Morphologies of caustics and dislocation lines: some clues about their interrelation.

According to singular optics, the phase and intensity that characterize structured electromagnetic beams can be understood in terms of concepts that involve subspaces where they or their derivatives exhibit a particular behavior, such as giving rise to extreme values or not being well defined. Caustics are a paradigmatic example of the former, while helical dislocation lines exemplify the latter. In this work the interrelation of the morphology of caustics and the morphology of dislocation lines is theoretically studied. The analysis for highly structured beams requires an efficient methodology that allows the identification of optical vortices, their topological charge, and the helical dislocation lines they belong to. Such a methodology is introduced and applied to paraxial elliptic umbilic beams and nonparaxial Airy symmetric three-dimensional beams. Nonparaxial beams exhibit caustic surfaces that delimit regions with a finite volume and different intensity average. It is shown that in the high intensity region so defined, the dislocation lines play the role of an internal skeleton, i.e., an endoskeleton, of the beam. The exoskeleton created in the low intensity regions shows subtle and interesting features that complement those of the endoskeleton; the caustics that delimit low intensity regions have a strong influence on the morphology of the exoskeleton.

Single-bubble and multibubble cavitation in water triggered by laser-driven focusing shock waves.

In this study a single laser pulse spatially shaped into a ring is focused into a thin water layer, creating an annular cavitation bubble and cylindrical shock waves: an outer shock that diverges away from the excitation laser ring and an inner shock that focuses towards the center. A few nanoseconds after the converging shock reaches the focus and diverges away from the center, a single bubble nucleates at the center. The inner diverging shock then reaches the surface of the annular laser-induced bubble and reflects at the boundary, initiating nucleation of a tertiary bubble cloud. In the present experiments, we have performed time-resolved imaging of shock propagation and bubble wall motion. Our experimental observations of single-bubble cavitation and collapse and appearance of ring-shaped bubble clouds are consistent with our numerical simulations that solve a one-dimensional Euler equation in cylindrical coordinates. The numerical results agree qualitatively with the experimental observations of the appearance and growth of large bubble clouds at the smallest laser excitation rings. Our technique of shock-driven bubble cavitation opens interesting perspectives for the investigation of shock-induced single-bubble or multibubble cavitation phenomena in thin liquids.

On the general properties of symmetric incomplete Airy beams.

We study the general properties of a class of Airy beams symmetric under reflection of the transverse coordinates. Following a recent proposal, their angular spectra depend on the absolute value of the third power of the transverse components of the wave vector. The proposed beams are shown to be described by symmetric superpositions of incomplete Airy special functions. Their angular spectra do not correspond to any of those described by standard catastrophe optics. However, the morphologies of the symmetric beams are similar to some of those already classified within that scheme, differing mainly on the scaling exponents. Finally, the structural stability of three-dimensional symmetric incomplete Airy beams is experimentally probed.

Manipulation and microrheology of carbon nanotubes with laser-induced cavitation bubbles.

Multiwalled carbon nanotubes (MWCNT) are exposed to a transient and strong liquid jet flow created by a pair of differently sized laser-induced cavitation bubbles. The position and size of the bubbles are controlled with a spatial light modulator within a 15 microm thick liquid gap. Depending on the tube's position with respect to this jet flow, rotation, translation, and a bending deformation is observed with a high-speed camera recording at up to 300,000 frames per second. By measuring the decay time of the respective bending modes we determine the flexural rigidity of MWCNTs to be on the range of 0.45-4.06x10(-19) N m2. The average diameter of the MWCNTs is 117.8+/-6.7 nm with a thickness of 4.6+/-0.75 nm, yielding a Young's modulus between 0.033-0.292 TPa.

Generation of laser-induced cavitation bubbles with a digital hologram.

We demonstrate a method using a spatial light modulator (SLM) to generate arbitrary 2-D spatial configurations of laser induced cavitation bubbles. The SLM acts as a phase hologram that controls the light distribution in the focal plane of a microscope objective. We generate cavitation bubbles over an area of 380 x 380 microm(2) with a 20x microscope objective through absorption of the pulsed laser light in a liquid ink solution. We demonstrate the ability to accurately position up to 34 micrometer sized bubbles using laser energies of 56 microJ.