Ablation of black-Si by (Gauss-)Bessel femtosecond laser beams

Laser machining and modification of black-Si (b-Si) by femtosecond laser Gaussian (G-) and Gauss-Bessel (GB-) beams are compared at a wavelength of 1030 nm. The GB-beam was generated using a diffractive axicon lens and 10 × demagnification optics. It was found that modification of b-Si well below (a factor ∼ 50 × ) the single pulse ablation fluence of 0.2 J/cm 2 was possible, corresponding to ablation/melting of nano-needles. The width of modification was almost independent of pulse energy/fluence and had a width of 1 / e 2 -intensity profile at the melting regime. For the GB-beam, the smallest width of laser modification at ∼ 0.2 J/cm 2 threshold (at the center core) was close to the FWHM of the core of the GB-beam. The aspect ratio of the ablated groove on the surface of b-Si made by GB-beam was twice as large –up to 8– compared to that achievable with G-beam, and it was at a lower fluence of ∼ 4 J/cm 2 ( ∼ 50 × reduction). Reflectivity of two-side nanotextured b-Si on plasma-thinned 70-µm thick Si was strongly reduced in the near-IR range, reaching transmittance > 95 % at 1.7–2.1 µm wavelengths.