Femtosecond Laser Generated Hierarchical Macropore/LIPSS Metasurfaces and Their Ultrabroadband Absorbance, Photothermal Properties, and Thermal-Induced Reflectance Oscillation

As an emerging versatile technique, femtosecond laser structuring has shown its superior capacity to develop hierarchical micro/nanostructures. In this work, macropore/LIPSS micro/nanostructures (LIPSS: laser induced periodic surface structure) prepared by femtosecond laser ablation (fs-LA) of titanium in air are confirmed to be multifunctional metasurfaces, enabling ultrabroadband UV–vis–NIR–MIR absorbance, photothermal conversion, and thermal-emission induced reflectance oscillation. Different laser powers (2, 5, 8, and 15 W) and scanning velocities (50, 100, 200, and 400 mm/s) were adopted to tune the density/size of macropores, which gives the opportunities to evaluate their impact on multifunctions. It is found that macropore/LIPSS structures with larger and deeper macropores possess the best performances for all functions. Reflectance (absorbance = 1 – reflectance) is used as the absorbance indicator for as-prepared opaque samples. A maximal >90% absorbance in the UV–vis–NIR range (0.2–2.5 μm) and >60% absorbance in the MIR range (2.5–16 μm) are achieved due to synergistic metasurface and structural effects, much better than many metal/dielectric metasurface absorbers prepared by lithography-based techniques. The photothermal conversion capacity of Ti substrates can be significantly enhanced after hierarchical micro/nanostructuring, as evidenced by the maximal detectable temperatures of 127 and 50 °C from the ablated/unablated samples after 8 min high-density light irradiation. Pre-coating of a 15 nm thick carbon layer on Ti substrates for fs-LA can effectively increase ablation efficiency to enlarge the macropore size and endow more carbon to the resultant structures, which are beneficial to achieve higher photothermal converters, as indicated by the temperature increase up to 147 °C, making them good candidates for sterilization applications. Multifunctions reported in this work including the first discovery of thermal-induced reflectance oscillation phenomenon for macropore/LIPSS structures prove that fs-LA is a one-step cost-effective technique for multifunctional metasurface manufacturing.