance of such configurations is dependent on internuclear separation. The ionization appears to be dominated by pairs of states with gerade and ungerade symmetry, as they have a large dipole coupling and the transition is near resonant with the strong laser field. In a oneelectron molecule, this pair consists of the ground and first excited state whereas in a two-electron molecule this corresponds to the lowest lying pair of ionic states. In this paper, we propose a framework for organizing the numerous ionization pathways based on the electronic configuration of the initial charge state.

The Poincare mapping and the corresponding mapping sections for global motions in a linear system possessing a dead-zone restoring force are developed through the switching planes pertaining to the two constraints. The global periodic motions based on the Poincare mapping are determined, and the analysis for the stability and bifurcation of periodic motion is carried out. From the global periodic motions, the global chaos in such a system is investigated numerically. The bifurcation scenario with varying parameters was presented. The mapping structures of periodic and chaotic motions are discussed. The Poincare mapping sections for global chaos are given for illustration. The grazing phenomenon embedded in chaotic motion is observed.