Time-resolved study of fs laser-induced plasma in bulk a-SiO_2

摘要

When focusing an ultrashort laser pulse inside a transparent material, a permanent alteration of the (complex) refractive index takes place in the irradiated region. In the tight focusing regime, a precise control over the energy deposition can be achieved. Based on this principle, point by point fabrication of embedded microoptical systems has been demonstrated in the bulk of amorphous fused silica (a-SiO_2). Although the permanent laser-induced structural changes are starting to be known, the mechanisms underlying material modification are not completely understood. A widely accepted scenario of the laser-matter interaction still has to be elaborated. Specifically, the identification and the relative importance of localized and delocalized energy relaxation channels are under debate. The laser energy is coupled into the material through free carriers generated via photoionization mechanisms. Our goal is to study the energy transfer from the free carriers to the bulky material by measuring the time-resolved absorption in the interaction region. Our setup, based on an optical microscope employed in a pump-probe scheme, allows for time-resolved imaging of the interaction region with a temporal resolution about 300 fs and a spatial resolution of 650 nm. By processing ultrafast photographs of the laser generated electron-hole plasma, we are able to monitor the absorbance in-situ. We present results obtained with two illumination probe wavelengths at 400 nm and 800 nm. The time span covered ranges from the very first moments of the laser-matter interaction up to 1 ns. We observe that the absorption kinetics is not homogeneous over the irradiated volume and depends strongly on the initial distribution of laser energy. We propose a correlation of the observed absorption with the creation and decay of optically active defects.