Femtosecond laser induced thermal damage in thin films: The role of heat dissipation

This thesis reports investigations of the thermal modifications induced by ultra-short pulsed laser irradiation in thin films at fluences near and above ablation threshold. Specific work was aimed at understanding the role of heat dissipation in determining the nature and spatial extent of thermal processes induced by femtosecond laser irradiation. Thermal damage in the form of microstructural modifications, phase transitions, and/or chemical reactions was analyzed within, outside, and below areas of femtosecond laser irradiated surfaces.; The thermal damage induced by femtosecond laser ablation of silicon thin foils was investigated by high resolution transmission electron microscopy. Extensive single-crystalline Si was observed at the ablated hole edges for irradiation near the ablation threshold. Higher fluence ablation produced hole edges exhibiting a strong amorphous content. The rate of lateral cooling following femtosecond laser irradiation is proposed to account for the resulting microstructure near femtosecond laser ablation.; A nanoscale technique was developed for direct measurement of heat affected zones resulting from femtosecond laser ablation. The lateral extent of cobalt silicidation was measured by selected area diffraction and morphological investigations using transmission electron microscopy. Silicidation extending 3 mum from the edge of an ablated hole provided evidence of a zone where the local temperature reached ∼500°C. Evidence of Si recrystallization was observed within 1 mum of the ablated hole edge and validated by thermal modeling.; Reactive multilayer films composed of Co/Al, Al/Pt, and Ti/Ni were irradiated by femtosecond and nanosecond pulsed lasers. Fluence thresholds for igniting self-propagating reactions were measured as a function of bilayer thickness. The resulting ignition thresholds versus bilayer thickness trends were nearly reciprocal that of reaction propagation speed versus bilayer thickness. The trend is indicative of a relationship between the ignition thresholds and the thermal damage necessary to achieve a critical volume of layer intermixing. Femtosecond laser ablation at lower fluences near ablation threshold showed evidence of single and multiple layer removal. Samples irradiated by nanosecond lasers of comparable fluence showed evidence of extensive intermixing with negligible material removal. Heat dissipation rates were observed to be an important factor for pulsed laser ignition of reactive multilayers based on numerical analysis and comparative ignition studies of three bimetallic systems.