Initiation of Femtosecond Laser Machined Ripples in Steel Observed by Scanning Helium Ion Microscopy (SHIM)

The physics of ablation has been studied experimentally with ultrashort laser pulses of 210 fs with a wavelength of 800 nm. A specimen of highly alloyed steel (alloy 800H) has been used. After the laser ablation experiments the surface has been inspected using SEM, AFM and scanning helium ion microscopy (SHIM). At the lowest fluence spherical bubbles are observed consisting of a mixture of gas and liquid having diameters in the range 20-90 nm. With increasing fluence the chain of events that has been observed consists of bubble forming and initiation of pre-ripples followed by a transformation towards regular course ripples with an orientation perpendicular to laser polarization direction. The pre-ripples have a different orientation that is not understood yet.