Photoacoustic cavitation in spherical and cylindrical absorbers |
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Authors: | G Paltauf H Schmidt-Kloiber |
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Institution: | Institute of Experimental Physics, Karl-Franzens-Universit?t Graz, Universit?tsplatz 5, 8010 Graz, Austria (Fax: +43-316/380-9816, E-mail: guenther.paltauf@kfunigraz.ac.at), AT
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Abstract: | Photomechanical damage in absorbing regions or particles surrounded by a non-absorbing medium is investigated experimentally
and theoretically. The damage mechanism is based on the generation of thermoelastic pressure by absorption of pulsed laser
radiation under conditions of stress confinement. Principles of photoacoustic sound generation predict that the acoustic wave
generated in a finite-size absorbing region must contain both compressive and tensile stresses. Time-resolved imaging experiments
were performed to examine whether the tensile stress causes cavitation in absorbers of spherical or cylindrical shape. The
samples were absorbing water droplets and gelatin cylinders suspended in oil. They were irradiated with 6-ns-long pulses from
an optical parametric oscillator. Photoacoustic cavitation was observed near the center of the absorbers, even if the estimated
temperature caused by absorption of the laser pulse did not exceed the boiling point. The experimental findings are supported
by theoretical simulations that reveal strong tensile stress in the interior of the absorbers, near the center of symmetry.
Tensile stress amplitudes depend on the shape of the absorber, the laser pulse duration, and the ratio of absorber size to
optical absorption length. The photoacoustic damage mechanism has implications for the interaction of ns and sub-nslaser pulses
with pigmented structures in biological tissue.
Received: 9 October 1998 / Accepted: 5 January 1999 / Published online: 31 March 1999 |
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Keywords: | PACS: 42 62 Be 43 35 Ud 87 51 |
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