Structural Coloration on Titanium Films by Direct Femtosecond Laser Patterning Empowered by Neural Networks

Pulsed laser radiation enables high-performing modification of material interfaces, yielding the formation of diverse nanostructures. In particular cases, the imprinted nanostructures permanently modify the optics of the patterned surface through interference, scattering, absorption, or diffraction effects, resulting in its structural coloration with nonfading, stable, and vivid hues. Despite substantial progress achieved in the field of laser-assisted structural coloration of surfaces, there is still a huge need for methods combining facile realization, high resolution unachievable with the best inkjet printers, as well as different coloration schemes realized within a single sample. In this paper, for a single sample representing a Ti-TiO₂-Ti sandwich structure, we unveiled the coexistence of parameter-regulated femtosecond-laser processing regimes yielding the formation of subwavelength ripples, regular thermo-chemical laser-induced periodic surface structures, and uniform oxide films rendering the surface with isotropic and anisotropic polarization-dependent colors at a wide gamut. We elaborated the neural network allowing analysis of the correlation between the processing parameters and color appearance, as well as the realization of predictable printing of multicolor images. Finally, we took advantage of ultrashort pulse duration to demonstrate the exceptional color printing resolution up to 20000 dpi with potential applications toward marking, labeling, and anticounterfeiting.