Photoacoustic cavitation in spherical and cylindrical absorbers

Photomechanical damage in absorbing regions or particles surrounded by a no n-absorbing medium is investigated experimentally and theoretically. The da mage mechanism is based on the generation of thermoelastic pressure by abso rption of pulsed laser radiation under conditions of stress confinement. Pr inciples of photoacoustic sound generation predict that the acoustic wave g enerated in a finite-size absorbing region must contain both compressive an d tensile stresses. Time-resolved imaging experiments were performed to exa mine whether the tensile stress causes cavitation in absorbers of spherical or cylindrical shape. The samples were absorbing water droplets and gelati n cylinders suspended in oil. They were irradiated with 6-ns-long pulses fr om 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 expe rimental findings are supported by theoretical simulations that reveal stro ng tensile stress in the interior of the absorbers, near the center of symm etry. Tensile stress amplitudes depend on the shape of the absorber, the la ser pulse duration, and the ratio of absorber size to optical absorption le ngth. The photoacoustic damage mechanism has implications for the interacti on of ns and sub-ns laser pulses with pigmented structures in biological ti ssue.