abstract

• Nanosecond long laser pulses are used in medical applications where precise tissue ablation with minimal thermal and mechanical collateral damage is required. When a laser pulse is incident on a material, optical energy will be absorbed by a combination of linear and nonlinear absorption according to both: laser light irradiance and material properties. In the case of water or gels, the first results in heat generation and thermoelastic expansion; while the second results in an expanding plasma formation that launches a shock wave and a cavitation/boiling bubble. Plasma formation due to nonlinear absorption of nanosecond laser pulses is originated by a combination of multiphoton ionization and thermionic emission of free electrons, which is enhanced when the material has high linear absorption coefficient. In this work, we present three experimental approaches to study pressure transients originated when 6 ns laser pulses are incident on agar gels and water with varying linear absorption coefficient, using laser radiant exposures above and below threshold for bubble formation: (a) PVDF sensors, (b) Time-resolved shadowgraphy and (c) Time-resolved interferometry. The underlying hypothesis is that pressure transients are composed of the superposition of both: shock wave originated by hot expanding plasma resulting from nonlinear absorption of optical energy and, thermoelastic expansion originated by heat generation due to linear absorption of optical energy. The objective of this study is to carry out a comprehensive experimental analysis of the mechanical effects that result when tissue models are irradiated with nanosecond laser pulses to elucidate the relative contribution of linear and nonlinear absorption to bubble formation. Furthermore, we investigate cavitation bubble formation with temperature increments as low as 3°C. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

authors

• Aguilar G.
• Camacho-López S.
• Pérez Gutiérrez Francisco Gerardo

publication date

• 2011-01-01

published in

• Progress in Biomedical Optics and Imaging - Proceedings of SPIE  Proceedings

keywords

• Agar gel; Artificial tissues; Cavitation bubble; Collateral damage; Expanding plasmas; Experimental analysis; Experimental approaches; Experimental studies; Free electron; Laser lights; Linear absorption; Linear absorption coefficient; Material property; Mechanical effects; Mechanical response; Multiphoton ionization; Nanosecond laser pulse; Nonlinear absorptions; Optical energy; Plasma formations; Pressure transient; PVDF sensor; Radiant exposure; Relative contribution; Shadowgraphy; Temperature increment; Thermoelastic expansion; Time-resolved; Time-resolved interferometry; Tissue ablations; Tissue models; Bubble formation; Cavitation; Gels; Incident solar radiation; Laser applications; Laser pulses; Laser tissue interaction; Medical applications; Multiphoton processes; Photoionization; Plasma diagnostics; Plasma simulation; Shock waves; Thermionic emission; Thermoelasticity; Tissue; Transients; Water absorption; Neodymium lasers

Digital Object Identifier (DOI)

• 10.1117/12.889686

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volume

• 8092