Ablation thresholds in ultrafast laser micro-machining of common metals in air

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/cm²), niobium (0.1460 J/cm²), titanium (0.1021 J/cm²) and copper (0.3529 J/cm²). The ablation depth per pulse was measured for laser pulse fluences ranging from the ablation threshold (of most metals) ∼ 0.1 J/cm² up to 10 J/cm². It has been shown previously that there are two different ablation regimes. 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, α^-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", 1. 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).