Title

Modeling of laser-induced damage in dielectrics with subpicosecond pulses

Date of Completion

January 1999

Keywords

Engineering, Electronics and Electrical|Physics, Condensed Matter

Degree

Ph.D.

Abstract

Theoretical study of ultrafast laser induced damage (LID) by short pulses (τ < 1 ps) is carried out on large-band-gap dielectric in an effort to understand the complex physical processes involved. The numerical method of solving a general time-dependent Fokker-Planck type equation is discussed in detail and some of the difficulties in the numerical implementation of its boundary conditions are pointed out. The calculation shows that, contrary to the accepted picture, the collisional avalanche ionization competes with the multiphoton ionization even for pulse length shorter than 25 fs. Sensitivity tests of all the rates in the equation are performed and critical comments are made on the theoretical derivations of the rates found to be most sensitive. From these tests we obtain information in developing new materials that have the desired damage fluence for specific applications. To describe the relaxation of electron plasma, a three body recombination rate (TBR) is included. Its effect during the pulse and long after the pulse has passes is examined. Thus, the temporal behavior of the electron density due to a single pulse is treated, as well as the case of exposure to two laser pulses with a time delay between them. A linear decay term associated with the exciton formation is included in the equation to explain recently obtained experimental data; the model is only partially successful in reproducing the data. The effect of the presence of optical defects on the damage threshold is considered in the context of the rate equation input. ^