  Transparent biological tissues can be precisely dissected with ultrafast
lasers using optical breakdown in the tight focal zone. Typically, tissues are
cut by sequential application of pulses, each of which produces a single
cavitation bubble. We investigate the hydrodynamic interactions between
simultaneous cavitation bubbles originating from multiple laser foci.
Simultaneous expansion and collapse of cavitation bubbles can enhance the
cutting efficiency by increasing the resulting deformations in tissue, and the
associated rupture zone. An analytical model of the flow induced by the bubbles
is presented and experimentally verified. The threshold strain of the material
rupture is measured in a model tissue. Using the computational model and the
experimental value of the threshold strain one can compute the shape of the
rupture zone in tissue resulting from application of multiple bubbles. With the
threshold strain of 0.7 two simultaneous bubbles produce a continuous cut when
applied at the distance 1.35 times greater than that required in sequential
approach. Simultaneous focusing of the laser in multiple spots along the line
of intended cut can extend this ratio to 1.7. Counter-propagating jets forming
during collapse of two bubbles in materials with low viscosity can further
extend the cutting zone - up to a factor of 1.54.
