Ultra-short-pulse laser irradiation and ablation of dielectrics

Increasing availability and reliability of ultra-short pulse laser systems is opening up the possibility of using such lasers for ablation and structuring of dielectrics and other materials (e.g. metals) which cannot be structured with high micro-metre precision using standard ns laser systems. In this chapter the advantage, of visible (800 nm) ultra-short laser pulses for micro-structuring of high band gap materials is discussed. Different phenomena are observed, related to three fluence regimes, (i) Fluence damage threshold fluence (Fth). In this regime it is possible to produce long channels with a high aspect ratio and little residual damage and stress in the material, (ii) Fluence = Fth. Here, two distinct ablation phases can be observed. A gentle and a strong ablation phase. The gentle phase leads to controlled melting and vaporisation of material, sometimes accompanied by ripple formation on the surface. At higher laser intensities (or after a sufficient number of incubation pulses ) the strong ablation phase is observed which we relate to phase explosion, (iii) Fluence < Fth. Self-focusing of the laser light due to the Kerr effect can occur in the dielectric materials for ps pulses. This leads to the possibility of direct writing of micro-structures in the material bulk without producing damage on the entrance or exit surfaces. Manipulating the laser pulse width and/or pulse energy can control the position of the structures caused in the bulk. Similar non-linear optical effects are responsible for material removal from the exit surface of the material to leave extremely smooth well-defined micro-metre sized cone-shaped holes.