IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 2, NO. 4, DECEMBER 1996 801 Plasma Mediated Ablation of Biological Tissues with Nanosecond-to-Femtosecond Laser Pulses: Relative Role of Linear and Nonlinear Absorption Alexander A. Oraevsky, Luiz B. Da Silva, Member, IEEE, Alexander M. Rubenchik, Michael D. Feit, M. E. Glinsky, Michael D. Perry, Beth M. Mammini, Ward Small, IV, and Brent C. Stuart Abstract— Plasma mediated ablation of collagen gels and porcine cornea was studied at various laser pulse durations in the range of 1 ns–300 fs at 1053-nm wavelength. It was found that pulsed laser ablation of transparent and weakly absorbing gels is always mediated by plasma. On the other hand, ablation of strongly absorbing tissues is mediated by plasma in the ultrashort-pulse range only. Ablation threshold along with plasma optical breakdown threshold decreases with increasing tissue absorbance for subnanosecond pulses. In contrast, the ablation threshold was found to be practically independent of tissue linear absorption for femtosecond laser pulses. The mechanism of optical breakdown at the tissue surface was theoretically investigated. In the nanosecond range of laser pulse duration, optical breakdown proceeds via avalanche ionization initiated by heating of electrons contributed by strongly absorbing impurities at the tissue surface. In the ultrashort- pulse range, optical breakdown is initiated by multiphoton ionization of the irradiated medium (six photons in case of tissue irradiated at 1053-nm wavelength), and is less sensitive to linear absorption. High-quality ablation craters with no thermal or mechanical damage to surrounding material were obtained with subpicosecond laser pulses. Experimental results suggest that subpicosecond plasma mediated ablation can be employed as a tool for precise laser microsurgery of various tissues. I. INTRODUCTION P LASMA optical breakdown is the only mechanism of laser energy deposition in transparent and low absorbing tissues. Therefore, plasma mediated ablation was studied by a number of groups working in the area of laser ophthalmology [1]–[8]. It has been found that picosecond and subpicosecond ablation produces lower collateral damage to adjacent tissues than nanosecond ablation. However, the mechanisms of laser- tissue interactions have not been thoroughly studied especially for the femtosecond pulse domain. Optical breakdown remains incompletely understood for water containing media including biological tissues [7]–[9]. For nanosecond pulse durations, optical breakdown threshold is determined by laser heating Manuscript received October 17, 1996; revised December 19, 1996. This work was supported by the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract W-7405-ENG-48, and was supported in part by The Whitaker Foundation. A. A. Oraevsky is with the Department of Electrical and Computer Engineering, Rice University, Houston, TX 77005 USA. L. B. Da Silva, A. M. Rubenchik, M. D. Feit, M. E. Glinsky, M. D. Perry, B. M. Mammini, W. Small, IV, and B. C. Stuart are with the University of California, Lawrence Livermore National Laboratory, Livermore, CA 94550 USA. Publisher Item Identifier S 1077-260X(96)09631-1. of absorbing impurities at the surface of irradiated medium and associated thermal losses. In the picosecond pulse dura- tion range, the additional mechanism of nonlinear absorption and subsequent multiphoton ionization of irradiated material begins to influence the ablation process. Our previous studies of plasma mediated damage of transparent dielectrics and bio- logical tissues suggested that further decrease in pulse duration leads to optical breakdown solely due to multiphoton ioniza- tion [10], [11]. This paper presents quantitative experimental and theoretical studies of ablation parameters as functions of pulse duration in the range of 1 ns–300 fs. Our intention was to observe the difference in tissue response due to the difference in optical breakdown mechanisms while irradiating with variable pulse duration and fluence [11]. A first-principles theory of optical breakdown at the surface of biological tissue is not yet developed. Therefore, the emphasis of our study was put on the theoretical description of optical breakdown, light penetration depth as a function of pulse duration, and the experimental quantification of ablation parameters, such as the threshold laser fluence, [J/cm ], the crater drilling rate, i.e., the crater depth per pulse with a unit fluence, [ m J cm ], the ablation efficiency, i.e., mass removed per unit of absorbed energy ( [ g mJ]), and the recoil stress amplitude ( [bar]) and the shock gradient, [bar/ m]. We have also qualitatively described ablation crater quality and collateral damage in gels with the aid of optical microscopy for nanosecond and femtosecond pulsed ablation. II. MATERIALS AND METHODS A. Collagen Gels and Porcine Corneas Collagen gels were made of 10-g gelatin G2625 (Sigma, St. Louis, MO) mixed with 100 cm hot distilled water at 70 C and then cooled down to room temperature. Four different gels were prepared for experiments: 1) clear gel with low absorption coefficient 0.1 cm defined by water absorbance at 1053 nm; 2) cupric sulfate colored gel with moderate absorbance 22 cm ; 3) turbid gel colored with cupric sulfate 22 cm with added polystyrene microspheres as a scatterer (effective light at- tenuation coefficient, 76 cm ; and 4) a gel with high absorbance 1000 cm colored with carbon 1077–260X/96$5.00  1996 IEEE