Figure 4.

Normalized intensity profiles of the focus spot along x-direction at respective wavelengths (a) y = 0, λ = 250 nm, z = 5.3 µm, (b) y = 0, λ = 300 nm, z = 4.4 µm, (c) y = 0, λ = 350 nm, z = 3.5 µm and (d) y = 0, λ = 400 nm, z = 2.9 µm.