Non-iridescent yet angle-dependent structural colors on titanium surfaces induced by laser oxidation.

Optically variable features are widely used in product design and anti-counterfeiting. However, current industrial methods rely heavily on chemical inks, which pose environmental concerns and suffer from poor wear and corrosion resistance. We experimentally demonstrate the generation of non-iridescent yet angle-dependent structural colors on titanium surfaces using a nanosecond laser-induced oxidation. Unlike conventional optical color-change methods that rely on multilayer interference, grating diffraction, or surface plasmons, this technique leverages a periodically arranged stepped structure to achieve abrupt color changes under small angle variations. The color shift originates from morphological differences among structures at different heights, which reflect light at distinct angles and produce varying colors through interference effects. The formation mechanism is elucidated through numerical simulations of the processing temperature, revealing that controlled laser ablation, oxidation, and thermal radiation on the sample surface create the unique structure. By tuning the point distance and dwell time, the affected area and intensity of these processes can be regulated. This advancement not only provides new ideas for anti-counterfeiting applications but also broadens the capabilities of laser coloring technology.