As ultrafast laser technology advances, it is of importance to evaluate the potential of sub-100-fs laser pulses for laser surgery. We have extended the investigation of laser- induced optical breakdown on hard and soft tissues down to laser pulse widths of 20 fs. Powerful 20-fs to 100-ps pulses from a single Ti:sapphire oscillator/amplifier laser source at 800 nm were focused in vitro onto the surface of fresh human corneas and human enamel to a spot of 60 - 70 micron in diameter. The threshold for ablation was determined by increasing the pulse energy while monitoring scattered probe light at ejected ablation particles. Our experiments show a slower decrease of the threshold fluence in dependence of the pulse width in the femtosecond regime than in the picosecond regime. Unlike previously suggested, no saturation behavior could be observed at the shortest available pulse widths. For the shortest pulses with 20 fs width, we measured a threshold of 0.38 J/cm<SUP>2</SUP> and 0.42 J/cm<SUP>2</SUP> for cornea and enamel, respectively. For the longest pulses at 100 ps, the threshold fluence was 4.3 J/cm<SUP>2</SUP> and 2.06 J/cm<SUP>2</SUP>, respectively. Comparison to theoretical models and to previous data determines the contribution of multi-photon and avalanche processes. Our results suggest an optimum laser pulse width of several hundred femtoseconds for most applications in ultrashort pulse laser surgery.
We evaluated in vivo wound healing responses to plasma- mediated ablation in skin as a function of laser pulsewidth and energy. Experiments utilized a regeneratively amplified Ti:Sapphire laser operating at 800 nm with pulsewidths varied from 7 ns to 100 fs. Skin incisions were created in mice by tightly focusing the laser beam on the tissue surface. Incisions of equal depth were compared at time points ranging from 6 hours to 3 weeks using standard histologic methods. Incision depth was proportional to pulse energy at each pulsewidth. Fluence threshold dependence on laser pulsewidth agreed with those predicted by ex vivo testing. Histologic analysis revealed minimal adjacent tissue damage at pulsewidths less than a few picoseconds and energies near the fluence threshold. Longer pulsewidths and higher fluence levels were associated with more significant collateral effects. These in vivo results suggest collateral tissue damage and secondary effects may be minimized by controlling laser pulsewidth and energy.