The study of ultrafast dynamics behind a solid-state shock front using optical nanogauges
Lee, I-Yin Sandy
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https://hdl.handle.net/2142/20967
Description
Title
The study of ultrafast dynamics behind a solid-state shock front using optical nanogauges
Author(s)
Lee, I-Yin Sandy
Issue Date
1995
Doctoral Committee Chair(s)
Dlott, Dana D.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Language
eng
Abstract
Photothermal laser ablation is studied using poly-(methyl methacrylate) films doped with a dye, IR-165, which functions as a molecular heater and thermometer. Direct optical measurements of temperature are performed on samples heated by 100 ns near-IR pulses at 1.064 $\mu\rm m,$ at rates dT/dt $\approx 5 \times 10\sp9$ deg/s. Below ablation threshold, the heat capacity measured by optical calorimetry is precisely the value obtained by conventional calorimetry. At ablation threshold, the peak surface temperature is $\rm T\sb{abl} = 600\sp\circ C$ and the weight-fraction of material decomposed at ablation time is $\chi\sb{th} = 0.02.$ With increasing pulse energy, the fraction decomposed increases and a more forceful ablation is observed, but the surface temperature does not continue to increase past $\rm T\sb{lim} = 715\sp\circ C,$ which is determined to be the limiting temperature for thermal decomposition.
Using picosecond optical microscopy on shock targets with different thickness aluminum layers, it was found that the shock required 0.5 ns to form and then it propagated for a few ns with a constant velocity of 8.3 km/s (8.3 nm/ps), indicating a shock pressure of 49 GPa. The arrival time jitter of many hundreds of shocks, at an aluminum/polymer interface was found to be $\pm$50 ps. The shock propagation through a polymer, polyester, was studied by observing the arrival of the front at a 50 nm thick nanogauge embedded in the polymer. When the shock was transmitted from the aluminum to a polymer layer, its velocity was 5.5 km/s, indicating a shock pressure of 14 GPa, in good agreement with shock impedance calculations.
Microfabricated shock target arrays with embedded thin layers of dye-doped polymer films, termed optical nanogauges, are used to measure the velocity and pressure $\rm (P\sim 2$ GPa) of picosecond laser driven shock waves in polymers. The $\sim$60 ps rise time of absorbance changes of the dye appears to be limited by the transit time of the shock across the 300 nm gauge. The rise time of the 2 GPa shock front in poly-methyl methacrylate is $<$60 ps. Picosecond dynamics behind the shock front, which are likely due to transient overheating and subsequent fast cooling of the dye molecules, are observed for the first time.
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