Cavitation dynamics and directional microbubble ejection induced by intense femtosecond laser pulses in liquids

We study cavitation dynamics when focusing ring-shaped femtosecond laser beams in water. This focusing geometry reduces detrimental nonlinear beam distortions and enhances energy deposition within the medium, localized at the focal spot. We observe remarkable postcollapse dynamics of elongated cavitation bubbles with high-speed ejection of microbubbles out of the laser focal region. Bubbles are ejected along the laser axis in both directions (away and towards the laser). The initial shape of the cavitation bubble is also seen to either enhance or completely suppress jet formation during collapse. In the absence of jetting, microbubble ejection occurs orthogonal to the laser propagation axis.

Figure 1

Experimental layout. The input Gaussian beam is transformed into a ring-shaped beam in reflection from the $W$ axicon. The ring is focused into a cuvette filled with water and the laser-pulse induced cavitation bubble is monitored using a stroboscopic LED that illuminates the bubble whose image is magnified and collected on a CCD camera.

Figure 2

Photograph of the water-filled cuvette. A central bright spot indicates the laser focus, $F$ (the laser beam is propagating from left to right). Bubbles are seen to be ejected over $\sim 1$-cm distances along the laser beam propagation axis, in both the forward and backward directions.

Figure 3

Evolution of the bubble position. Time zero corresponds to the instant in which the laser pulse is focused in the medium. (a) The dark region between time 0 and $21\mu$s, which is the main cavitating bubble that shrinks and then, after a slight rebound, breaks up into two clusters of microbubbles whose trajectories are highlighted by dashed lines. (b) The microbubble average ejection velocity, as given by the gradient of the bubble positions shown in (a).

Figure 4

Measured bubble evolution (a) for a slightly asymmetric initial bubble and (b) for a symmetric bubble. Arrows indicate the bubble cluster ejection directions in the two cases. The times in each figure indicate the time lapsed after the laser-pulse passage, defined to be time zero.

Figure 5

(a) Time evolution of the bubble longitudinal (Dz) and transverse (Dx) dimensions. (b and c) Evolution (b) of the bubble wall velocities and (c) of the bubble volume for input pulse energies indicated in the figure.

Figure 6

Details of jet evolution at the moment of impact with the colliding opposite bubble wall. Input energy is $100\mu$J. The arrow indicates the position of the jet tip.