Real time ultrafast spectroscopy of shock front pore collapse

Shock-wave induced nanopore collapse (average diameter 100 nm) at 4.2 GPa in a 3-μm-thick poly-methyl methacrylate (PMMA) layer is measured in real time using coherent anti-Stokes Raman spectroscopy (CARS). Pore collapse is monitored via CARS transitions of a dye probe embedded in the porous medium. A pore collapse time constant of 3 ns in PMMA is in poor agreement with hydrodynamic pore collapse models but in excellent agreement with a viscoplastic model that uses the “shock viscosity” determined from the PMMA viscoelastic response to shock. The shock viscosity is more than 12 orders of magnitude smaller than the ordinary viscosity. A downstream gauge of polycrystalline anthracene monitors changes in the steeply rising shock front (<25 ps rise time) after passing through the porous medium or a scattering medium with 100-nm-diam scatterers. The anthracene is a two-dimensional (2D) shock gauge that provides a time sequence of CARS spectra S(t,λ). The 2D gauge is shown to be capable of discriminating between a shock front that gradually rises with time constant tr or a bunch of steeply rising shocklets with an arrival time spread equal to tr. The transmitted shock front is shown to consist of a bunch of steep shocklets with an arrival time spread of 550 ps.