Figure 5.

(a) Top-view electron density distribution calculated by 3D-FDTD coupled with electron density equation at the pulse peak and (b) the corresponding Fourier transform (FT). Pulse duration is 120 fs (FWHM) and laser wavelength λ = 800 nm in air. In-plane distribution of the electron density are taken at $z=200$ μnm from the silica-air interface. The pulse energy $2\mu J$ corresponds to a peak fluence of 5 J/cm² with a beam waist $w_0=5$ μm. Concentration of inhomogeneities on the surface $C_i=\mathrm{0.1\%}$. Initial size of inhomogeneities on the surface $r=10$ nm. (c) SEM image of a c-SiO₂ surface after the irradiation with $N=20$ pulses of $F_0=7.2$ J/cm² peak fluence. Laser wavelength $\lambda =800$ nm in air. Pulse duration is 150 fs. The beam waist radius is 18 μm.