Mechanism study of skin tissue ablation by nanosecond laser pulses

Understanding the fundamental mechanisms in laser tissue ablation is essential to improve clinical laser applications by reducing collateral damage and laser pulse energy requirements. In this dissertation, skin tissue ablation by nanosecond laser pulses has been studied from near infrared to ultraviolet for a clear understanding of the mechanism that can be used to enhance future design of pulsed lasers for dermatology and plastic surgery. Multiple laser and optical configurations have been constructed to generate 9–12 ns laser pulses at 1064, 532, 266, and 213 nm. Through histology and optical transmission measurements of ablation depth as a function of laser pulse energy, the 589 nm spectral line in the secondary radiation from ablated skin tissue samples was identified as the signature of the occurrence of ablation. Subsequently, this spectral signature has been used to investigate the probabilistic process of the ablation near the threshold at the four wavelengths. In addition, optical breakdown and backscattering in water were investigated along with a nonlinear refraction index measurement using a z ‐scan technique. Preliminary studies on ablation of a gelatin based tissue phantom are also presented. The current theoretical models describing soft tissue ablation by short laser pulses were critically reviewed. A new plasma‐mediated model has subsequently been developed and a laser‐induced localized thermal ionization pathway has been investigated. It was found to have significant influence on the initial free electron density during plasma formation due to the combination of strong light absorption and subsequent confined temperature rise in chromophores. Good agreement has been found between the model and experimental results.