a – Department of Engineering, University of Liverpool, Brownlow Hill, Liverpool L69 3GH, UK
b – Center for Manufacturing Engineering, Tennessee Technological University, 115 West Tenth Street, Cookesville, TN 38505, USA
c – Department of Engineering, Institute For Manufacturing, University of Cambridge, Mill Lane, Cambridge CB2 11RX, UK

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 cm2 and the volume ablation rate was measured to be 30 < V < 450 mm3 per pulse in the fluence range 1:4 < F < 21 J cm2. The presence of helium avoids gas breakdown above the substrate and leads to improved surface micro-structure by minimising surface oxidation and debris re-deposition. At 1 kHz rep. rate, with fluence F > 7 J cm2 and >85 W cm2 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 cm2 with low power density,
3 W cm2 produces much superior surface micro-structuring with minimum melt and measured surface roughness Ra
1:1  0:1 mm at a depth D
50 mm. 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 cm2 combined with single pulse fluence <4 J cm2 produces near optimum microstructuring. The debris under these conditions contains pure aluminium nanoparticles carried with the helium stream. # 2004 Elsevier B.V. All rights reserved.