Direct time resolved observation of energy deposition by ultrashort laser pulses in fused silica

The highly nonlinear interaction of femtosecond laser pulses with transparent materials (glasses) allows the deposition of optical energy into a restricted volume inside the dielectric. This gives the possibility to induce micrometer-sized structural changes which are associated with local refractive index changes n. For n>0, the writing of waveguides with ultrashort laser pulses has been demonstrated successfully in various glasses. However, neither the dynamics nor the processes leading to the local refractive index changes are fully understood which prevents an optimum control of the laser-induced modification process. In the following article, we describe a technique for time resolved side-picturing allowing ultrafast monitoring of the laser-induced modification inside transparent material. The experimental observation of a strong absorptive electronic cloud along with the launch of a pressure wave is reported. Comparison between femtosecond and picosecond pulse modification is reported under time resolved imaging of the affected volume. Experimental evidences of a higher energy confinement in the case of picosecond irradiations are clearly shown