Figure 3.

Top-view electron density distributions calculated by 3D nonlinear Maxwell’s equations coupled with electron density equation 40 fs before the pulse peak, corresponding to the interference patterns (a–c). Pulse duration is 120 fs (FWHM) and laser wavelength λ is 800 nm in air. In-plane distributions of the electron density are taken at (a) $z=20$ nm, (b) $z=100$ nm and (c) $z=200$ nm from the silica-air interface. The pulse energy 2 μJ 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. The electron density is normalized to the critical value $N_{cr}=1.74\cdot {10}^{21}$ cm⁻³.