Interferometric analysis of sub-nanosecond laser-induced optical breakdown dynamics in the bulk of fused-silica glass.

Dynamics of laser-induced optical breakdown in the bulk of fused-silica glass irradiated by a sub-nanosecond laser pulse at a wavelength of 790 nm with a fluence of 522 J/cm was studied by the femtosecond time-resolved complex interferometry in Nomarski arrangement utilising a Fresnel bi-prism. Evolution of the plasma channel and the development of the free electron density were in focus of the investigation. The measured ultimate length of the plasma channel was equal to 30 μm and almost doubled the length estimated within the moving breakdown model. The history of the transient electron density distribution in the plasma was reconstructed from the phase shift maps using the inverse Abel transform and it revealed further deviation from this model. The core of the plasma channel exhibited at the last stages of the development a considerable level of the electron density up to 2.4×10 cm. The signature of the pre-breakdown phase has been identified as radiation caused by ionization-released electrons interacting with ions and has been demonstrated in solids for the first time in this way. Origin of the discrepancy between the theoretical prediction of the moving breakdown model and the measured values of the channel length is discussed as well.