TRANSFORMATION OF CIRCULAR INTO TRIANGULAR CRATERS AS A FUNCTION OF PARAMETERS IN A TRAIN OF LASER-PULSES

In the pulsed laser-material interaction with the pulse-repetition rat e, nu, in the KHz range, and the scan velocity v approximate to 25.4-1 27 mm/s (1-5 ''/s), the morphology of the laser trace changes drastica lly. The trace morphology arises from few simultaneous processes: mate rial vaporization and crater formation with surface hydrodynamic flow. Depending on nu and v, the crater form changes from circular to trian gular, passing through a generation of coherent hydrodynamic and chaot ic structures finishing with pairing or merging of three vortices to f orm a triangular one; the laser trace changes from the discontinuous t o continuous. The phenomena observed are important from the theoretica l aspect for the study of interaction of 2, 3 or more vortices on a sh ort time scale, and the applied aspect is important for deep laser scr ibing and surface modification uses. Copyright (C) 1996 Elsevier Scien ce Ltd.