Highspeed holography of laserinduced breakdown in liquidsW. Lauterborn and K. J. Ebeling Citation: Applied Physics Letters 31, 663 (1977); doi: 10.1063/1.89495 View online: http://dx.doi.org/10.1063/1.89495 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/31/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High-speed video study of laser-induced forward transfer of silver nano-suspensions J. Appl. Phys. 114, 064910 (2013); 10.1063/1.4817494 High-speed scanning laser-induced breakdown spectroscopy at 1000 Hz with single pulse evaluation for thedetection of inclusions in steel J. Laser Appl. 17, 183 (2005); 10.2351/1.1961738 Highspeed electron microscopy of laserinduced vaporization of thin films J. Appl. Phys. 69, 2581 (1991); 10.1063/1.348647 Laserinduced dielectric breakdown in cryogenic liquids J. Appl. Phys. 52, 1004 (1981); 10.1063/1.328796 Highspeed photography of laserinduced breakdown in liquids Appl. Phys. Lett. 21, 27 (1972); 10.1063/1.1654204

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BE.E. Bergmann, Bull. Am. Phys. Soc. 21, 600 (1976). fE.E. Bergmann and G.P. Kolleogy, Rev. Sci. Instrum. (to be publtshed).

SP.B. Stephenson and V.P. McDowell, Rev. Sci. Instrum. 45, 427 (1974). ~.R.Abraham and S.R. Smith, Phys. Rev. A 15, 421 (1977).

High-speed holography of laser-induced breakdown in liquids w. Lauterborn and K. J. Ebeling

Drittes Physikalisches lnstitvt, UniYersitiit GOttingell, D-34 GOttingell, Federal Republic of Germany (Received 13 June 1977; accepted for publication 31 August 1977)

The phenomenon of ruby laser-induced breakdown in liquids is investigated by high-speed holography. The advantage of using holography instead of ordinary photography lies in the possibility to easily suppress the bright white light emitted during the breakdown process. This light constitutes an incoherent background on the holographic plate and does not reproduce upon reconstruction of the recorded scene.

PACS numbers: 42.40.My. 47.SS.Bx. 79.20.08

The phenomenon of laser -induced breakdown in liquids has found applications l - 3 in the old field of cavitation because the bubbles formed during breakdown4

-T are

extremely well suited to systematically study bubble dynamics. The new area of research has been called optic cavitation3,s in analogy to the well-established field of acoustic cavitation where intense sound waves lead to the formation of cavities in a liquid.

One main topic of current interest in cavitation physics is the collapse of cavitation bubbles. Bubble collapse is a very fast process (in at least the sub­microsecond range), and the bubbles shrink to very small sizes (in at least the submicrosecond range). The collapse phase of a bubble is thus not easily acces­sible. Now, the expansion phase of the bubble immedi­ately after generation by the laser light is the inverse of the collapse phase as long as the bubble remains spherical and damping is negligible. 2,9

This has been shown to be true for laser-generated bubbles in water2 and is predicted by numerical calcula· tions of simple2 (incompressible liquid) as well as more sophisticated9 (compressible liquid) models of cavity dynamics. For instance, the absolute values of the bubble wall velocity compared at the same bubble radius during expansion and contraction agree to within a few percent down to very small radii. Thus, inferences can be drawn on cavitation bubble collapse by investigat­ing optic breakdown phenomena in liquids.

Figure 1 shows the experimental setup used in the present investigations. Two ruby lasers are used, one for breakdown and another one for holography.

The ruby laser for breakdown emits giant pulses of 30-50-ns duration (Kerr cell Q switch) and some tenth of a joule total energy. These pulses are focused into the liquid under study (usually water) with a single lens of short focal length. In most cases, the lens was sub­merged in the liquid and had a focal length of 44 mm in water.

