Laser-induced breakdown and damage to transparent materials has remained an active area of research for four decades. In this paper we review the basic mechanisms that lead to laser-induced breakdown and damage and present a summary of some open questions in the field. We present a method for measuring the threshold intensity required to produce breakdown and damage in the bulk, as opposed to on the surface, of the material. Using this technique, we measure the material band-gap and laser-wavelength dependence of the threshold intensity for bulk damage using femtosecond laser pulses. Based on these thresholds, we determine the relative role of different nonlinear ionization mechanisms for different laser and material parameters.

Using tightly focused femtosecond laser pulses of just 5 nJ, we produce optical breakdown and structural change in bulk transparent materials and demonstrate micromachining of transparent materials by use of unamplified lasers. We present measurements of the threshold for structural change in Corning 0211 glass as well as a study of the morphology of the structures produced by single and multiple laser pulses. At a high repetition rate, multiple pulses produce a structural change dominated by cumulative heating of the material by successive laser pulses. Using this cumulative heating effect, we write single-mode optical waveguides inside bulk glass, using only a laser oscillator.

We report an investigation of white-light continuum generation and self-focusing by 140-fs Ti:sapphire laser pulses in extended transparent media. It is found that continuum generation is triggered by self-focusing and that both phenomena depend on the medium's bandgap. There is a bandgap threshold for continuum generation. Above that threshold the continuum's width increases with increasing bandgap. Furthermore, the beam's self-focal diameter is discontinuous across the threshold. To explain the observations a mechanism is proposed that involves multiphoton excitation of electrons into the conduction band at the self-focus; the generated free electrons cause spectral superbroadening and limit the self-focal diameter. The continuum beam's surprisingly low divergence is then investigated and explained in terms of a Kerr lensing effect.

The long light filaments generated in air by powerful ultrashort laser pulses, previously attributed to self-channeling, were investigated by use of gigawatt pulses from a Ti:sapphire chirped-pulse-amplification laser system. A filament contained only a small fraction of the pulse energy and always ended at the diffraction length of the beam (~100 m), independently of the pulse energy. These features are explained by the moving-focus model, which is presented as an alternative to the self-channeling model. Computer simulations involving ionization of the air also support the moving-focus model.

We report an investigation of white-light continuum generation during self-focusing in extended transparent media using 140-fs Ti:sapphire laser pulses. A band-gap threshold is found above which the width of the continuum tends to increase with increasing band gap and below which there is no continuum generation. This is, to our knowledge, the first report of a parameter predicting the width of the continuum in condensed media. Multiphoton excitation of electrons into the conduction band is proposed as the primary mechanism reponsible for the observations. [S0031-9007(98)06156-0]