Figure 6.

Top-view electron density distributions calculated by 3D-FDTD coupled with electron density equation at the pulse peak with pulse energy $2\mu J$. Pulse duration is 120 fs (FWHM). In-plane distribution of the electron density are taken at (a) $z=50$ nm and (b) $z=250$ nm from the silica-air interface. The pulse energy 2 μJ corresponds to a fluence of 5 μJ/cm² with a beam waist radius $w_0=5$ μm. Laser wavelength λ is 800 nm in air. Concentration of inhomogeneities on the surface is fixed at $C_i=\mathrm{0.1\%}$. Initial size of inhomogeneities on the surface $r=10$ nm. (c) SEM image of the c-SiO₂ surface after the irradiation with $N=20$ pulses of $F_0=5.8$ J/cm² peak fluency. Laser wavelength $\lambda =800$ nm in air. Pulse duration is 150 fs. The beam waist radius is 18 m. (d,e) Calculated 3D electron density profiles showing both types of periodic structures (HSFL and LSFL-||) colored as a function of the depth Z (d) and electron density N _e (e). The cutoff for the electron densities is fixed to $N_e>{10}^{20}$ cm⁻³.