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 diamond-like nanocomposite (DLN) a-C:H:Si:O films with femtosecond laser pulses (wavelength λ=515 nm, pulse duration τ=320 fs, pulse repetition rate f=100 kHz) under different scanning conditions (single spots, linear structures). The pulse separation in the bursts is 25 ns (intra-burst frequency f=40 MHz) and the pulse number is varied from 1 to 8. The ablation depth and specific ablation rates (μm3/μJ) 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 amorphous film structure with the pulse number in the bursts (at 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 high
precision surface microstructuring of DLN films in the burst mode are presented.