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

Abstract

Different applications of TiO 2 metasurfaces, such as dynamic metadevices, [8] metalens, [10] color nanoprinting, [2,[11][12][13] vacuum ultraviolet third harmonic generation, [14] and holo graphy [15,16] have been demonstrated. The fabrication of delicate TiO 2 nanostructures is the basis for the applications of TiO 2 metasurfaces.Laser processing is a maskless and path-directed technique, and it has been used in cutting, welding, drilling, and surface coloring. [17][18][19][20][21][22] However, the resolution of laser processing is usually limited by the diffraction limit. [23,24] Compared with the long pulsed laser, ultrafast laser has attracted attentions in both academic research and engineering for its sub-diffraction-limited spatial resolution. [25][26][27] Femtosecond laser induced two-photon polymerization (TPP) can be used for the fabrication of nanostructured components. [28] For example, fibroin nanowires with widths down to 350 nm was achieved by aqueous multiphoton lithography. [29] Polymer nanowires with widths smaller than 175 nm was achieved by a femtosecond two-photon lithography technique. [30] Unfortunately, TPP cannot be used for processing of bulk materials and are required to be fabricated in liquids. Another method for processing nanostructures is laser-induced periodic surface structuring (LIPSS). LIPSS can be fabricated on almost all types of materials (metals, semiconductors, dielectrics). [31][32][33] For instance, uniform periodic nanostructures with width of 375 nm were formed on titanium using a femtosecond laser. [34] However, the period of the nanostructures processed by LIPSS is normally uncontrollable. The method is lack of flexibility for fabrication of TiO 2 metasurfaces. In recent years, nano-patterning on TiO 2 , [35] silicon, [36] and silk fiber, [37] have been achieved by near-field enhanced ultrafast laser induced localized breakdown. Nano-patterning on metal surface has been achieved by spatially modulated ultrafast laser, [38] microspheres assisted nano-patterning, [39] and femtosecond laser direct writing. [40] Nevertheless, these ultrafast laser nanopatterning methods can only be used for drilling and grooving by ablation. An optical nano-patterning method for the preparation of TiO 2 metasurfaces on titanium surface is still desired.Herein, we report an ultrafast laser nano-patterning approach of writing direction controllable TiO 2 nanostructures, which can be used for surface coloring and optical encryption. The dynamic process of TiO 2 nanostructures growth and mechanisms were studied. A diverse of patterns of TiO 2 Metasurfaces constructed from subwavelength nanostructures provide a route for surface coloring and optical encryption. Because of the high transmittance and refractive index, titanium dioxide (TiO 2 ) is a candidate for visible metasurfaces, and its nano-processing is essential for TiO 2 metasurfaces. Herein, a direction-controllable ultrafast laser nano-patterning approach is reported for writing TiO 2 nanostructures on titanium. The writing directio...

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