A study of the fundamental mechanisms governing the femtosecond laser damage process is presented in this thesis. In particular, these studies are carried out at mid-infrared (IR) wavelengths, a regime that has not been investigated previously for femtosecond pulses. However, the increased interest in the interaction between mid-IR pulses and solid crystals, as well as the greater availability of short-pulse laser systems operating at these wavelengths, has made such studies paramount. Even beyond the practical applications of preventing laser damage in relevant laser systems, an understanding of the damage process offers insight into the fundamental mechanisms of mid-IR laser-interactions in general.;This was achieved through measurements of the single-pulse laser induced damage and ablation thresholds of the semiconductors Si, Ge, and ZnSe across a range of wavelengths extending from the near-IR to the mid-IR. Additionally, the morphology of many damage sites was imaged through a wide variety of techniques. The results of these measurements were compared with existing theoretical models, testing their validity with low-bandgap semiconductors and in the mid-IR. These comparisons show that modifications to these models are necessary, particularly to account for effects that become especially relevant in the mid-IR. Such modifications are presented and shown to be in reasonable agreement with the experimental data.;In addition to this, multi-pulse studies were performed in order to determine the nature of microstructure formation at mid-IR wavelengths. Laser induced periodic surface structures (LIPSS or ripples) were generated on Ge after irradiation with multiple mid-IR, femtosecond pulses at oblique incidence. Low spatial frequency LIPSS (LSFL) were observed for two types of laser polarization on the material surface. The quantitative and qualitative features of the LSFL were found to be consistent with a currently established model of their formation, after generalization to arbitrary laser polarization. Additionally, a new method is proposed to determine the propagation length of surface-bound electromagnetic waves. This is used along with the measured LSFL period to estimate the density of free carriers, and their collision frequency with phonons, within the laser-excited surface.;Finally, high spatial frequency LIPSS (HSFL) formation was studied across a wide range of wavelengths for multiple angles of incidence and two types of laser polarization. It is shown that their formation is consistent with a similar, but distinct mechanism as the LSFL. This mechanism is found to only give rise to HSFL formation at mid-IR wavelengths due to the transparency of Ge in this regime. Also observed are a novel type of HSFL, only present in the periphery of the damage spot with half the period of the ordinary HSFL. A new mechanism for their formation is proposed and shown to be consistent with the quantitative and qualitative features of these peripheral HSFL.
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