Direction Controllable Nano-Patterning of Titanium by Ultrafast Laser for Surface Coloring and Optical Encryption

Metasurfaces constructed from subwavelength nanostructures provide a route for surface coloring and optical encryption. Because of the high transmittance and refractive index, titanium dioxide (TiO₂) is a candidate for visible metasurfaces, and its nano-processing is essential for TiO₂ metasurfaces. Herein, a direction-controllable ultrafast laser nano-patterning approach is reported for writing TiO₂ nanostructures on titanium. The writing direction is controlled by adjusting the polarization of incident laser. Due to the localization of pulse energy inside the formed nanostructures, diverse planar patterns with feature sizes smaller than the diffraction limit are written. A strip-shaped nanostructure with the width of 344 nm, parallel nanostructures with the center-to-center distance down to 370 nm, and a “THU” pattern with the height of 790 nm are achieved. Grating TiO₂ nanostructures with various periods are fabricated for surface coloring and printing. Based on the anisotropic optical behaviors of grating-type nanostructures, encrypting information by orthogonal scans is demonstrated as an optical encryption strategy. This nano-patterning approach opens a route for the fabrication of TiO₂ metasurfaces, and provides a new method for surface coloring and optical encryption.