TiO₂ nanostructures via controlled oxidation of titanium LIPSSs using a picosecond pulse train and their coloration properties (<italic>invited</italic>)

ObjectiveStructural coloration, characterized by high brightness, high saturation, and excellent resistance to fading, shows broad application potential in the fields of information display and information hiding. However, when structural designs rely solely on a single dielectric or metal material, it is often difficult to achieve diverse color appearances under normal incidence observation conditions. Moreover, the colorimetric properties are easily limited by changes in observation angle, making it difficult to meet the diverse needs in practical applications. The rational integration of two distinct material types can effectively overcome the constraints of single-component systems, and significantly improve the richness and diversity of color rendering effects. Ultrafast laser-induced periodic surface structures (LIPSSs), as a highly efficient and precise nanofabrication technique, offer substantial research significance and application value. Further precise regulation of the composition of nanoripple materials by laser technology, such as combining laser-induced nanoripple structures with oxide layers, can not only achieve surface coloration but also realize dynamically tunable colors with changes in observation angle and incident angle. This capability underpins the core functions of information display and hiding. Therefore, in this paper, titanium metal is used as the research object, and a surface layer featuring titanium dioxide nanoripple structures is fabricated using LIPSSs combined with picosecond laser-induced oxidation via pulse trains. The color rendering characteristics of the resulting structures are analyzed in depth.MethodsThe experimental procedure is illustrated in Fig.1(a). First, the “cold” processing characteristics of picosecond laser were utilized to fabricate LIPSSs on the titanium surface. Subsequently, the “thermal” processing characteristics of picosecond laser pulse trains were employed to prepare TiO2 nanoripple structures originated from Ti LIPSSs. A picosecond laser processing system with a wavelength of 532 nm and a pulse width of 10 ps was used in the experiment, as shown in Fig.1(b). The parameters for laser-induced nanoripple structures were fixed as follows: A single pulse energy of 0.87 μJ, pulse repetition frequency of 400 kHz, scanning interval of 5 μm, and scanning speed of 2000 mm/s. During the laser thermal oxidation process, pulse train processing was adopted with a pulse burst of 6 pulses, an inter-pulse interval of 50 ns, a pulse repetition frequency of 400 kHz, and a scanning interval of 2 μm. The experiments were conducted under varying scanning speed and laser power. The evolution law of nanostructures under different scanning speeds was observed by scanning electron microscopy (SEM), while the surface color change and oblique angle diffraction characteristics were analyzed by a digital microscope. Based on these analyses, the color variation patterns were obtained, as shown in Fig.2.Results and DiscussionsNumerical simulation analysis of the optical properties of the color rendering unit was carried out using the finite-difference time-domain (FDTD) method (Fig.3). The results show that the color evolution law with varying height variation of TiO2 nanostructures is similar to that of TiO2 thin films. In addition, the color changes follow the principle of grating diffraction under oblique incidence conditions. Experimental studies on laser-induced oxidization show that when the number of pulse sequences is increased to six, various TiO2 nanoripple structures can be fabricated by adjusting the laser power and scanning speed. As the degree of oxidation increases, the width of the TiO2 nanostructures gradually increases, leading to the formation of nanostructures with dual-period characteristics, as shown in Fig.4. Coloration analysis demonstrates that the dual-periodic nanostructures exhibit anisotropic color rendering characteristics in two mutually perpendicular directions. Furthermore, the influence of incident angle on surface color was investigated. The results show that when the surface appears yellow or light green, the color gradually changes to blue with the increase of incident angle; while when the surface appears blue, increasing the incident angle results in a sequential color transition from blue to green, yellow, orange, and finally red, as shown in Fig.5 and Fig.6.ConclusionsAiming to address the limitations of single coloration and restricted observation angles of structural colors in single dielectric or metallic materials, this paper proposes an innovative approach that combines laser cold-induced fabrication of metallic LIPSSs with thermally induced oxidation processes. By achieving precise oxidation on the basis of metallic nanoripples, titanium dioxide nanoripple structures are successfully constructed. This method not only enables surface colorization, but also endows the material with rich and tunable structural colors at different incident angles. The results show that when the initial surface colors are different, there are significant differences in their color evolution laws with the variation of incident angle. This study provides important theoretical support and application insights for color printing on metal surfaces, as well as for information display and hiding technologies.