Self-limited underdense microplasmas in bulk silicon induced by ultrashort laser pulses

Abstract

Two-photon ionization by focused femtosecond laser pulses initiates the development of micrometer-scale plasmas in the bulk of silicon. Using pump-and-probe transmission microscopy with infrared light, we investigate the space-time characteristics of these plasmas for laser intensities up to 1012 W/cm2. The measurements reveal a self-limitation of the excitation at a maximum free-carrier density of ≅1019 cm−3, which is more than one order of magnitude below the threshold for permanent modification. The plasmas remain unchanged in the ∼100 ps timescale revealing slow carrier kinetics. The results underline the limits in local control of silicon dielectric permittivity, which are inherent to the use of single near-infrared ultrashort Gaussian pulses.