Raman spectroscopy of femtosecond multi-pulse irradiation of vitreous silica: experiment and simulation

We report an experimental and numerical study of femtosecond multi-pulse laser-induced densification in vitreous silica (v-SiO2) and its signature in Raman spectra. We compare the experimental findings to recently developed molecular dynamics (MD) approach accounting for bond-breaking due to laser irradiation, together with a dynamical matrix approach and bond polarizability model based on first-principle calculations for the estimation of Raman spectra. We observe two stages of the laser-induced densification and Raman spectrum evolution: growth during several hundreds of pulses followed by further saturation. At the medium-range, the network connectivity change in v-SiO2 is expressed in reduction of major ring fractions leading to more compacted structure. With the help of Sen & Thorpe model, we also study the short-range order transformation and derive the interbonding Si-O-Si angle change from the Raman measurements. Experimental findings are in excellent agreement with our MD simulations, and, hence, support bond-breaking mechanism of laser-induced densification. Thus, our modeling explains well the laser-induced changes both in the short-range order caused by the appearance of Si-coordination defects and medium-range order connected to evolution of the ring distribution. Finally, our findings disclose similarities between sheared-, permanently-densified- and laser-induced-glass and suggest interesting future experiment in order to clarify the impact of the thermo-mechanical history on glasses under shear, cold- and hot-compression, and laser-induced densification.