Time-Resolved Third Order Harmonic Generation on Shocked Silicon Crystals

We are using nonlinear optical diagnostics to probe the shock-induced melt transition in silicon. Pump-probe shock experiments on [100] Si crystals were carried out using the Ti:Sapphire THOR laser (800 nm, 1 J, 600 ps-chirped, 40 fs-compressed). Two dimensional interferometry was used to map rear surface displacement at discrete times to infer a peak shock pressure. Third order harmonic generation (THG) is used to probe the bulk material's long range order, while a reflectivity diagnostic is used in conjuction with the THG diagnostic to determine it's validity. Preliminary evidence shows the anomalous response that at shock pressures $<$100 kbar ($\sim $elastic limit) the THG signal does not decrease; however, at higher pressures of $\sim $300-400 kbar the THG signal falls dramatically indicating fast crystalline disordering.