Pulsed laser induced breakdown in solid state materials and air

Laser induced breakdown is important in applications ranging from laser machining to laser induced breakdown spectroscopy to laser induced damage. Laser induced breakdown in dielectric materials results from a combination of multiphoton absorption (MPA), tunneling and avalanche ionization. Aspects of these processes have been studied through measurements of the laser induced damage threshold (LIDT) and its dependence on pulse duration, polarization and wavelength. At long pulse durations (> 1ns), relaxation of conduction band electrons during the pulse is responsible for the increased LIDT compared to scaling derived from sub-picosecond pulses. Linear polarization produces lower LIDT than circular polarization for MPA order of m=3 and 4 with femtosecond and picosecond pulses. Laser damage tests with two-color pulse pairs show that the damage threshold depends on the order of the pulse pair. The measured difference between single-pulse and multiple-pulse MPA of sapphire and fused silica coefficients was explained with the creation of laser induced defects. Measurements of damage and ablation thresholds of dielectrics and metals for different numbers of pulses show the effect of laser induced material modifications, also called material incubation. A phenomenological incubation model based on changes of (i) absorption coefficient and (ii) critical energy is introduced and successfully explained the dependence of the ablation threshold on the number of pulses for metals and dielectrics. We explore LIDTs of bulk metals using the plasma emission method. Nickel alloy shows good agreement between the crater size method and the plasma emission method. The transient electron temperature in a femtosecond air plasma (filament) was determined from absorption and optical diffraction experiments. The electron temperature and plasma density decay on similar time scales of a few hundred picoseconds. Comparison with plasma theory reveals the importance of inelastic collisions that lead to energy transfer to vibrational degrees of freedom of air molecules during the plasma cooling.Optical Science and EngineeringDoctoralUniversity of New Mexico. Dept. of Physics & AstronomyRudolph, WolfgangBecerra Chavez, Francisco ElohimSchwoebel, PaulCremers, Davi