The influence of the ionization regime on femtosecond laser beam machining mono-crystalline diamond

Abstract

Diamond is widely used in a series of industrial and scientific applications, including the micro-electro-mechanical systems, optical elements, ultra-precision machining tools, etc. Femtosecond laser beam machining is promising for high precision and high accuracy machining diamond material for its famous non-thermal machining regime. Initially, the laser ablation threshold was determined for the low and high laser fluence stage, respectively. Then, the ionization regime of femtosecond pulsed laser beam machining mono-crystalline diamond with different laser power was discussed. The influence of the ionization regime on laser machining also was analyzed basing on Raman analysis and laser machined surface topography. The laser ablation threshold was 0.29 J/cm2 for the low laser fluence stage and 2.35 J/cm2 for the high laser fluence stage, respectively. The calculation of the Keldysh parameter indicated that the multi-photon ionization prevailed in the low laser fluence stage while the tunneling ionization became dominant in the high laser fluence stage. The transition of the ionization regime from the multi-photon ionization to the tunneling ionization occurred around the laser fluence 3.2 J/cm2. Graphitization of diamond material was discovered on laser machined diamond surface by means of Raman analysis. Graphitization would be enhanced by the tunneling ionization regime in the high laser fluence stage. A strong surface cracking tendency was also observed in laser machining in the high fluence stage.