Laser processing of sapphire with picosecond and sub-picosecond pulses

Laser processing of sapphire using a Ti:sapphire laser at 790 and 395 nm and pulse widths varying between 0.2 and 5 ps is reported. A clear improvement in quality is demonstrated for multi-shot processing with sub-ps laser pulses. For fluences between 3 and 12 Jrcm2 two ablation phases were observed, in agreement with previous work from Tam et al. using 30 ps, 266 nm laser pulses wA.C. Tam, J.L. Brand, D.C. Cheng, W. Zapka, Appl. Phys. Lett. 55 20. 1994. 2045x. During the ‘gentle ablation’ phase periodic wavelike structures, i.e. ripples, were observed on the Al2O3 surface, perpendicular to the laser polarisation and with a spacing almost equalling the laser wavelength, indicating metallic-like behaviour. The ripple modulation depth was in the order of a few tens of nm. For fluences between 1 and 2.5 Jrcm2, below the single-shot surface damage threshold and at a pulse width above 200 fs, microstructures could be produced at the rear side of a 1 mm thick sapphire substrate without affecting the front surface. q1997 Elsevier Science B.V.