We present two laser driven shock wave loading techniques utilizing long pulse lasers, laser-launched flyer plate
and confined laser ablation, and their applications to shock physics. The full width at half maximum of the drive
laser pulse ranges from 100 ns to 10 μs, and its energy, from 10 J to 1000 J. The drive pulse is smoothed with
a holographic optical element to achieve spatial homogeneity in loading. We characterize the flyer plate during
flight and dynamically loaded target with temporally and spatially resolved diagnostics. The long duration
and high energy of the drive pulse allow for shockless acceleration of thick flyer plates with 8 mm diameter
and 0.1-2 mm thickness. With transient imaging displacement interferometry and line-imaging velocimetry, we
demonstrate that the planarity (bow and tilt) of the loading is within 2-7 mrad (with an average of 4±1 mrad),
similar to that in conventional techniques including gas gun loading. Plasma heating of target is negligible in
particular when a plasma shield is adopted. For flyer plate loading, supported shock waves can be achieved.
Temporal shaping of the drive pulse in confined laser ablation enables flexible loading, e.g., quasi-isentropic,
Taylor-wave, and off-Hugoniot loading. These dynamic loading techniques using long pulse lasers (0.1-10 μs)
along with short pulse lasers (1-10 ns) can be an accurate, versatile and efficient complement to conventional
shock wave loading for investigating such dynamic responses of materials as Hugoniot elastic limit, plasticity,
spall, shock roughness, equation of state, phase transition, and metallurgical characteristics of shock-recovered
samples, in a wide range of strain rates and pressures at meso- and macroscopic scales.
We have used the thermal modeling tool in COMSOL Multiphysics to investigate factors that affect the thermal
performance of the optical refrigerator. Assuming an ideal cooling element and a non-absorptive dielectric trapping
mirror, the three dominant heating factors are blackbody radiation from the surrounding environment, conductive heat
transfer through mechanical supports, and the absorption of fluoresced photons transmitted through the thermal link.
Laboratory experimentation coupled with computer modeling using Code V optical software have resulted in link
designs capable of reducing the transmission to 0.04% of the fluoresced photons emitted toward the thermal link. The
ideal thermal link will have minimal surface area, provide complete optical isolation for the load, and possess high
thermal conductivity. Modeling results imply that a 1cm3 load can be chilled to 102 K with currently available cooling
efficiencies using a 100 W pump laser on a YB:ZBLANP system, and using an ideal link that has minimal surface area
and no optical transmission. We review the simulated steady-state cooling temperatures reached by the heat load for
several link designs and system configurations as a comparative measure of how well particular configurations perform.
Dielectric mirror leakage at large angles of incidence limits the effectiveness of solid state optical refrigerators due to
reheating caused by photon absorption in an attached load. In this paper, we present several thermally conductive link
solutions to greatly reduce the net photon absorption. The Los Alamos Solid State Optical Refrigerator (LASSOR) has
demonstrated cooling of a Yb3+ doped ZBLANP glass to 208 K. We have designed optically isolating thermal link
geometries capable of extending cooling to a typical heat load with minimal absorptive reheating, and we have tested the
optical performance of these designs. A surrogate source operating at 625 nm was used to mimic the angular distribution
of light from the LASSOR cooling element. While total link performance is dependent on additional factors, we have
found that the best thermal link reduced the net transmission of photons to 0.04%, which includes the trapping mirrors
8.1% transmission. Our measurements of the optical performance of the various link geometries are supported by
computer simulations of the designs using Code V, a commercially available optical modeling software package.
We present an overview of laser cooling of solids. In this
all-solid-state approach to refrigeration, heat is removed radiatively when an engineered material is exposed to high power laser light. We report a record amount of net cooling (88 K below ambient) that has been achieved with a sample made from doped fluoride glass. Issues involved in the design of a practical laser cooler are presented. The possibility of laser cooling of semiconductor sensors is discussed.
Transient surface deformations in dielectric materials induced by laser irradiation were investigated with time-resolved interferometry. Deformation images were acquired at various delay times after exposure to single pulses (100 ps at 1.064 micrometer) on fresh sample regions. Above the ablation threshold, we observe prompt ejection of material and the formation of a single unipolar compressional surface acoustic wave propagating away from the ablation crater. For calcite, no deformation -- either transient or permanent -- is discernable at laser fluences below the threshold for material ejection. Above and below-threshold behavior was investigated using a phosphate glass sample with substantial near infrared absorption (Schott filter KG3). Below threshold, KG3 exhibits the formation of a small bulge roughly the size of the laser spot that reaches its maximum amplitude by approximately 5 ns. By tens of nanoseconds, the deformations become quite complex and very sensitive to laser fluence. The above-threshold behavior of KG3 combines the ablation-induced surface acoustic wave seen in calcite with the bulge seen below threshold in KG3. A velocity of 2.97 +/- 0.03 km/s is measured for the KG3 surface acoustic wave, very close to the Rayleigh wave velocity calculated from material elastic parameters. Details of the transient interferometry system are also given.
Transient surface morphology changes in dielectric materials induced by laser irradiation were investigated with time- resolved interferometry. Deformation images were acquired at various delay times after exposure to single pulses on fresh sample regions. Above the ablation threshold, we observe prompt ejection of material and the formation of a single unipolar compressional surface acoustic wave propagating away from the ablation crater. For calcite, no deformation - either transient or permanent - is discernable at laser fluences below the threshold for material ejection. Below- threshold behavior was investigated using a phosphate glass sample with substantial near IR absorption. KG3 exhibits the formation of a small bulge roughly the size of the laser spot that reaches its maximum amplitude by approximately 5 ns. At lower laser fluences, diffusion of thermal energy away from that region causes a much weaker and boarder bulge to appear on a slower time scale. At higher laser fluences, a pair of strong, unipolar rarefaction surface acoustic waves is launched, separating from the central region at roughly 17 and 22 ns. Details of the transient interferometry system will also be given.
The first infrared vibrational photon echo experiments conducted in a liquid and a glass are reported. The experiments were performed on the CO stretching mode of tungsten hexacarbonyl at 5.1 micrometers (1960 cm-1) in 2-methyltetrahydrofuran over the temperature range 300 K to 16 K using picosecond pulses from the free electron laser at Stanford University. In addition, the first vibrational population relaxation measurements spanning a temperature range that takes a system from a liquid to a supercooled liquid to a glass are reported.
Measurements of orientational relaxation over 6 decades in time have been made on the liquid crystal Methoxy Benzylidene butyl aniline (MBBA) using a Transient Grating Optical Kerr effect experiment (TG-OKE). The Slower dynamics have been shown to fit to Landau-de Gennes modified Debye Stoke Einstein Hydrodynamic equation. The faster dynamics show a power law behavior that is temperature independent for 43 degree(s) above the nematic-isotropic phase transition. The slower dynamics deviate from Landau-de Gennes behavior at the same temperature that the faster dynamics become temperature dependent. This is attributed to the domain size, the factor controlling the slow decay, becoming small enough that the local structure is disturbed. Two possible sets of processes are proposed for the power law dependence of the faster dynamics. A parallel process, the Forster direct transfer model, where there is a distribution of potential surfaces for the system to propagate along and the serial process (or Hierarchically constrained dynamics model) where some degrees of freedom are suppressed unless other degrees of freedom are in particular states. These results are compared to earlier work on pentylcyanobiphenyl(5CB). The same behavior is seen in both 5CB and MBBA.