Time-resolved Raman Spectroscopy On Nanosecond Laser-driven Shock Wave In Organic Thin Film

When a beam of laser pulse with the power density higher than GW/cm2 irradiates a metal, the metal in the surface will be gasified instantaneously and a plasma will be ignited. The following laser energy continues to support the expansion of the plasma, resulting in the formation of a series of compression waves inside the irradiated materials. These compression waves continuously chase, superpose, and enhance till the laser-driven shock wave forms. Nowadays, laser-driven shock wave is one of the most widely used dynamic high-pressure loading techniques in the laboratories. Laser-driven shock waves have various applications in the fields of shock-induced chemistry, microstructure processing, laser supporting detonation waves and inertial confinement nuclear fusion. As a transient phenomenon, to characterize the laser-driven shock wave and the dynamic response of the material under shock wave loading remains the focus in the relevant research fields. With time resolved Raman spectroscopy, we not only can investigate the characteristics of the shock wave propagating through materials, but also can synchronously study the structural dynamics of the shocked materials in real-time. However, the excitation efficiency of spontaneous Raman scattering is so poor that it is truly hard to extract the spontaneous Raman signal from the noise signals in the shocked materials, which is a major problem to be solved desperately in the researches. This work aims to improve the signal to noise ratio of Raman spectra in the laser-driven shock wave experiments through designing and optimizing the optical layout for excellent time resolved Raman spectral signals. The results of this work will provide valuable reference data and experimental method for further studies on micro-structural dynamics of organic thin films under laser-driven shock loading.By combining both the ultrafast-laser-driven shock wave and time resolved Raman spectroscopy techniques, we designed and built time resolved Raman spectroscopy system applicable in laser-driven shock wave experiments. We studied the influence of the main components with various parameters in the optical system upon signal-to-noise ratio and the spatial resolution of the experimental spectra. Using the optimized experimental system, single Raman spectral signal acquisition of polycrystalline anthracene thin film has been realized under nanosecond-laser-driven shock wave loading with time resolution of 10 ns. Moreover, we systematically studied the whole impact process of nanosecond laser-driven shock wave in polycrystalline anthracene organic thin film including formation, loading and unloading. Through the analysis of time resolved Raman spectroscopy data, we found out that the shock wave propagated through polycrystalline anthracene film(thickness of 160±10 μm) with the velocity of 3.04±0.19 km/s, the peak pressure up to ~2.3 GPa, the shock wave front width of ~4.8 μm, and the pressure rise time of ~1.54 ns. The shock wave took ~50 ns to propagate through the whole thin film, while the shock pressure unloading time is as long as ~170 ns.