“…In contrast, if the length L of the birefringent microtracks ( Figure 3 ) is explicitly taken into account, the derivation for the first time average refractive index change, Δn (633 nm) = Ret / L , indicates very different dependence on laser pulsewidth τ ( Figure 4 c,d). Here, the maximal value of Δn max ≈ (4 ± 2) × 10 −3 at 1030 nm ( Figure 4 c) and Δn max ≈ (1.5 ± 0.5) × 10 −3 at 515 nm ( Figure 4 d) appear pulsewidth-independent but strongly wavelength-dependent, as expected according to the nanoplasmonic mechanism of the form-birefringence inscription in the form of bulk nanogratings [ 19 , 22 ]. Moreover, the maximal values are achieved in the threshold-like manner versus E ( Figure 4 c,d), as expected within the latter mechanism; they will be discussed below.…”