R. L. Rabie

Spectrally modified chirped pulse generation of sustained shock waves

A method is described for generating shock waves with 10–20 ps risetime followed by >200 ps constant pressure, using spectrally modified (clipped) chirped laser pulses. The degree of spectral clipping alters the chirped pulse temporal intensity profile and thereby the time-dependent pressure (tunable via pulse energy) generated in bare and nitrocellulose-coated Al thin films. The method is implementable in common chirped amplified lasers, and allows synchronous probing with a <200 fs pulse.

Ultrafast interferometric microscopy for laser-driven shock wave characterization

We have applied ultrafast time-resolved two-dimensional interferometric microscopy to the measurement of shock wave breakout from thin metal films. This technique allows the construction of a two-dimensional breakout profile for laser generated impulsive shocks with temporal resolution of <300 fs and out-of-plane spatial resolution of 0.5 nm using 130 fs, 800 nm probe pulses. Constraints placed on the spatial extent of the probed region and on the spatial resolution of the technique by the short duration of the probe pulses are discussed. In combination with other techniques, such as spectral interferometry, this technique provides a powerful means of investigating shock dynamics in a variety of materials.

Ultrafast nonlinear optical method for generation of planar shocks

Planar shocks generated by short pulse lasers are useful in studies of shock compression phenomena and may have applications in materials science, biology, and medicine. We have found the fluence profiles of 120–400 fs duration Gaussian spatial mode incident laser pulses are reproducibly flattened via surface optical breakdown and Kerr focusing in thin dielectric substrates at fluences just above the ablation threshold. These flat laser profiles have been used to produce planar shocks that are flat to 0.7 nm root-mean-square over a 75–100 μm diameter.

High-pressure thermodynamic, electronic and magnetic properties of Ni

The thermodynamic, electronic and magnetic properties of Ni at high pressures have been calculated using the ab initio pseudopotential plane-wave method and the density-functional theory. The P-V-T equation of state is obtained from the Helmholtz free energy of the crystal in the quasiharmonic approximation. The pressure dependence of the thermal expansion coefficient, bulk modulus, electronic band structure, phonon spectrum and the magnetic moment are presented. The calculated results are in good agreement with the available experiment measurements.

Ultrafast Spectroscopic Investigation of Shock Compressed Glycidyl Azide Polymer and Nitrocellulose Films

As a first experiment to observe the initial chemical reactions induced by sub-picosecond laser driven shocks in energetic materials, we have investigated thin films of glycidyl azide polymer (GAP) and nitrocellulose (NC). The GAP and NC films were spin coated onto thin film metal layers that had been vapor coated onlo transparent substrates. Sub-picosecond laser pulses were used to launch planar shock waves thro,ugh the metal layer and into the GAP and NC films. Time-resolved displacement measurements of both GAP and NC films suggest that shock-induced chemical reaction may be occurring. While the GAP film velocity falls as expected between that of the A1 free surface and the particle velocity (using the free surface approximation), the displacement measurements in NC films yield velocities lower than the particle velocity. While data modeling discounting reaction has been unsuccessful, WIR microscopy confirms the presence of unreacted GAP and NC in the shocked regions of the films.

Sub‐picosecond Laser‐Driven Shocks in Metals and Energetic Materials

A high‐energy sub‐picosecond laser was used both to drive a shock into thin film targets and to spectroscopically interrogate the shocked material. Targets were thin films of molecular materials coated or grown upon thin vapor‐plated metal films on thin glass substrates, or neat metal films on thin glass substrates. The non‐linear optical interaction of the shock‐driving laser with the thin glass substrate produced surprisingly flat shock waves. Sub‐picosecond time‐resolved frequency‐ and spatial‐domain interferometries were used to characterize the shock wave as it transited from the thin metal film into the thin molecular material layer. Overviews of the effect of the pressure‐dependent complex index of refraction of the shocked thin film metal layer, ultrafast interferometric interrogation of shocked molecular materials (examples: glycidyl azide polymer and nitrocellulose thin films), and progress in preparation of, as well as the need for, uniform, well oriented, thin energetic material layers appropr...

Ultrafast time-resolved 2D spatial interferometry for shock wave characterization in metal films

We discuss the application of ultrafast time‐resolved two‐dimensional interferometric microscopy to the measurement of shock wave breakout from thin metal films. This technique allows the construction of a two‐dimensional breakout profile for laser generated impulsive shocks with temporal resolution of < 300 fs and out‐of‐plane spatial resolution of 1.5 nm using 130 fs, 800 nm probe pulses. Constraints placed on the spatial extent of the probe region and on the spatial resolution of the technique by the short duration of the probe pulses will be discussed. In combination with other techniques, such as spectral interferometry, this technique provides a powerful means of investigating shock dynamics in a variety of materials.

ULTRAFAST MEASUREMENT OF THE OPTICAL PROPERTIES OF SHOCKED NICKEL AND LASER HEATED GOLD

We have used high‐resolution Frequency Domain Interferometry (FDI) to make the first ultrafast measurement of shock‐induced changes in the optical properties of thin nickel (∼500 nm) targets. Data taken at several angles of incidence allowed the separation of optical effects from material motion, yielding an effective complex index for the shocked material. In contrast to our previous studies of aluminum, measurements with an 800 nm probe wavelength found a phase shift attributable to optical property changes with the same sign as that due to surface motion, during an 11.5 GPa shock breakout. A similar experiment was attempted with thin gold films (∼180 nm) using Ultrafast Spatial Interferometry (USI). However, since the electron‐phonon coupling in gold is extremely weak, a shock is observed as it “forms”. Ballistic electrons and electron‐electron equilibrium cause fast heating of the electrons in the entire thickness of the thin film, followed by lattice excitation through electron‐phonon coupling, event...

Time- and space-resolved optical probing of the shock rise time in thin aluminum films

The time resolution of most common methods (various forms of interferometry, e.g. VISAR, Fabry-Perot, ORVIS) used to measure the free surface particle velocity as a shock exits from a shocked material is from a few ns down to about 200 ps. Dlott et al. have recently obtained a shock rise time resolution of about 25 ps using a method involving singular value decomposition of coherent anti-Stokes Raman (CARS) spectra from thin film nanogauges. We are using frequency domain interferometric methods utilizing 120 fs pulses from a chirped-pulse amplified Ti:sapphire laser to measure the particle velocity rise time as a shock (driven by ca. 0.5 mJ pulses focussed to 75 μm diameter) exits the free surface of 0.5–2 μm aluminum films on thin glass substrates, following the example of Evans, et al. Some preliminary results of these investigations are reported.

Ultrafast nonlinear optical method for generation of flat-top shocks

Flat top shocks generated reproducibly by short pulse lasers are useful in studies of shock compression phenomena and may have applications in materials science, biology, and medicine. We have found the fluence profiles of Gaussian spatial mode 120 - 400 fs duration incident laser pulses are reproducibly flattened via surface optical breakdown in dielectric substrates at fluences just about the breakdown threshold. These flat top laser profiles have been used to produce shocks flat to 0.7 nm RMS over a 75 - 100 micrometer diameter.