For high -speed holography of the breakdown and the

663 Applied Physics Letters, Vol. 31, No. 10, 15 November 1977

subsequent events, a usual holographic setup is em­ployed with an off-axis reference beam and diffuse back lighting of the object. A specialty of the arrangement is that the ruby laser for holography may be multiply Q switched, and a series of holograms may be taken by spatial multiplexing through the apertures of a rotating disk directly in front of the holographic plate. 9,10

Figure 2 shows some examples of reconstructions from holograms obtained in this way. Four different breakdown scenes in water are shown at a time shortly after the breakdown (first column: 150 ns, 1. 75 /lS, 3 /lS, and 4. 1 J,ls after breakdown) and some time later (second column: 250 /lS, 67 /ls, 133 /lS, and 154 /lS after breakdown). Bubble formation and shock-wave emission can be observed simultaneously. This is dif­ficult to achieve with ordinary photography T because of the bright white light emitted during the breakdown process and the laser light scattered at the grOwing cavity. But if the breakdown is recorded holographical­ly I this light constitutes an incoherent background on the holographic plate and does not reprOduce upon re­construction of the scene. The incoherent background

'.240mm

ruby laser for breakdown

FIG. 1. Block diagram of the experimental setup for high­speed holography of laser-induced breakdown in liquids.

Copyright © 1917 American Institute of Physics 663

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FIG. 2. Bubble and shock-wave formation after laser-induced breakdown in water as obtained via high-speed holography. Each row shows a different scene at two different times each which are (150 ns, 250 j,ls), (1. 75 j,lS, 67 j,ls), (3 j,lS, 133 j,ls), and (4.1 j,lS, 154 j,ls) after breakdown. The scale can be taken from the 500-j,lm pitch of the screw thread visible in the frames together with purposely attached air bubbles. The ruby laser light is focused from the right.

may lower the maximum obtainable diffraction efficiency of the hologram, but, as Fig. 2 shows, high-quality reconstructions are possible by proper exposure of the holographic plate. It should thus be possible to study the detachment of the shock wave from the bubble and to measure the bubble size at the end of the laser pulse. Picosecond holography would be deSirable for this purpose.

The left-hand photograph in the upper row of Fig. 2 was taken only 150 ns after the breakdown. There are three points of breakdown. At each point, a bubble is formed and a spherical shock wave is emitted. The detachment of the shock wave from the expanding bub­ble seems to be asymmetrical. An explanation cannot be given. When the velocity of the bubble wall and shock wave are compared, it is noted that during the initial

664 Appl. Phys. Lett., Vol. 31, No. 10,15 November 1977

expansion phase of the bubble its wall velocity must be higher than half the shock -front velocity. As can be seen in the upper right-hand photograph in Fig. 2, all three bubbles coalesce in the course of the expansion to one big asymmetrical bubble.

The second row of Fig. 2 shows a similar Situation, but the first photograph (holographic reconstruction) was taken 1. 75 /lS after the breakdown, and there are four or five points of breakdown. A comparison with the photograph directly above shows that the high initial bubble wall velocity is rapidly slowed down, whereas the shock wave goes out with the velocity of sound in the liquid. A careful examination of the picture reveals more than four or five shock waves. These are renec­tions of the primary Shock waves at the other bubbles. The right-hand photograph in the second row gives the situation only about 65 /lS later. A rather complicated bubble scene has formed with many small additional bubbles.

The third and fourth rows are examples of later stages subsequent to the breakdown when the shock waves have reached the air bubbles attached to screws. These bubbles were introduced in the vicinity of the region of breakdown to study the influence of a collapSing cavita­tion bubble on other bubbles. It can be seen that there is a strong interaction back and forth via shock waves (see lower left-hand photograph of Fig. 2). Neverthe­less, the laser -generated Single bubble retains its spherical shape, whereas the attached air bubble is strongly distorted (lower right-hand photograph of Fig. 2). Similar observations were made with laser­generated spherical bubbles in the vicinity of plane solid boundaries. During the expansion phase, the bubble remains almost spherical. 3

Holography has proven to be an easy means to dis­pense with the bright white light that accompanies laser-induced breakdown. Sophisticated measurements like shock-wave detachment from the simultaneously formed bubble should become feasible.

This work was sponsored by the Deutsche Forschun­gsgemeinschaft and the Fraunhofer-GeseUschaft. We thank Dr. K. Hinsch for many discussions on the subject.

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BW. Lauterborn, Phys. Bl. 32, 553 (1976). 9K. J. Ebeling, Ph. D. dissertation (University of Gottingen, Germany, 1976)(unpublished).

loK.J. Ebeling and W. Lauterborn, Opt. Commun. 21, 67 (1977).

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