Xianxu Zheng

Shock induced damage and damage threshold of optical K9 glass investigated by laser-driven shock wave

The shock wave driven by short laser pulse is used to study the damage of brittle material K9 glass. The damage morphology of K9 glass surface indicates that the material has experienced different loading modes, respectively, at the central area and the surrounding area of the shock wave. At the central area of shock wave, the wavefront is plane and has a uniform pressure distribution, the material mainly suffers a longitudinal shock pressure; but on the edge the shock wave, the wavefront is approximately spherical, besides longitudinal pressure, transverse tensile stress will emerge inside the material. In the latter case, the damage threshold of the material is much smaller than that in the case of compressing by longitudinal pressure only. According to the relationship between damage area and shock pressure, an experimental method is proposed to measure the damage threshold of materials under shock loading. The damage threshold of K9 glass under spherical shock wave is measured to be about 1.12 GPa; an...

Visualizing Intramolecular Vibrational Redistribution in Cyclotrimethylene Trinitramine (RDX) Crystals by Multiplex Coherent Anti-Stokes Raman Scattering

The femtosecond time-resolved multiplex coherent anti-Stokes Raman scattering (CARS) technique has been performed to investigate intramolecular vibrational redistribution (IVR) through vibrational couplings in 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) molecules. In the multiplex CARS experiment, the supercontinuum (SC) was used as broad-band Stokes light to coherently and collectively excite multiple vibrational modes, and quantum beats arising from vibrational couplings among these modes were observed. The IVR of RDX is visualized by a topological graph of these vibrational couplings, and with analysis of the topological graph, two vibrational modes, both of which are assigned to ring bending, are confirmed to have coupling interactions with most of the other vibrational modes and are considered to have a tendency of energy transfer with these vibrational modes. We suggest that the mode at 466 cm-1 is a portal of energy transfer from outside to inside of the RDX molecule and the mode at 672 cm-1 is an important transit point of energy transfer in the IVR.

Investigation of laser induced breakdown in liquid nitromethane using nanosecond shadowgraphy

A nanosecond time-resolved shadowgraphy is performed to observe a laser-induced breakdown in nitromethane. The digital delays are introduced between a pump beam and an illumination light to achieve a measuring range from 40 ns to 100 ms, which enable us to study the shock wave propagation, bubble dynamics, and other process of the laser-induced breakdown. Compared with distilled water, there are two obvious differences observed in nitromethane: (1) the production of a non-evaporative gas at the final stage, and (2) an absence of the secondary shock wave after the first collapse of the bubble. We also calculated the bubble energy in nitromethane and distilled water under a different incident energy. The results indicate that the bubble energy in nitromethane is more than twice as large as that in water. It is suggested that chemical reactions contribute to the releasing of energy.

Observation of laser-driven shock propagation by nanosecond time-resolved Raman spectroscopy

An improved nanosecond time-resolved Raman spectroscopy is performed to observe laser-driven shock propagation in the anthracene/epoxy glue layer. The digital delay instead of optical delay line is introduced for sake of unlimited time range of detection, which enables the ability to observe both shock loading and shock unloading that always lasts several hundred nanoseconds. In this experiment, the peak pressure of shock wave, the pressure distribution, and the position of shock front in gauge layer were determined by fitting Raman spectra of anthracene using the Raman peak shift simulation. And, the velocity of shock wave was calculated by the time-dependent position of shock front.

Phonon-assisted anti-Stokes excitation: Mechanism for the unusual temperature dependence of the Ce3+ luminescence in yttrium aluminum garnet

Luminescence emission spectra and decay kinetics of Ce3+ in YAG were investigated as a function of temperature (300–800 K) under excitation in the long wavelength tail of the lowest 4f–5d absorption spectrum. Both the anti-Stokes and Stokes emission intensity were found to exhibit an unusual non-monotonic temperature dependence compared to those commonly excited in the absorption band, where luminescence intensity decreases monotonically with increasing temperature. This phenomenon is well explained by a phonon-assisted anti-Stokes excitation model with strong vibronic interaction. It reveals that the intensity enhancement of Ce3+ anti-Stokes and Stokes emission is caused by the anti-Stokes excitation efficiency. Excitation efficiency is enhanced with increasing temperature, and thermal ionization of the 5d electron into the conduction band remains as the quenching mechanism of Ce3+ anti-Stokes and Stokes luminescence at high temperatures.Luminescence emission spectra and decay kinetics of Ce3+ in YAG were investigated as a function of temperature (300–800 K) under excitation in the long wavelength tail of the lowest 4f–5d absorption spectrum. Both the anti-Stokes and Stokes emission intensity were found to exhibit an unusual non-monotonic temperature dependence compared to those commonly excited in the absorption band, where luminescence intensity decreases monotonically with increasing temperature. This phenomenon is well explained by a phonon-assisted anti-Stokes excitation model with strong vibronic interaction. It reveals that the intensity enhancement of Ce3+ anti-Stokes and Stokes emission is caused by the anti-Stokes excitation efficiency. Excitation efficiency is enhanced with increasing temperature, and thermal ionization of the 5d electron into the conduction band remains as the quenching mechanism of Ce3+ anti-Stokes and Stokes luminescence at high temperatures.