Femtosecond laser ablation properties of borosilicate glass

We study the femtosecond laser ablation properties of borosilicate glass using atomic force microscopy and laser pulses of 200 fs duration, centered at 780 nm wavelength. We show that both single-shot and multishot ablation threshold fluences can be determined by studying the diameter and the depth of single-shot ablated craters. The linear relationship between the square of the crater diameter and the logarithm of the laser fluence in the form of D-2=2w(0)(2)ln(F-0/F-th(N=1)) provides the single-shot ablation threshold, F-th(N=1), whereas the linear relationship between the ablation depth and the logarithm of laser fluence in the form of h(a)=alpha(eff)(-1)ln(F-0/F-th(N>1)) provides the multishot ablation threshold, F-th(N>1). The results depict a multishot ablation threshold of approximate to1.7 J/cm(2) independent of the atmospheric conditions. The slopes of the linear fits also provide a precise estimate of the beam radius at the surface, w(0)approximate to5.9 mum, and the "effective optical penetration depth," alpha(eff)(-1)approximate to238 nm in air. The method is systematic, provides results that are consistent with the literature, and eliminates uncertainties because of instrument sensitivities. We also show that threshold measurement based on the extrapolation of volume to zero, a method used often in previous studies, is somewhat questionable. Finally, the measured dimensions of ablated craters reveal that the ablation volume per unit input energy is about 1.3-1.5 mum(3)/muJ at an intermediate fluence regime of 10<F-0(av)<40 J/cm(2). This value represents an order of magnitude larger ablation efficiency when compared to the ablation of glass with nanosecond ultraviolet laser pulses. (C) 2004 American Institute of Physics.