Femtosecond laser micro-structuring of aluminium under helium

The interaction of 180 fs, 775 nm laser pulses with aluminium under a flowing stream of helium at ambient pressure have been used to study the material re-deposition, ablation rate and residual surface roughness. Threshold fluence Fth∼0.4 J cm−2 and the volume ablation rate was measured to be 307 J cm−2 and >85 W cm−2 average power density, residual thermal effects result in melt and debris formation producing poor surface micro-structure. On the contrary, surface micro-machining at low fluence F∼1.4 J cm−2 with low power density, ∼3 W cm−2 produces much superior surface micro-structuring with minimum melt and measured surface roughness Ra∼1.1±0.1 μm at a depth D∼50 μm. By varying the combination of fluence/scan speed during ultra-fast ablation of aluminium at 1 kHz rep. rate, results suggest that maintaining average scanned power density to <5 W cm−2 combined with single pulse fluence <4 J cm−2 produces near optimum micro-structuring. The debris under these conditions contains pure aluminium nanoparticles carried with the helium stream.