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In the current work ablation of metal targets in air with femtosecond laser pulses is studied. The laser pulses used for the study were 775 nm in wavelength, 150 fs in pulse duration and the repetition rate was 100 Hz. Ablation thresholds have been measured for a number of metals including stainless steel (0.1600 J/cm 2 ), niobium (0.1460 J/cm 2 ), titanium (0.1021 J/cm 2 ) and copper (0.3529 J/cm 2 ). The ablation depth per pulse was measured for laser pulse fluences ranging from the ablation threshold (of most metals) ~ 0.1 J/cm 2 up to 10 J/cm 2 . It has been shown previously that there are two different ablation regimes. 1 In both cases the ablation depth per pulse depends logarithmically on the laser fluence. Whilst operating in the first ablation regime the ablation rate is low and is dependant on the optical penetration depth, . -1 . While in the second ablation regime the ablation rate is greater and is characterized by the "electron heat diffusion length" or the "effective heat penetration depth", l. In the present study good qualitative agreement in the ablation curve trends was observed with the data of other authors, e.g. Nolte et al (1997). 1

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In this paper the interaction of ultra-short pulses (150fs) of laser radiation (wavelength 775nm) over a range of fluences (0.1 -10Jcm -2 ) with wafer grade Silicon material [100] in air was analysed using optical and electron microscopy. Optical microscopy was performed by the use of a white light interferometer and a high power optical microscope (magnification 100X). The resolution of both these methods was only sufficient to resolve large dimensions relative to the wavelength of light. For smaller geometries and greater detail, electron microscopy (resolution 1.5nm, 1KV) was used to obtain more information due to its greater resolution and depth of focus. When used in conjunction with surface, cross sectional and transmission imaging, this technique provided the greatest level of detail on the physical processes involved. Using these analysis techniques it was possible to provide a qualitative understanding of the ablation process as a function of laser fluence and to quantitatively describe the depth per pulse over a range of laser fluences, from which a value for the ablation threshold for Silicon (0.17Jcm -2 ) could be derived.