Burst-mode fs-laser direct writing for full-thickness oxidation of Ta thin films

Direct fs-laser (1030~nm/200~fs) write of a throughout oxide Ta$_{2}$O$_{5}$ on a 200~nm Ta film was achieved using a combined ps- and ns- burst mode (Burst-in-Burst or BiB) of fs-pulse exposure at a high 0.6~MHz repetition rate. Few micrometers-wide lines were formed at the center of 12~$\mu$m focal spot by controlled oxidation without ablation. The oxidized regions were flat and optically transparent. Wavelength-scale self-organized ripples of oxidized Ta$_{2}$O$_{5}$ sub-1~$\mu$m gratings were recorded by rastering a $1\times 1$~mm$^2$ area. The oxidized ripples with periodic pattern $\sim wavelength$ were aligned with the polarization of the writing beam. Energy deposition in the burst-mode oxidation is discussed by comparing 200~fs and 20~ps BiB-mode writing modes. The presented strategy of self-guided oxidation with heat deposition by BiB fs-laser opens an opportunity for debris-free and annealing-free oxidation on a sub-wavelength scale.