Abstract

The ablation mechanism is explained by considering the absorption characteristic and the band-gap energy of ceramics in which the incident laser radiation induces the transition across the band gap, resulting in the production of electron-hole pairs and the generation of the appreciable heat necessary for electronic excitation. As in the case of the ablation of ceramics with IR lasers, viz. Nd:YAG and CO₂ lasers, the photon energy is smaller than the characteristic band-gap energy E_g (i.e., hη < E_g), and with sufficient coherence of the irradiating beam, absorption in the laser-induced plasma is mainly governed by the inverse Bremsstrahlung. However, for UV lasers the photon energy lies in the band-gap energy, hη = E_g and therefore in addition to being governed by the inverse Bremsstrahlung, the absorption mechanism is governed by multiphotons and resonance absorption. As shown in the Fig. 1, the band-gap energy of typical ceramic materials is within the range of the photon energies of various excimer lasers, so that ablation with minimal thermal effects would be possible provided that the short-duration photon energy matches the band-gap energy of the material. However, in most cases superfluous energy is radiated; therefore, quasi-thermal ablation effects may also occur. If the photon energy is less than the band-gap energy, as in the case of wide-band-gap materials, photon processes of two or more steps become necessary.

© 1995 Optical Society of America