Materials Under Extreme Conditions

Laser-induced focusing shock waves -- 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 alternate approaches to laser shock, in which the shock wave propagates laterally within a thin layer 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. We are developing 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.

Shock wave focusing in water -- Direct real-time visualization of converging shock waves in a few micron-thick liquid layer was demonstrated in an all-optical experiment. The set-up includes an axicon that focuses an intense picosecond excitation pulse into a ring-shaped pattern in a water layer. Optical excitation induces a shock wave that propagates in the plane of the sample and converges toward the center resulting in cylindrical focusing of the shock front. Streak-camera images with a quasi-cw probe beam yield real-time records of the entire shock. Using a Mach-Zehnder interferometer and time-delayed femtosecond laser pulses, we obtain a series of images tracing the shock wave as it converges at the center of the ring before reemerging as a diverging shock, resulting in the formation of a cavitation bubble. Through quantitative analysis of the interferograms, density profiles of shocked samples are extracted (see Fig. 2, left). The experimental geometry used in our study opens prospects for spatially resolved spectroscopic studies of cavitation and (see Fig. 2, right) materials under shock compression.

Laser-induced particle impact experiments -- High velocity ballistic impact phenomena range in scale from catastrophic events such as impact of asteroids on planets to impact of micron– and sub–micron–sized particles, which, while attracting less public attention, plays a large role in many areas of science and technology. In space exploration, micrometeoroids present both an object for investigation and a hazard to spacecraft. In many earthly technologies microparticle impact often poses a problem by causing erosion, but can also be put to a good use, for example in powder blasting and cold spraying. In medicine and biology ballistic microparticles are used for needle–free gene and drug delivery. Despite the wide range of applications, the dynamics of microparticle impact remain unexplored. While macroscale projectile impact has been studied in real time using high–speed imaging techniques, investigations of microscale impact have been limited to post–mortem analysis of impacted samples.

We have devised a novel, bench-top technique, called laser-induced particle impact test (LIPIT), to investigate material responses to high strain rate deformation, as well as material behavior under high speed impact at the micron scale (see Fig. 3).

(*) Mono-disperse micro-particles are deposited on a laser-absorbing polymer-coated glass substrate (launch pad) to form a monolayer on top of it.
(*) Upon intense laser excitation, vaporization and the rapid expansion of gas causes acceleration of the particles to speeds up to 4 km/s.
(*) Sample response to impact is observed using a high-speed camera (>30 million fps) revealing dynamics of microparticle impact in real-time.

The described method is applicable to a wide range of materials (see Fig. 4). The particle motion, deformation of the specimen and stress waves in the material can be imaged with a temporal resolution of 3 ns – or better if a femtosecond pulse sequence synchronized with the camera frame rate is used to illuminate the sample. The experimental tool will be capable of guiding the modeling effort and supporting the design of novel high-performance materials through direct experimental validation.