Experimental study on 785 nm femtosecond laser ablation of sapphire in air

Abstract

Changes in surface morphology and ablation rate induced on sapphire were investigated after interaction with femtosecond laser pulses in air at variable fluence (2 to 77 J cm−2) and repetition rate (10 to 1000 Hz). Multiple laser pulses at a wavelength of 785 nm and pulse width of 130 fs were fired at the surface of sapphire to produce craters whose depth, size and morphology were evaluated using optical and scanning electron microscopy. Ablation rate was found to depend on laser fluence, number of laser pulses and repetition rate. A rapid increase in ablation rate with fluence was observed for fluences lower than 5.9 J cm−2, followed by a slow increase up to fluence of 40.7 J cm−2. A drop in ablation rate occurred at fluence greater than 40.7 J cm−2. Craters produced at high repetition rate (1000 Hz) at fluence of 11.8 J cm−2 were deeper than those produced at low repetition rate (10 Hz) during the first 40 to 50 pulses. The situation was reversed for craters produced by greater than 50 laser pulses. The drop in ablation rate observed at high fluence and repetition rate can be attributed to attenuation of the laser energy due to plasma and particle shielding that result from interactions with the laser-generated particles that cannot be completely removed from the ablated crater. Defocusing effects associated with the non-equilibrium ionization of air which causes a divergence to the laser beam and consequently a reduction in the laser intensity at the sample surface can be another reason for the observed drop in the ablation rate at high fluence.