Evolution of Cone-Shaped Damage Channels in Aluminum Nanofilm Induced by Repeated High-Power Subpicosecond Terahertz Pulses

We investigate the formation of surface periodic structures during ablation of a 20 nm aluminum film on a glass substrate by high-power terahertz pulses. Using subpicosecond pulses in the 0.5–3 THz range with a field strength of 15 MV/cm (fluence 0.3 J/cm2) generated in a DSTMS crystal pumped by a femtosecond Cr:Forsterite laser, we observe discrete growth of cone-shaped damage channels with a period of 20 µm at an energy density below the single pulse ablation threshold (Fa≈0.15 J/cm2). The channel length increases from pulse to pulse (for 8, 20, and 100 pulses) due to local current density enhancement at the channel tip. This enhancement scales inversely with the square root of the tip radius and reaches an order of magnitude. Surface morphology analysis reveals a thermomechanical mechanism governing film destruction. The observed self-organized periodic structures, whose orientation is strictly perpendicular to the THz electric field, hold promise for functional devices in the terahertz band, such as polarizers, near-field sensors, and spatially selective absorbers, provided the formation process can be regulated.