Novel laser-induced cavitation: the constrained ring bubble

Using high speed photography at sub-microsecond temporal resolution, we observed laser-induced cavitation within a thin film of liquid. In accordance with the literature, focusing a pulse of a gaussian-like intensity distribution into the liquid, instigated a disk-shaped cavity. The propagation of an acoustic transient, generated during the formation and expansion of the cavity, is evidenced by secondary cavitation stimulated in the surrounding liquid. Introducing a laguerre-gauss holographic diffractive optic element into the beam path, re-distributes the optical energy into the so-called 'doughnut mode', with an axial intensity minimum. Focusing this modulated pulse into the liquid induced a ring-shaped cavity with a notably different dynamic to that of the disk cavity, due primarily to the encapsulated droplet, which is present from cavity initiation through expansion and subsequent deflation. In this paper we present initial observations on the novel dynamics of the ring-shaped cavity and discuss several of the distinctive features. Particularly the secondary cavitation induced in the surrounding liquid, and the implicated multiplexing of the acoustic transient generated during the ring-cavity expansion, is of interest.