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 J/cm ² two ablation phases were observed, in agreement with previous work from Tam et al. using 30 ps, 266 nm laser pulses [A.C. Tam, J.L. Brand, D.C. Cheng, W. Zapka, Appl. Phys. Lett. 55 (20) (1994) 2045]. During the 'gentle ablation' phase periodic wavelike structures, i.e. ripples, were observed on the Al ₂ O ₃ 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 J/cm ² , 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.