Analysis of USP Laser Induced Ablation Threshold in Transparent Aqueous Tissue

The ultrashort pulsed (USP) laser induced plasma-mediated ablation in transparent media is modeled and studied in this work. We propose that a certain number of free electrons are required to trigger the avalanche ionization for the first time. Based on this assumption, the ablation process is postulated as two separate processes — the multiphoton and avalanche ionizations. For USP laser induced ablation in the transparent corneal epithelium at 800 nm, the critical seed free-electron density and the time to initialize the avalanche ionization for pulse widths from picoseconds down to the femtoseconds range are calculated. It is found that the critical seed free-electron density decreases as the pulse width increases, obeying a t_p^−5.65 rule. Moreover, this model is also extended to the estimation of crater sizes in USP laser ablation of polydimethylsiloxane (PDMS). The crater sizes ablated in a PDMS by a 900 fs pulsed laser at wavelength 1552 nm are modeled using the present model, and the results match with the existing experimental measurements.