Laser scribing of thin dielectrics with polarised ultrashort pulses

• Observation of damage regions occurring on rear surface of dielectric substrates.
• Demonstration of dependence of damage on laser polarisation state.
• Demonstration of improvement in ablation rate accompanying reduction in damage.
• Model indicating increase in ablation due to polarisation dependent reflections.

Laser processing of thin glass has proven problematic due to the inefficient coupling of optical energy into glass and difficulty achieving economical processing speed while maintaining cut quality. Laser glass processing is pertinent to touch screen display, microfluidic, microoptic and photovoltaic applications. The results of the laser scribing of 110 μm thick alkali free glass with ultrashort polarised laser radiation are presented. A novel ex-situ characterisation procedure is reported which enables effective characterisation of the scribe depth, width, shape and the morphology of the laser material interaction zone. A study of glass cross sections scribed with laser polarisation oriented parallel to the plane of incidence (P polarised) showed damage regions extending away from the trench walls and correlated damage on the rear surface; these regions are indicative of damage caused by light transmission. The damage was significantly reduced by altering the polarisation from parallel to be perpendicular to the plane of incidence (S polarised) due to the increased reflectance from the trench walls. The processing speed was also impacted by the laser polarisation; it was found that S polarised light required less passes to fully ablate through the glass substrate. A processing window capturing the peak of the polarisation effect was identified. An optical model was developed to predict the effect of polarisation on the intensity distribution reaching the rear surface of the glass. S polarised light increases the reflectance resulting in a waveguiding effect which confines a larger amount of the light in the trench. Consequently we see an increased fluence incident on the central region of the trench.