Electron emission and ultrafast low-fluence plasma formation during single-shot femtosecond laser surface ablation of various materials

Emission of erosive plasma has been observed during electric probe and optical emission spectral measurements of plumes produced by single-shot femtosecond laser ablation of optical-quality surfaces of various materials—copper, titanium, and silicon—at laser fluences well below the corresponding thermal ablation thresholds, replacing presumably electron emission at lower fluences. The onset of erosive plasma correlates on the fluence scale with saturation of dependences of self-reflectivity of the pumping femtosecond laser pulses, reflecting the “freezing” of electron dynamics (variation of electron density or temperature) during the pumping pulses, despite the monotonically increasing laser fluences.