Relaxation of Dense Electron Plasma Induced by Femtosecond Laser in Dielectric Materials

Electron plasma induced by a focused femtosecond pulse (130fs, 800nm) in dielectric materials (Soda Lime glass, K9 glass, and SiO₂ crystal) is investigated by pump-probe shadow imaging technology. The relaxation of the electron plasma in the conduction band is discussed. In SiO₂ crystals, a fast self-trapping process with a trapping time of 150fs is observed, which is similar to that in fused silica. However, in Soda Lime glass and K9 glass, no self-trapping occurs, and two decay processes are found: one is the energy relaxation process of conduction electrons within several picoseconds, another is an electron-hole recombination process with a timescale of 100ps. The electron collision time τ in the conduction band is also measured to be in the order of 1fs in all of these materials.