Light nurtures plants: The picosecond laser-induced lithops-like microstructures on titanium alloy surface with broad-band ultra-low reflectivity

Micro-nanostructures on metal surfaces have received significant attention due to their widespread applications. Although ultrafast laser processing was a feasible method to fabricate micro-nanostructures, some issues involving the formation and application of novel micro-nanostructures still exist. Here, we proposed the unique lithops-like microstructures on titanium alloy (Ti-6Al-4V) by ultraviolet picosecond (UV-ps) laser treatment. These structures were either separate tomb-like bumps or bump clusters consisting of several interconnected bumps, which were similar in appearance to lithops. Due to nanoparticle deposition, these bumps were also covered with nanostructures. Titanium alloy was oxidized to form Ti2O3 and TiO2 during treatment. The formation of lithops-like microstructures involved the laser-induced physical/chemical processes of melting, flowing, vaporization, oxidation, solidification, and deposition of material. As laser energy increased, the smooth surface gradually evolved into lithops-like microstructures under the synergistic effects of electromagnetic interference, the Marangoni effect, preferential valley ablation, and thermal effects. By combining geometric light trapping within microstructures with surface plasmon resonance (SPR) absorption within nanostructures, these structures exhibited ultra-low reflectivity (0.02%∼2.31%) in the ultraviolet–visible-near-infrared band (200 ∼ 2500 nm). They also showed ultra-low reflectance (<3%) over a wide range of incidence angles (5°∼70°). The anti-reflection properties of lithops-like microstructures were superior to those of some typical micro-nanostructures.