Characterization of Femtosecond Laser-induced Structural Changes in CVD Diamond by Raman Spectroscopy

Multiple pulses from a femtosecond laser were irradiated onto a diamond grown by chemical vapor deposition, and the dependence of the structural changes of the affected layer on laser parameters was investigated by Raman spectroscopy. A graphite peak was observed in the Raman spectra. The thickness of the graphitized layer was estimated considering the absorption coefficient of graphite. Based on the intensity decrease of diamond peaks, which was the most pronounced change, the dependence of the graphitized layer thickness on the fluence and number of pulses was investigated. As the fluence and/or accumulated number of pulses increased, the thickness of the graphite layer increased. At an accumulated number of pulses of 250 and 2000, the threshold fluence of graphitization was similar to that of laser-induced periodic surface structure formation and smaller than that of ablation. At a fluence below the ablation threshold and a sufficiently large number of pulses, the thickness of the graphite layer remained constant, as determined by the fluence.