Fig. 1. Schematic diagram of the ultrafast double pulse femtosecond lasers fabrication system. M, mirror; ND, neutral density attenuator.

Fig. 2. Top view (XZ plane) SEM images (a) where Φ is 0°; (b) where Φ is 90°.

Fig. 3. (a) Microscopic images of the etched micro-channel irradiated by different time delays. (b) Etching rate of a single pulse and a double pulse.

Fig. 4. Side view (YZ plane) the light entrance SEM images of long range periodic nanostructures formed by different time delays after being polished and shortly etched. (a) Δt=0fs; (b) Δt=10ps; (c) Δt=100ps.

Fig. 5. Relationship between the etching depth and the pulse energy ratio.