Figure 1.
Schematic depiction of the experimental setup of the laser path (red) and the illumination path (orange).
Single pulses of the fs-laser are selected by an acousto-optic modulator (AOM), half-wave plate and polarizing beam-splitter cube allow for laser power adjustment. Subsequent laser pulses are spatially separated via polygon scanner and a Keplerian telescope imaging (see also magnified image detail). The focal region inside the sample medium-filled cuvette is illuminated homogeneously by Koehler illumination and a magnified image of the cavitation bubble is reproduced on the chip of the CCD camera.
Figure 2.
Schematic depiction of the adjustment of subsequent generated cavitation bubbles.
The first bubble is induced on the left side at a time defined as t1 = 0.0 µs. The second cavity is generated at t2 after a constant delay of 10.0 µs. The bubble size is characterized as its radius RCav while the distance between the focal spots amounts to Δr.
Figure 3.
Cavitation bubble dynamics of different observable interaction mechanisms.
The first cavitation bubble occurs at about 0.0 µs for every image series. Its single bubble dynamics is shown in two more frames at 5.0 µs and 9.0 µs. The second cavity with defined temporal and spatial separation appears at 10.0 µs next to the first one. Afterwards the dynamics of the cavitation bubble interaction is shown at selected points in time. A more detailed depiction with equidistant time steps can be seen in S2 Figure. Especially, the jet formation of interaction mechanism 7 is shown in Fig. 4 for the whole duration of oscillation and for the total jet length.
Figure 4.
Detailed bubble dynamics of two cavities in the observable interaction mechanism 7.
The parameters to observe mechanism 7 were here a focus separation of Δr = 71.8 µm and a laser pulse energy of Epulse = 10.7-times Eth. The image series begins with the occurrence of the second cavity at 10 µs. Afterwards, the dominating jet formation in laser scanning direction is shown with the overall jet length by composing two images covering different imaging regions within the cuvette at the same time delay. A more detailed time evolution of the effects and the whole dynamics in equidistant time steps is shown in the S3 Figure).
Figure 5.
Characteristic interaction effects of two spatially and temporally separated laser pulses and their cavitation bubbles.
The depicted interaction effects of two spatially as well as temporally separated laser pulses and their cavitation bubbles are clearly represented in this depiction. The characteristic effect is specified on the left side and shown as an example snapshot on the right. Effect E is shown in form of two different snapshots to reveal the diversity of jet characteristics and strength, respectively.
Figure 6.
Observable interaction mechanisms of two spatially and temporally separated laser pulses and their cavitation bubbles.
(a) Overview of occurring interaction effects A to J as part of the observable mechanisms 1 to 11. The depiction shows, which effects are combined as a superposition within each of the experimentally observable interaction mechanisms. (b) Bar diagram of observable interaction mechanism depending on the stepwise adjusted distance between the two foci Δr and the continuously varied applied pulse energy. Vertical lines mark the measured limit between two significantly different mechanisms. The error bars show the standard deviation resulting from the experimental measured pulse energy over 50 pulses as well as energy losses due to a vignetting of the scanning setup. The selected colors differentiate between weak interaction mechanisms (blue), strong interaction mechanisms within the scanning plane (green) and suppressed or axially medium-affecting interaction mechanisms (red). The dashed horizontal line denotes the applied pulse energy, above which the resulting cavitation bubble had a lifetime>10 µs.
Figure 7.
Color-coded map of the different interaction mechanisms in the effective parameter space of pulse overlap.
The map includes the parameter space of spatial overlap parameter ηr and temporal overlap parameter ηt. The dots mark the experimentally measured limit of different interaction mechanisms. For visual assistance they were connected by lines while the space in between was filled with the color belonging to the prevalent mechanism. The mechanisms are described in Fig. 4 regarding their combination of characteristic interaction effects.
Figure 8.
Contour depiction of the jet characteristics scaled with the applied laser pulse energy.
Jet length within the parameter space of (a) focus separation and pulse energy (scaling the temporal overlap) as well as (b) spatial overlap parameter ηr and temporal overlap parameter ηt, and jet velocity as a function of (c) focus separation and pulse energy as well as (d) the overlap parameters ηr and ηt. The cross signs the maximum impact on the untreated medium (here water) at a maximum value for jet length and velocity at the same time. The dashed and dotted lines show supposed borders between the previously introduced interaction scenarios for visual assistance.
Figure 9.
Snapshots of the bubble-to-bubble interaction within different sample media.
Images show representative interactions after application of constant laser parameters in: (a) De-ionized water, (b) porcine vitreous body (increased artefacts due to inhomogeneities inside the biological tissue), (c) 1% gelatin solution, (d) 2% gelatin solution, and (e) 5% gelatin solution. The pulse energy Epulse corresponds to 8.5-times breakdown threshold Eth in water and the focus separation confirms to Δx = 46.1 µm. All pictures are taken at Δt = 13.6 µs. While (a), (b), and (c) show the typical interaction effects of mechanism 7, in (c) and (d) mechanism 2 conveys.
Table 1.
Overview of the analyzed sample media with constant spot separation.
Figure 10.
Single snapshots of the cavitation bubble interaction dynamics in a 5% porcine gelatin solution.
The pulse energy is 10.5-times breakdown threshold in water and the focus separation confirms to 30.4 µm; the spatial overlap parameters is ηr = 1.41: (a) Formation of second cavity close to first one, (b) jet formation through first bubble, and (c) jet through right cavitation bubble along the direction of laser scanning.
Table 2.
Overview of the analyzed sample media with adapted spot separation.
Figure 11.
Cavitation bubble dynamics after five subsequent laser pulses in water.
Single pictures of the cavitation bubble dynamics due to the application of five subsequent laser pulses using the optimum parameters for water (see Section 3.3). The second and fourth cavities lead to jet formation (see pictures at 15 and 35 µs). Due to the modified overlap by the previous interaction mechanisms the second jet is weaker.