Laser radiation interactions with solids

The chapters of this thesis are linked by virtue of the fact that each is an individual investigation of a particular area of current interest related to the interaction of intense laser beams with solids.

The first chapter stands as a general introduction to the area of laser induced damage to solids. The currently accepted theories regarding the fundamental processes leading to damage are discussed as are the definition of laser induced damage, how it is detected and why it is important to eliminate it.

Chapter two is an investigation into the basic formation mechanism behind the generation of laser induced ripple structures during laser irradiation of a surface. With the relevance of this phenomenon to laser damage and pattern formation the work is of great importance and interest to other workers in the field. For the first time direct observations confirming the currently accepted theory of ripple formation are presented, which at the same time extend the formation of laser induced ripples into the ultraviolet. The work is one of the first to systimatically compare theory and experiment. The work is placed in the context of previous work reported in the literature and the main physics is made clear. The work also puts foreward an addition to the currently accepted theory of ripple formation in the form of an intensity interference mechanism. This addition can be used to successfully explain what were previously termed as anomalous ripples.

Chapter three relates directly to the subject of laser induced damage. Detailed within is an investigation of the so called 'laser annealing' effect. By preconditioning or laser annealing a surface to be damage tested with various regimes of low fluence, non damaging pulses, its subsequent single shot damage threshold can be significantly enhanced. A review of previous work is given. Quantitative measurements are presented for several UV and IR optical materials. Attempts are made to explain the observations in terms of the removal or reduction of surface roughness and contamination and changes to the crystal structure of the surfaces under test. The dominant mechanism would appear to be the reduction, by laser annealing, of surface roughness leading to an enhancement of the laser induced damage threshold.

To be able to make reliable and meaningful damage threshold measurements it is necessary to carefully characterize the spatial intensity profile of the damaging laser's output. Chapter four contains a review and discussion of various methods of beam profiling and details the work done in extending the Universities currently existing UV laser beam profiling system. The construction of a new system for profiling the output of a CO₂ IR laser is also discussed. This system provides a single shot 2-dimensional realtime method of profiling. The system is of low cost, but has specifications close to those of commercial systems.

Chapter five presents the results of an investigation into one of the fundamental problems encountered in the theories of laser damage formation. By using so called crossed beams configurations of the output of syncronized UV and IR lasers an attempt is made to determine the role of so called 'seed electrons ' in the damage mechanism. A review of previous work is given. Both UV and IR materials are irradiated and the results of the work used in an attempt to find which form of damage is predominant at each wavelength.