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 10x demagnification optics. It was found that modification of b-Si well below (a factor 50x) the single pulse ablation fluence of 0.2 J/cm2 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/e2-intensity profile at the melting regime. For the GB-beam, the smallest width of laser modification at 0.2 J/cm2 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/cm2 (50x reduction). Reflectivity of two-side nanotextured b-Si on plasma-thinned 70-micrometers thick Si was strongly reduced in the near-IR range, reaching transmittance >95% at 1.7-2.1 micrometres wavelengths.