Heat accumulation effects in laser processing of diamond-like nanocomposite films with bursts of femtosecond pulses

In this paper, we have investigated the burst mode (BM) ablation and surface structuring of diamondlike nanocomposite (DLN) a-C:H:Si:O films with femtosecond laser pulses (wavelength lambda = 515 nm, pulse duration tau = 320 fs, and pulse repetition rate f = 100 kHz) under different scanning conditions (single spots and linear structures). The pulse separation in the bursts is 25 ns (intraburst frequency f = 40 MHz), and the pulse number is varied from 1 to 8. The ablation depth and specific ablation rates (mu m(3)/mu J) are found to be higher for the burst mode compared to single-pulse irradiation, increasing with the pulse number in the burst. The obtained experimental data of the higher ablation efficiency are shown to correlate with computer simulations of the BM ablation. In correlation with the ablation findings, Raman spectra of single spots and microgrooves have evidenced a growing graphitization of the amorphous film structure with the pulse number in the bursts (at an equal energy deposited into the films). Contact-mode atomic force microscopy (AFM) is applied to reveal an influence of the BM processing on the surface properties (nanoscale relief, friction) of laser-structured films. Based on the ablation and Raman data analysis, AFM examination of ablated/redeposited layers, and computer simulations of the burst mode ablation, the heat accumulation is identified as the main factor responsible for the enhanced ablation efficiency during the BM processing of DLN films. In addition, results of the high precision surface microstructuring of DLN films in the burst mode are presented.