Femtosecond laser nanostructuring of titanium metal towards fabrication of low-reflective surfaces over broad wavelength range

Highlights: • Single step sub-wavelength structures were fabricated on the surface of bulk titanium (Ti) metal in air and water environment over an area of 10 × 10 mm² for reflection studies. • High spatial frequency laser induced periodic structures (HSFL) with spatial periodicity of λ/15–λ/12 were fabricated in air and water environments. • The periodicity of low spatial frequency laser induced periodic structures (LSFL) decreases with increase in the number of irradiated pulses. • Smaller structures were observed in water as compared to air. • The fs laser induced surface modifications were found to suppress the specular reflectance of the Ti surface over a wide wavelength range of 250–2000 nm to a great extent. - Abstract: We investigated experimentally the formation of laser induced periodic surface structures (LIPSS) on titanium (Ti) metal upon irradiation with linearly polarized Ti:Sapphire femtosecond (fs) laser pulses of ∼110 fs pulse width and 800 nm wavelength in air and water environments. It is observed that initially formed random and sparsely distributed nano-roughness (nanoholes, nanoparticles and nanoprotrusions) gets periodically structured with increase in number of laser pulses. In air at lower fluence, we observed the formation of high spatial frequency-LIPSS (HSFL) oriented parallel to the laser polarization direction, whereas at higher fluence formation of low spatial frequency-LIPSS (LSFL) were observed that are oriented perpendicular to the incident laser polarization. In water two types of subwavelength structures were observed, one with spatial periodicity of ∼λ/15 and oriented parallel to laser polarization, while the other oriented perpendicular to laser polarization with feature size of λ/4. The optimal conditions for fabricating periodic sub-wavelength structures are determined by controlling the fluence and pulse number. The fs laser induced surface modifications were found to suppress the specular reflection of the Ti surface over a wide wavelength range of 250–2000 nm to a great extent.