G. Toker, V. Bulatov, T. Kovalchuk, and I. Schechter (Israel)
1064 nm, laser breakdown, ethanol, optical imaging
Study of processes accompanying optical breakdowns in tap water and ethanol may lead to better understanding laser methods applicable in Biomedical Engineering. Structure and dynamics of the breakdown in tap water and ethanol at λ=1064 nm were investigated by applying a high spatial resolution shadow, Schlieren and Mach Zehnder interference diagnostic techniques. It was found that a laser spark column has essentially discrete character in virtue of the breakdown mechanism on inclusions: it consists of cavities over inclusion’s particles and associated shock waves. The comparable analysis of the pictures of breakdown in water and ethanol was performed. Image processing of interferograms allows determining dynamics and evolution of heated region of the liquids in the focal volume of a singlet. In tap water it deactivated by thermal conductivity; in ethanol by generating a cylindrical shock wave. Despite the fact that ethanol is a weakly absorbing liquid at 1064 nm it effectively heated in the focal volume of a focusing singlet. Heated in axial direction the alcohol generates cylindrical shock wave in radial direction, which creates positive changes of the index of refraction on its front due to the shock compression of the liquid. The positive changes of the refraction index are favorable for supporting the effect of self-focusing and canalization of the heating laser radiation. In experiments effect of bleaching the NIR laser radiation accompanies the self focusing in ethanol is also observed.
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