Papers by Hiroki Fujiwara
The picosecond thermal response of a normal octadecanethiol self-assembled monolayer on a gold su... more The picosecond thermal response of a normal octadecanethiol self-assembled monolayer on a gold surface was studied using pump-probe, broadband-infrared, background-suppressed and vibrational sum frequency generation spectroscopy. The orientation fluctuations in response to flash heating were characterized from the intensity kinetics of the terminal methyl C-H stretching vibrations. An intensity decrease (5.7 +/- 1.3 ps) in the asymmetric modes indicated that stochastic processes of molecular conformations developed during multiple light-matter interactions. Including torsional diffusion into the rotation term of second order electrosusceptibility explained the experimental surface temperature increment two order-of-magnitudes smaller than the activation energy of the dihedral angles (~15 K).
AIP Conference Proceedings, 2009
AIP Conference Proceedings, 2012
A method is described where a tabletop laser-driven flyer plate apparatus using ~8 ns launching p... more A method is described where a tabletop laser-driven flyer plate apparatus using ~8 ns launching pulses from a Nd:YAG laser is used to determine the Hugoniot of a thin layer of transparent materials impacted by 25 μm thick flyers moving up to 2 km/s, producing shocks that were fully supported for 8 ns. In this method, a high-speed 8 GHz fiberoptic displacement interferometer looks through the transparent sample, which has an ultrathin 50% reflecting mirror deposited on its surface, to simultaneously measure U s and U p on a single shot. The initial results obtained on a 31 μm thick film of PMMA were in excellent agreement with the Hugoniot literature.
AIP Conference Proceedings, 2012
Nitro stretch probing of a single molecular layer to monitor shock compression with picosecond ti... more Nitro stretch probing of a single molecular layer to monitor shock compression with picosecond time resolution
Transient grating acoustic spectroscopy has been applied to studies on the vibrational energy rel... more Transient grating acoustic spectroscopy has been applied to studies on the vibrational energy relaxation process of the electronic ground state of azulene, two 1-alkylazulenes, and five bridged azulene-anthracenes in three different solvents: 1,1,1-trichloro-1,2,2-trifluoroethane, acetonitrile, and xenon. The solute molecule was vibrationally excited by the photo-excitation of the auzlenyl group to the S1 state through the fast internal conversion, and the rate of solvent thermalization due to the vibrational energy relaxation was determined. The thermalization rates for 1-alkylazulenes and bridged azulene-anthracenes were faster than that of azulene. Based on the results of the thermalization rates of 1-alkylazulenes, we concluded that the acceleration of the energy dissipation from the azulenyl group induced the faster energy dissipation from the solute to the solvent. The vibrational normal mode analysis suggests that the density of the vibrational modes and anharmonic coupling b...
Applied Optics, 2010
Converting a Gaussian to a flat-top beam is useful for many applications including laser-launched... more Converting a Gaussian to a flat-top beam is useful for many applications including laser-launched thin-foil flyer plates. A flat-top beam is needed to maintain a constant launch velocity across the flyer; otherwise, the flyer can disintegrate in flight. Here we discuss and demonstrate the use of a variable reflectivity mirror (VRM) with a Gaussian reflectivity profile with an additional hard aperture and compare it to a refractive beam shaper. An ideal VRM would generate a flat-top beam with 37% efficiency. Readily available high-power Gaussian or super-Gaussian mirrors create an approximate flat-top profile, but there is a trade-off between flatness and efficiency. We show that a super-Gaussian mirror can, in principle, convert an input Gaussian beam with 30% efficiency to a flat-top beam with 3% (maximum-tominimum) variation. With a Gaussian mirror and a high-energy pulsed Nd:YAG laser having relatively poor beam quality, we generate flat-top beams with 25% conversion efficiency having 6% variation (standard deviation σ ¼ 4:2%). The beams are used to launch 400 μm diameter, 25 μm thick Al flyer plates, whose flight was monitored by a high-speed displacement interferometer. The plates flew across a 300 μm gap at 1:3 km=s. The distribution of arrival times at the witness plate was 5 ns, as determined by the rise time of the impact emission. Compared to a total flight time of 260 ns, the velocity spread of different parts of the flyer plate was 2%.
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Papers by Hiroki Fujiwara