Because of the outstanding photochemical and optical properties, the titanium dioxide (TiO₂) has been used for applications such as energy materials and optical information. Compared to other semiconductor photocatalysts, TiO₂ has attracted more attentions since the report of its ability for water splitting. In the recent half century, TiO₂ is considered one of the ideal semiconductor photocatalysts because of its abundance, high stability, nontoxicity, and low cost. Because of its characteristics of high transmittance and high refractive index, TiO₂ also has application prospects in the field of optical information. With the development of micro/nano technology, advanced micro/nano processing technologies have been used to prepare TiO₂ micro/nano structures and functional devices (Fig. 1). In the field of energy materials, TiO₂ micro/nano structures can be used for photoelectrochemistry water-splitting, dye-sensitized solar cells, photocatalysis, photovoltaic cells, photodetectors, and lithium-ion batteries. In the field of optical information, TiO₂ micro/nano structures can be used for metasurface structure color, optical encryption, holography, and optical metadevices. The micro/nano processing of TiO₂ is essential for the applications of TiO₂ micro/nano functional devices.

The ultrafast laser processing minimizes heat affected zone due to its ultra-short interaction time with the material. The ultrafast laser can be used to process almost any material due to the ultra-high power density. This provides an ideal choice for the processing of TiO₂ micro/nano structures and functional devices. The ultrafast laser micro/nano processing of TiO₂ has attracted attention in recent years. In this review, we introduce the research progress of ultrafast laser processing of TiO₂ micro/nano structures and functional devices. Processing approaches and applications of TiO₂ micro/nano structures are discussed. In addition, the prospects of ultrafast laser processing of TiO₂ micro/nano structures and functional devices are discussed.

Four ultrafast laser processing approaches are commonly used for TiO₂ micro/nano structures (Fig. 4), including ultrafast laser ablation of material surface, laser-induced periodic surface structure, ultrafast laser induced phase transformation of TiO₂ and ultrafast laser writing TiO₂ micro/nano structures on titanium. Numerical calculation models for ultrafast laser ablation of TiO₂ have been established. The ablation thresholds are calculated, and the results are consistent with the experimental results (Fig. 5). A method is developed to achieve processing of amorphous TiO₂ nanotubes and their transformation to anatase using the ultrafast laser (Fig. 6). Compared with thermal annealed pure TiO₂ nanotubes, TiO₂ nanotubes with exposed reactive anatase {010} facets are prepared after the ultrafast laser induced phase transformation. Based on the method of laser induced in situ growth of TiO₂, a "THU" pattern with height smaller than the laser wavelength is realized (Fig. 7).

TiO₂ micro/nano functional devices, such as devices for photoelectrochemistry water-splitting, structural color, and optical encryption, are also discussed. TiO₂ nanotubes with exposed reactive anatase {010} facets can be prepared by the ultrafast laser. Due to the exposure of reactive facets, the TiO₂ nanotubes show an improved photocurrent density about five times of those of the thermal annealed pure anatase TiO₂ nanotubes (Fig. 9). Based on the method of laser induced in situ growth of TiO₂, grating-type TiO₂ nanostructures with various periods can be processed. The grating-type TiO₂ nanostructures is used for grating-based structural colors in the visible range. The observed colors are determined by the grating period with a fixed incident angle (Fig. 10). Due to the anisotropic optical behavior of grating-type structures, an optical encryption strategy is developed. The horizontal and vertical nanostructures demonstrate a remarkable difference in scattered intensity when visualized under a dark field optical microscope. The scattered intensity is the strongest when the nanostructures are illuminated along the direction perpendicular to the nanostructures. While it is the weakest when the nanostructures are illuminated along the direction parallel to them. Two different images can be encrypted on the same position (Fig. 11).

We introduce the research progress of ultrafast laser processing of TiO₂ micro/nano structures and functional devices. The photochemical and optical properties of TiO₂ are introduced. The photochemical and optical properties of TiO₂ can be adjusted by preparing TiO₂ with different micro/nano structures. A model of transient local electron density is established. The theoretical predictions of ablation threshold and diameters are realized. Different approaches, including ultrafast laser ablation of material surface, laser-induced periodic surface structure, ultrafast laser induced phase transformation of TiO₂ and ultrafast laser writing TiO₂ micro/nano structures on the surface of titanium plate, have been studied for the processing of TiO₂ micro/nano structures and functional devices. Ultrafast laser provides a choice for the processing of TiO₂ micro/nano structures and functional devices. The method shows promising applications in fields such as photoelectrochemistry water-splitting, structural color, and optical encryption devices. The processing of TiO₂ micro/nano structures and functional devices using the ultrafast laser also has some challenges, for example, the efficiency needs to be improved. With the further research on the mechanism and processing technology, it is expected to further expand the application ranges of TiO₂ micro/nano structures and functional devices.