Laser-induced focusing shock waves, and study of materials under shock conditions
The lab is located in NE47-577 (617-324-6526)
Cylindrically or spherically focusing shock waves have been of keen interest for the past several decades. In addition to the fundamental study of materials under extreme conditions, cavitation, and sonoluminescence, focusing shock waves enable a myriad of applications including hypervelocity launchers, synthesis of new materials, production of high-temperature and high-density plasma fields, applications in controlled thermonuclear fusion, and a variety of medical therapies. The use of pulsed lasers to excite shock waves has considerably widened the possibilities for study of shock propagation and the dynamic properties of materials under shock loading.
We are exploring an alternate approach to laser shock, in which the shock wave propagates laterally within a thin layer of liquid confined between solid walls. The confined layer is amply accessible for optical diagnostics enabling the direct visualization of shock waves. This approach is especially beneficial for studying cylindrically focused shock waves. The next stage in advancing the “in-plane” laser shock experiment is to develop a range of spectroscopic probes for studying not only shock wave propagation but also the material evolution under shock, including phase transitions, chemical reactions, and other shock-assisted effects. We plan to couple shock waves with ultrafast optical excitation and probing so that material processes under shock loading can be observed with femtosecond time resolution.
Description of experiment:
Following pulsed laser excitation of a thin sample, the shock wave propagates laterally in the plane of the sample rather than through the sample plane as in the front-back approach described above. In the present case, the optical excitation or laser ‘‘shock’’ pulse is focused to a circular ring pattern at the sample, launching a shock wave that propagates and focuses inward toward the center.
A 300-ps, 800-nm laser shock pulse was directed onto an axicon conical prism, and focused by a lens with a 3 cm focal length to form a ring pattern at the sample layer. The ring had a 200 μm diameter, and the beam at any part of the ring had a width of ∼10 μm. 2D spatial images of the propagating shock waves were recorded with a variably delayed 180-fs, 400-nm probe pulse that was directed through the sample and a conventional two-lens imaging system to a CCD camera. For interferometric imaging, as shown in Fig.1, the imaging probe was recombined interferometrically with a reference pulse before reaching the camera. In some measurements, a streak camera and a 200-ns, 532-nm probe pulse derived from the laser that pumps the Ti:sapphire amplifier were used for continuous-time imaging of a linear segment of the shock-wave trajectory extending from opposite sides of the irradiated ring to the focus. In the present experiments, the sample consisted of a 5-μm thick water layer with absorbing suspended carbon nanoparticles made from ink diluted 10x so that the nanoparticle loading was about 2 wt%. The liquid was sandwiched between two 100-μm thick glass substrates using a polymer spacer. The shock wave was generated directly within the sample layer through absorption of the picosecond laser shock pulse by carbon nanoparticles that undergo photoreactive energy release and vaporization to generate high pressure.
Figure 1: Experimental set-up
The present imaging setup allows one exposure at a specified probe pulse time delay to be recorded each time the sample is irradiated and a shock wave is launched.A time history is built up by single exposures taken at different sample positions with different time delays of the femtosecond probe pulse. See Fig.2. Repeated measurements with the same time delay showed no significant variation. The recorded images can be used to extract the shock-wave propagation distance as a function of time, from which we can determine the shock speed during the time interval between any two successive probe pulse delays or averaged over the entire traversal.
Figure 2: Interferometric images of shock waves produced by 0.05 mJ excitation pulse taken at increasing time delays.
White arrows indicate the direction of propagation of shock fronts.
The total time duration for shock propagation to the focus is many nanoseconds, making it impractical to record 2D CCD images (each from a distinct sample region) on a near-continuous basis. However, once it is established that the response is cylindrically symmetric as expected, a single spatial dimension is sufficient and the second dimension can be used for time in a streak camera recording that provides a continuous-time-resolved picture of the entire shock event as shown in Fig. 3-a,b,c.
At 2.5 mJ excitation pulse energy, the shock speed rapidly increases and reaches about 9 km/s, or Mach 6, near the focus at 28 ns. The corresponding pressure is ~30 GPa as shown in Fig. 3-f. The corresponding temperature is ~2500 K based on the known properties of water under shock loading, described by P-T Hugoniot curve. The pressure reached exactly at the focus is expected to be even higher, but as in single-bubble sonoluminescence experiments the peak pressure cannot be determined accurately.
Figure 3: Single-shot streak images of cylindrically focusing shock waves propagating in water at shock laser pulse energies of (a) 0.15, (b) 2.0, and (c) 2.5 mJ. (d) Snapshot image of the shocked sample at 500 ns time delay for a laser shock pulse energy of 0.8 mJ. Cavitation is responsible for bubble formation at the focus. (e) Trajectories of the converging shock waves extracted from streak images a) b) and c) by a best fit polynomial equation. The radial distance on the vertical axis is measured from the center of convergence. (f) Pressure values calculated from fits of the trajectories in (e). The traces above and below the zero-pressure horizontal line represent the pressure values for the upper and lower trajectories shown in (e), derived from the upper and lower streak camera images of shock propagation from opposite sides of the excitation ring toward the center. The y-axis pressure values that appear above and below the zero-pressure line are all positive.
Using interferometric measurements of the focusing shock wave enabled a quantitative analysis of the density profiles of the in-plane propagating waves. Indeed, starting with parallel fringes, any subsequent shift in the fringes on the interferogram can be related to a change in the refractive index of the media, which can be translated into the density variation using an empirical linear relationship. Thus, on a single shot, it is possible to plot the full density profile of the entire shock event. See Fig. 4.
Figure 4: Diameter profiles of the density change corresponding to (a) 34.6 ns and (b) 78.2 ns images in Fig. 2.
Although we do not have a reliable experimental measurement of the pressure at the focus, numerical simulations indicate pressures of up to 30 GPa, and there are clear indications of sharply increased pressure there including severe sample damage, namely a crack in the glass substrate whose nucleation occurred at the center of focus.
Imaging of in-plane propagating shock waves offers an interesting alternative to traditional laser shock experiments. We have demonstrated quantitative interferometric measurements of the density profiles in shock waves, which will allow us to obtain density vs. shock velocity Hugoniot data after some improvements such as precise measurement of the liquid layer thickness.
Adding spectroscopic probes such as IR/Raman/THz will enhance our capability to characterize shock- compressed materials in this geometry. We believe that the technique can be extended to “soft” solid materials such as polymers that would allow effective confinement of the shock wave between walls of much higher elastic modulus such as glass or sapphire. The question of what takes place in the exact focus is of particular interest; the size of the focal area is expected to be smaller than the optical wavelength, which calls for experimental ingenuity. Numerical simulations with high spatial resolution will be helpful in guiding the interpretation. In addition, the capability to capture crack propagation indicates the potential of this method for studying dynamics of brittle fracture.
For more information, see  and 
1. “Direct Visualization of Laser-Driven Focusing Shock Waves,” T. Pezeril, G. Saini, D. Veysset, S. Kooi, P. Fidkowski, R. Radovitzky, and K. A. Nelson, Physical Review Letters, vol. 106, May. 2011, pp. 1-4. http://prl.aps.org/abstract/PRL/v106/i21/e214503
2. "Interferometric analysis of cylindrically focused laser-driven shock waves in a thin liquid layer," D. Veysset, A.A. Maznev, G. Saini, S.E. Kooi, T. Pezeril, and K.A. Nelson, in Shock Compression of Condensed Matter, AIP Conf. Proc. 1426, 1597 (2012). http://proceedings.aip.org/resource/2/apcpcs/1426/1/1597_1